PERLFUNC(1) Perl Programmers Reference Guide PERLFUNC(1)NAMEperlfunc - Perl builtin functions
DESCRIPTION
The functions in this section can serve as terms in an
expression. They fall into two major categories: list opera-
tors and named unary operators. These differ in their pre-
cedence relationship with a following comma. (See the pre-
cedence table in perlop.) List operators take more than one
argument, while unary operators can never take more than one
argument. Thus, a comma terminates the argument of a unary
operator, but merely separates the arguments of a list
operator. A unary operator generally provides a scalar con-
text to its argument, while a list operator may provide
either scalar or list contexts for its arguments. If it
does both, the scalar arguments will be first, and the list
argument will follow. (Note that there can ever be only one
such list argument.) For instance, splice() has three
scalar arguments followed by a list, whereas gethostbyname()
has four scalar arguments.
In the syntax descriptions that follow, list operators that
expect a list (and provide list context for the elements of
the list) are shown with LIST as an argument. Such a list
may consist of any combination of scalar arguments or list
values; the list values will be included in the list as if
each individual element were interpolated at that point in
the list, forming a longer single-dimensional list value.
Commas should separate elements of the LIST.
Any function in the list below may be used either with or
without parentheses around its arguments. (The syntax
descriptions omit the parentheses.) If you use the
parentheses, the simple (but occasionally surprising) rule
is this: It looks like a function, therefore it is a func-
tion, and precedence doesn't matter. Otherwise it's a list
operator or unary operator, and precedence does matter. And
whitespace between the function and left parenthesis doesn't
count--so you need to be careful sometimes:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.
If you run Perl with the -w switch it can warn you about
this. For example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
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A few functions take no arguments at all, and therefore work
as neither unary nor list operators. These include such
functions as "time" and "endpwent". For example,
"time+86_400" always means "time() + 86_400".
For functions that can be used in either a scalar or list
context, nonabortive failure is generally indicated in a
scalar context by returning the undefined value, and in a
list context by returning the null list.
Remember the following important rule: There is no rule that
relates the behavior of an expression in list context to its
behavior in scalar context, or vice versa. It might do two
totally different things. Each operator and function decides
which sort of value it would be most appropriate to return
in scalar context. Some operators return the length of the
list that would have been returned in list context. Some
operators return the first value in the list. Some opera-
tors return the last value in the list. Some operators
return a count of successful operations. In general, they
do what you want, unless you want consistency.
A named array in scalar context is quite different from what
would at first glance appear to be a list in scalar context.
You can't get a list like "(1,2,3)" into being in scalar
context, because the compiler knows the context at compile
time. It would generate the scalar comma operator there,
not the list construction version of the comma. That means
it was never a list to start with.
In general, functions in Perl that serve as wrappers for
system calls of the same name (like chown(2), fork(2),
closedir(2), etc.) all return true when they succeed and
"undef" otherwise, as is usually mentioned in the descrip-
tions below. This is different from the C interfaces, which
return "-1" on failure. Exceptions to this rule are "wait",
"waitpid", and "syscall". System calls also set the special
$! variable on failure. Other functions do not, except
accidentally.
Perl Functions by Category
Here are Perl's functions (including things that look like
functions, like some keywords and named operators) arranged
by category. Some functions appear in more than one place.
Functions for SCALARs or strings
"chomp", "chop", "chr", "crypt", "hex", "index", "lc",
"lcfirst", "length", "oct", "ord", "pack", "q/STRING/",
"qq/STRING/", "reverse", "rindex", "sprintf", "substr",
"tr///", "uc", "ucfirst", "y///"
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Regular expressions and pattern matching
"m//", "pos", "quotemeta", "s///", "split", "study",
"qr//"
Numeric functions
"abs", "atan2", "cos", "exp", "hex", "int", "log",
"oct", "rand", "sin", "sqrt", "srand"
ARRAYs@u-3p Functions for real @ARRAYs
"pop", "push", "shift", "splice", "unshift"
Functions for list data
"grep", "join", "map", "qw/STRING/", "reverse", "sort",
"unpack"
Functions for real %HASHes
"delete", "each", "exists", "keys", "values"
Input and output functions
"binmode", "close", "closedir", "dbmclose", "dbmopen",
"die", "eof", "fileno", "flock", "format", "getc",
"print", "printf", "read", "readdir", "rewinddir",
"seek", "seekdir", "select", "syscall", "sysread", "sys-
seek", "syswrite", "tell", "telldir", "truncate",
"warn", "write"
Functions for fixed length data or records
"pack", "read", "syscall", "sysread", "syswrite",
"unpack", "vec"
Functions for filehandles, files, or directories
"-X", "chdir", "chmod", "chown", "chroot", "fcntl",
"glob", "ioctl", "link", "lstat", "mkdir", "open",
"opendir", "readlink", "rename", "rmdir", "stat", "sym-
link", "sysopen", "umask", "unlink", "utime"
Keywords related to the control flow of your Perl program
"caller", "continue", "die", "do", "dump", "eval",
"exit", "goto", "last", "next", "redo", "return", "sub",
"wantarray"
Keywords related to scoping
"caller", "import", "local", "my", "our", "package",
"use"
Miscellaneous functions
"defined", "dump", "eval", "formline", "local", "my",
"our", "reset", "scalar", "undef", "wantarray"
Functions for processes and process groups
"alarm", "exec", "fork", "getpgrp", "getppid", "get-
priority", "kill", "pipe", "qx/STRING/", "setpgrp",
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"setpriority", "sleep", "system", "times", "wait",
"waitpid"
Keywords related to perl modules
"do", "import", "no", "package", "require", "use"
Keywords related to classes and object-orientedness
"bless", "dbmclose", "dbmopen", "package", "ref", "tie",
"tied", "untie", "use"
Low-level socket functions
"accept", "bind", "connect", "getpeername", "getsock-
name", "getsockopt", "listen", "recv", "send", "set-
sockopt", "shutdown", "socket", "socketpair"
System V interprocess communication functions
"msgctl", "msgget", "msgrcv", "msgsnd", "semctl",
"semget", "semop", "shmctl", "shmget", "shmread",
"shmwrite"
Fetching user and group info
"endgrent", "endhostent", "endnetent", "endpwent", "get-
grent", "getgrgid", "getgrnam", "getlogin", "getpwent",
"getpwnam", "getpwuid", "setgrent", "setpwent"
Fetching network info
"endprotoent", "endservent", "gethostbyaddr", "gethost-
byname", "gethostent", "getnetbyaddr", "getnetbyname",
"getnetent", "getprotobyname", "getprotobynumber", "get-
protoent", "getservbyname", "getservbyport", "getser-
vent", "sethostent", "setnetent", "setprotoent", "set-
servent"
Time-related functions
"gmtime", "localtime", "time", "times"
Functions new in perl5
"abs", "bless", "chomp", "chr", "exists", "formline",
"glob", "import", "lc", "lcfirst", "map", "my", "no",
"our", "prototype", "qx", "qw", "readline", "readpipe",
"ref", "sub*", "sysopen", "tie", "tied", "uc",
"ucfirst", "untie", "use"
* - "sub" was a keyword in perl4, but in perl5 it is an
operator, which can be used in expressions.
Functions obsoleted in perl5
"dbmclose", "dbmopen"
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Portability
Perl was born in Unix and can therefore access all common
Unix system calls. In non-Unix environments, the func-
tionality of some Unix system calls may not be available, or
details of the available functionality may differ slightly.
The Perl functions affected by this are:
"-X", "binmode", "chmod", "chown", "chroot", "crypt",
"dbmclose", "dbmopen", "dump", "endgrent", "endhostent",
"endnetent", "endprotoent", "endpwent", "endservent",
"exec", "fcntl", "flock", "fork", "getgrent", "getgrgid",
"gethostbyname", "gethostent", "getlogin", "getnetbyaddr",
"getnetbyname", "getnetent", "getppid", "getpgrp", "get-
priority", "getprotobynumber", "getprotoent", "getpwent",
"getpwnam", "getpwuid", "getservbyport", "getservent", "get-
sockopt", "glob", "ioctl", "kill", "link", "lstat",
"msgctl", "msgget", "msgrcv", "msgsnd", "open", "pipe",
"readlink", "rename", "select", "semctl", "semget", "semop",
"setgrent", "sethostent", "setnetent", "setpgrp", "setprior-
ity", "setprotoent", "setpwent", "setservent", "setsockopt",
"shmctl", "shmget", "shmread", "shmwrite", "socket", "sock-
etpair", "stat", "symlink", "syscall", "sysopen", "system",
"times", "truncate", "umask", "unlink", "utime", "wait",
"waitpid"
For more information about the portability of these func-
tions, see perlport and other available platform-specific
documentation.
Alphabetical Listing of Perl Functions
-X FILEHANDLE
-X EXPR
-X A file test, where X is one of the letters listed
below. This unary operator takes one argument,
either a filename or a filehandle, and tests the
associated file to see if something is true about
it. If the argument is omitted, tests $_, except
for "-t", which tests STDIN. Unless otherwise docu-
mented, it returns 1 for true and '' for false, or
the undefined value if the file doesn't exist.
Despite the funny names, precedence is the same as
any other named unary operator, and the argument may
be parenthesized like any other unary operator. The
operator may be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
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-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size (is empty).
-s File has nonzero size (returns size in bytes).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO), or Filehandle is a pipe.
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is an ASCII text file (heuristic guess).
-B File is a "binary" file (opposite of -T).
-M Script start time minus file modification time, in days.
-A Same for access time.
-C Same for inode change time (Unix, may differ for other platforms)
Example:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file permission operators
"-r", "-R", "-w", "-W", "-x", and "-X" is by default
based solely on the mode of the file and the uids
and gids of the user. There may be other reasons
you can't actually read, write, or execute the file.
Such reasons may be for example network filesystem
access controls, ACLs (access control lists), read-
only filesystems, and unrecognized executable for-
mats.
Also note that, for the superuser on the local
filesystems, the "-r", "-R", "-w", and "-W" tests
always return 1, and "-x" and "-X" return 1 if any
execute bit is set in the mode. Scripts run by the
superuser may thus need to do a stat() to determine
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the actual mode of the file, or temporarily set
their effective uid to something else.
If you are using ACLs, there is a pragma called
"filetest" that may produce more accurate results
than the bare stat() mode bits. When under the "use
filetest 'access'" the above-mentioned filetests
will test whether the permission can (not) be
granted using the access() family of system calls.
Also note that the "-x" and "-X" may under this
pragma return true even if there are no execute per-
mission bits set (nor any extra execute permission
ACLs). This strangeness is due to the underlying
system calls' definitions. Read the documentation
for the "filetest" pragma for more information.
Note that "-s/a/b/" does not do a negated substitu-
tion. Saying "-exp($foo)" still works as expected,
however--only single letters following a minus are
interpreted as file tests.
The "-T" and "-B" switches work as follows. The
first block or so of the file is examined for odd
characters such as strange control codes or charac-
ters with the high bit set. If too many strange
characters (>30%) are found, it's a "-B" file; oth-
erwise it's a "-T" file. Also, any file containing
null in the first block is considered a binary file.
If "-T" or "-B" is used on a filehandle, the current
IO buffer is examined rather than the first block.
Both "-T" and "-B" return true on a null file, or a
file at EOF when testing a filehandle. Because you
have to read a file to do the "-T" test, on most
occasions you want to use a "-f" against the file
first, as in "next unless -f $file && -T $file".
If any of the file tests (or either the "stat" or
"lstat" operators) are given the special filehandle
consisting of a solitary underline, then the stat
structure of the previous file test (or stat opera-
tor) is used, saving a system call. (This doesn't
work with "-t", and you need to remember that
lstat() and "-l" will leave values in the stat
structure for the symbolic link, not the real file.)
(Also, if the stat buffer was filled by an "lstat"
call, "-T" and "-B" will reset it with the results
of "stat _"). Example:
print "Can do.\n" if -r $a || -w _ || -x _;
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stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
abs VALUE
abs Returns the absolute value of its argument. If VALUE
is omitted, uses $_.
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as the
accept(2) system call does. Returns the packed
address if it succeeded, false otherwise. See the
example in "Sockets: Client/Server Communication" in
perlipc.
On systems that support a close-on-exec flag on
files, the flag will be set for the newly opened
file descriptor, as determined by the value of $^F.
See "$^F" in perlvar.
alarm SECONDS
alarm Arranges to have a SIGALRM delivered to this process
after the specified number of wallclock seconds has
elapsed. If SECONDS is not specified, the value
stored in $_ is used. (On some machines, unfor-
tunately, the elapsed time may be up to one second
less or more than you specified because of how
seconds are counted, and process scheduling may
delay the delivery of the signal even further.)
Only one timer may be counting at once. Each call
disables the previous timer, and an argument of 0
may be supplied to cancel the previous timer without
starting a new one. The returned value is the
amount of time remaining on the previous timer.
For delays of finer granularity than one second, you
may use Perl's four-argument version of select()
leaving the first three arguments undefined, or you
might be able to use the "syscall" interface to
access setitimer(2) if your system supports it. The
Time::HiRes module (from CPAN, and starting from
Perl 5.8 part of the standard distribution) may also
prove useful.
It is usually a mistake to intermix "alarm" and
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"sleep" calls. ("sleep" may be internally imple-
mented in your system with "alarm")
If you want to use "alarm" to time out a system call
you need to use an "eval"/"die" pair. You can't
rely on the alarm causing the system call to fail
with $! set to "EINTR" because Perl sets up signal
handlers to restart system calls on some systems.
Using "eval"/"die" always works, modulo the caveats
given in "Signals" in perlipc.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For more information see perlipc.
atan2 Y,X
Returns the arctangent of Y/X in the range -PI to
PI.
For the tangent operation, you may use the
"Math::Trig::tan" function, or use the familiar
relation:
sub tan { sin($_[0]) / cos($_[0]) }
Note that atan2(0, 0) is not well-defined.
bind SOCKET,NAME
Binds a network address to a socket, just as the
bind system call does. Returns true if it suc-
ceeded, false otherwise. NAME should be a packed
address of the appropriate type for the socket. See
the examples in "Sockets: Client/Server Communica-
tion" in perlipc.
binmode FILEHANDLE, LAYER
binmode FILEHANDLE
Arranges for FILEHANDLE to be read or written in
"binary" or "text" mode on systems where the run-
time libraries distinguish between binary and text
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files. If FILEHANDLE is an expression, the value is
taken as the name of the filehandle. Returns true
on success, otherwise it returns "undef" and sets $!
(errno).
On some systems (in general, DOS and Windows-based
systems) binmode() is necessary when you're not
working with a text file. For the sake of portabil-
ity it is a good idea to always use it when
appropriate, and to never use it when it isn't
appropriate. Also, people can set their I/O to be
by default UTF-8 encoded Unicode, not bytes.
In other words: regardless of platform, use bin-
mode() on binary data, like for example images.
If LAYER is present it is a single string, but may
contain multiple directives. The directives alter
the behaviour of the file handle. When LAYER is
present using binmode on text file makes sense.
If LAYER is omitted or specified as ":raw" the
filehandle is made suitable for passing binary data.
This includes turning off possible CRLF translation
and marking it as bytes (as opposed to Unicode char-
acters). Note that, despite what may be implied in
"Programming Perl" (the Camel) or elsewhere, ":raw"
is not the simply inverse of ":crlf" -- other layers
which would affect binary nature of the stream are
also disabled. See PerlIO, perlrun and the discus-
sion about the PERLIO environment variable.
The ":bytes", ":crlf", and ":utf8", and any other
directives of the form ":...", are called I/O
layers. The "open" pragma can be used to establish
default I/O layers. See open.
The LAYER parameter of the binmode() function is
described as "DISCIPLINE" in "Programming Perl, 3rd
Edition". However, since the publishing of this
book, by many known as "Camel III", the consensus of
the naming of this functionality has moved from
"discipline" to "layer". All documentation of this
version of Perl therefore refers to "layers" rather
than to "disciplines". Now back to the regularly
scheduled documentation...
To mark FILEHANDLE as UTF-8, use ":utf8".
In general, binmode() should be called after open()
but before any I/O is done on the filehandle. Cal-
ling binmode() will normally flush any pending
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buffered output data (and perhaps pending input
data) on the handle. An exception to this is the
":encoding" layer that changes the default character
encoding of the handle, see open. The ":encoding"
layer sometimes needs to be called in mid-stream,
and it doesn't flush the stream. The ":encoding"
also implicitly pushes on top of itself the ":utf8"
layer because internally Perl will operate on UTF-8
encoded Unicode characters.
The operating system, device drivers, C libraries,
and Perl run-time system all work together to let
the programmer treat a single character ("\n") as
the line terminator, irrespective of the external
representation. On many operating systems, the
native text file representation matches the internal
representation, but on some platforms the external
representation of "\n" is made up of more than one
character.
Mac OS, all variants of Unix, and Stream_LF files on
VMS use a single character to end each line in the
external representation of text (even though that
single character is CARRIAGE RETURN on Mac OS and
LINE FEED on Unix and most VMS files). In other sys-
tems like OS/2, DOS and the various flavors of MS-
Windows your program sees a "\n" as a simple "\cJ",
but what's stored in text files are the two charac-
ters "\cM\cJ". That means that, if you don't use
binmode() on these systems, "\cM\cJ" sequences on
disk will be converted to "\n" on input, and any
"\n" in your program will be converted back to
"\cM\cJ" on output. This is what you want for text
files, but it can be disastrous for binary files.
Another consequence of using binmode() (on some sys-
tems) is that special end-of-file markers will be
seen as part of the data stream. For systems from
the Microsoft family this means that if your binary
data contains "\cZ", the I/O subsystem will regard
it as the end of the file, unless you use binmode().
binmode() is not only important for readline() and
print() operations, but also when using read(),
seek(), sysread(), syswrite() and tell() (see perl-
port for more details). See the $/ and "$\" vari-
ables in perlvar for how to manually set your input
and output line-termination sequences.
bless REF,CLASSNAME
bless REF
This function tells the thingy referenced by REF
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that it is now an object in the CLASSNAME package.
If CLASSNAME is omitted, the current package is
used. Because a "bless" is often the last thing in
a constructor, it returns the reference for conveni-
ence. Always use the two-argument version if a
derived class might inherit the function doing the
blessing. See perltoot and perlobj for more about
the blessing (and blessings) of objects.
Consider always blessing objects in CLASSNAMEs that
are mixed case. Namespaces with all lowercase names
are considered reserved for Perl pragmata. Builtin
types have all uppercase names. To prevent confu-
sion, you may wish to avoid such package names as
well. Make sure that CLASSNAME is a true value.
See "Perl Modules" in perlmod.
caller EXPR
caller Returns the context of the current subroutine call.
In scalar context, returns the caller's package name
if there is a caller, that is, if we're in a subrou-
tine or "eval" or "require", and the undefined value
otherwise. In list context, returns
($package, $filename, $line) = caller;
With EXPR, it returns some extra information that
the debugger uses to print a stack trace. The value
of EXPR indicates how many call frames to go back
before the current one.
($package, $filename, $line, $subroutine, $hasargs,
$wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
Here $subroutine may be "(eval)" if the frame is not
a subroutine call, but an "eval". In such a case
additional elements $evaltext and $is_require are
set: $is_require is true if the frame is created by
a "require" or "use" statement, $evaltext contains
the text of the "eval EXPR" statement. In particu-
lar, for an "eval BLOCK" statement, $filename is
"(eval)", but $evaltext is undefined. (Note also
that each "use" statement creates a "require" frame
inside an "eval EXPR" frame.) $subroutine may also
be "(unknown)" if this particular subroutine happens
to have been deleted from the symbol table. $hasargs
is true if a new instance of @_ was set up for the
frame. $hints and $bitmask contain pragmatic hints
that the caller was compiled with. The $hints and
$bitmask values are subject to change between ver-
sions of Perl, and are not meant for external use.
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Furthermore, when called from within the DB package,
caller returns more detailed information: it sets
the list variable @DB::args to be the arguments with
which the subroutine was invoked.
Be aware that the optimizer might have optimized
call frames away before "caller" had a chance to get
the information. That means that caller(N) might
not return information about the call frame you
expect it do, for "N > 1". In particular, @DB::args
might have information from the previous time
"caller" was called.
chdir EXPR
chdir FILEHANDLE
chdir DIRHANDLE
chdir Changes the working directory to EXPR, if possible.
If EXPR is omitted, changes to the directory speci-
fied by $ENV{HOME}, if set; if not, changes to the
directory specified by $ENV{LOGDIR}. (Under VMS, the
variable $ENV{SYS$LOGIN} is also checked, and used
if it is set.) If neither is set, "chdir" does noth-
ing. It returns true upon success, false otherwise.
See the example under "die".
On systems that support fchdir, you might pass a
file handle or directory handle as argument. On
systems that don't support fchdir, passing handles
produces a fatal error at run time.
chmod LIST
Changes the permissions of a list of files. The
first element of the list must be the numerical
mode, which should probably be an octal number, and
which definitely should not be a string of octal
digits: 0644 is okay, '0644' is not. Returns the
number of files successfully changed. See also
"oct", if all you have is a string.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
$mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
# --w----r-T
$mode = '0644'; chmod oct($mode), 'foo'; # this is better
$mode = 0644; chmod $mode, 'foo'; # this is best
On systems that support fchmod, you might pass file
handles among the files. On systems that don't sup-
port fchmod, passing file handles produces a fatal
error at run time.
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open(my $fh, "<", "foo");
my $perm = (stat $fh)[2] & 07777;
chmod($perm | 0600, $fh);
You can also import the symbolic "S_I*" constants
from the Fcntl module:
use Fcntl ':mode';
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# This is identical to the chmod 0755 of the above example.
chomp VARIABLE
chomp( LIST )
chomp This safer version of "chop" removes any trailing
string that corresponds to the current value of $/
(also known as $INPUT_RECORD_SEPARATOR in the
"English" module). It returns the total number of
characters removed from all its arguments. It's
often used to remove the newline from the end of an
input record when you're worried that the final
record may be missing its newline. When in para-
graph mode ("$/ = """), it removes all trailing new-
lines from the string. When in slurp mode ("$/ =
undef") or fixed-length record mode ($/ is a refer-
ence to an integer or the like, see perlvar) chomp()
won't remove anything. If VARIABLE is omitted, it
chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash's values,
but not its keys.
You can actually chomp anything that's an lvalue,
including an assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and
the total number of characters removed is returned.
If the "encoding" pragma is in scope then the
lengths returned are calculated from the length of
$/ in Unicode characters, which is not always the
same as the length of $/ in the native encoding.
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Note that parentheses are necessary when you're
chomping anything that is not a simple variable.
This is because "chomp $cwd = `pwd`;" is interpreted
as "(chomp $cwd) = `pwd`;", rather than as "chomp(
$cwd = `pwd` )" which you might expect. Similarly,
"chomp $a, $b" is interpreted as "chomp($a), $b"
rather than as "chomp($a, $b)".
chop VARIABLE
chop( LIST )
chop Chops off the last character of a string and returns
the character chopped. It is much more efficient
than "s/.$//s" because it neither scans nor copies
the string. If VARIABLE is omitted, chops $_. If
VARIABLE is a hash, it chops the hash's values, but
not its keys.
You can actually chop anything that's an lvalue,
including an assignment.
If you chop a list, each element is chopped. Only
the value of the last "chop" is returned.
Note that "chop" returns the last character. To
return all but the last character, use
"substr($string, 0, -1)".
See also "chomp".
chown LIST
Changes the owner (and group) of a list of files.
The first two elements of the list must be the
numeric uid and gid, in that order. A value of -1
in either position is interpreted by most systems to
leave that value unchanged. Returns the number of
files successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
On systems that support fchown, you might pass file
handles among the files. On systems that don't sup-
port fchown, passing file handles produces a fatal
error at run time.
Here's an example that looks up nonnumeric uids in
the passwd file:
print "User: ";
chomp($user = <STDIN>);
print "Files: ";
chomp($pattern = <STDIN>);
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($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the
ownership of the file unless you're the superuser,
although you should be able to change the group to
any of your secondary groups. On insecure systems,
these restrictions may be relaxed, but this is not a
portable assumption. On POSIX systems, you can
detect this condition this way:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
chr NUMBER
chr Returns the character represented by that NUMBER in
the character set. For example, "chr(65)" is "A" in
either ASCII or Unicode, and chr(0x263a) is a
Unicode smiley face. Note that characters from 128
to 255 (inclusive) are by default not encoded in
UTF-8 Unicode for backward compatibility reasons
(but see encoding).
If NUMBER is omitted, uses $_.
For the reverse, use "ord".
Note that under the "bytes" pragma the NUMBER is
masked to the low eight bits.
See perlunicode and encoding for more about Unicode.
chroot FILENAME
chroot This function works like the system call by the same
name: it makes the named directory the new root
directory for all further pathnames that begin with
a "/" by your process and all its children. (It
doesn't change your current working directory, which
is unaffected.) For security reasons, this call is
restricted to the superuser. If FILENAME is omit-
ted, does a "chroot" to $_.
close FILEHANDLE
close Closes the file or pipe associated with the file
handle, returning true only if IO buffers are suc-
cessfully flushed and closes the system file
descriptor. Closes the currently selected filehan-
dle if the argument is omitted.
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You don't have to close FILEHANDLE if you are
immediately going to do another "open" on it,
because "open" will close it for you. (See "open".)
However, an explicit "close" on an input file resets
the line counter ($.), while the implicit close done
by "open" does not.
If the file handle came from a piped open, "close"
will additionally return false if one of the other
system calls involved fails, or if the program exits
with non-zero status. (If the only problem was that
the program exited non-zero, $! will be set to 0.)
Closing a pipe also waits for the process executing
on the pipe to complete, in case you want to look at
the output of the pipe afterwards, and implicitly
puts the exit status value of that command into $?.
Prematurely closing the read end of a pipe (i.e.
before the process writing to it at the other end
has closed it) will result in a SIGPIPE being
delivered to the writer. If the other end can't
handle that, be sure to read all the data before
closing the pipe.
Example:
open(OUTPUT, '|sort >foo') # pipe to sort
or die "Can't start sort: $!";
#... # print stuff to output
close OUTPUT # wait for sort to finish
or warn $! ? "Error closing sort pipe: $!"
: "Exit status $? from sort";
open(INPUT, 'foo') # get sort's results
or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be
used as an indirect filehandle, usually the real
filehandle name.
closedir DIRHANDLE
Closes a directory opened by "opendir" and returns
the success of that system call.
connect SOCKET,NAME
Attempts to connect to a remote socket, just as the
connect system call does. Returns true if it suc-
ceeded, false otherwise. NAME should be a packed
address of the appropriate type for the socket. See
the examples in "Sockets: Client/Server Communica-
tion" in perlipc.
continue BLOCK
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"continue" is actually a flow control statement
rather than a function. If there is a "continue"
BLOCK attached to a BLOCK (typically in a "while" or
"foreach"), it is always executed just before the
conditional is about to be evaluated again, just
like the third part of a "for" loop in C. Thus it
can be used to increment a loop variable, even when
the loop has been continued via the "next" statement
(which is similar to the C "continue" statement).
"last", "next", or "redo" may appear within a "con-
tinue" block. "last" and "redo" will behave as if
they had been executed within the main block. So
will "next", but since it will execute a "continue"
block, it may be more entertaining.
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back the top to re-check EXPR
}
### last always comes here
Omitting the "continue" section is semantically
equivalent to using an empty one, logically enough.
In that case, "next" goes directly back to check the
condition at the top of the loop.
cos EXPR
cos Returns the cosine of EXPR (expressed in radians).
If EXPR is omitted, takes cosine of $_.
For the inverse cosine operation, you may use the
"Math::Trig::acos()" function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt PLAINTEXT,SALT
Creates a digest string exactly like the crypt(3)
function in the C library (assuming that you actu-
ally have a version there that has not been extir-
pated as a potential munitions).
crypt() is a one-way hash function. The PLAINTEXT
and SALT is turned into a short string, called a
digest, which is returned. The same PLAINTEXT and
SALT will always return the same string, but there
is no (known) way to get the original PLAINTEXT from
the hash. Small changes in the PLAINTEXT or SALT
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will result in large changes in the digest.
There is no decrypt function. This function isn't
all that useful for cryptography (for that, look for
Crypt modules on your nearby CPAN mirror) and the
name "crypt" is a bit of a misnomer. Instead it is
primarily used to check if two pieces of text are
the same without having to transmit or store the
text itself. An example is checking if a correct
password is given. The digest of the password is
stored, not the password itself. The user types in
a password that is crypt()'d with the same salt as
the stored digest. If the two digests match the
password is correct.
When verifying an existing digest string you should
use the digest as the salt (like "crypt($plain,
$digest) eq $digest"). The SALT used to create the
digest is visible as part of the digest. This
ensures crypt() will hash the new string with the
same salt as the digest. This allows your code to
work with the standard crypt and with more exotic
implementations. In other words, do not assume any-
thing about the returned string itself, or how many
bytes in the digest matter.
Traditionally the result is a string of 13 bytes:
two first bytes of the salt, followed by 11 bytes
from the set "[./0-9A-Za-z]", and only the first
eight bytes of the digest string mattered, but
alternative hashing schemes (like MD5), higher level
security schemes (like C2), and implementations on
non-UNIX platforms may produce different strings.
When choosing a new salt create a random two charac-
ter string whose characters come from the set
"[./0-9A-Za-z]" (like "join '', ('.', '/', 0..9,
'A'..'Z', 'a'..'z')[rand 64, rand 64]"). This set
of characters is just a recommendation; the charac-
ters allowed in the salt depend solely on your
system's crypt library, and Perl can't restrict what
salts "crypt()" accepts.
Here's an example that makes sure that whoever runs
this program knows their password:
$pwd = (getpwuid($<))[1];
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system "stty -echo";
print "Password: ";
chomp($word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $pwd) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever
asks you for it is unwise.
The crypt function is unsuitable for hashing large
quantities of data, not least of all because you
can't get the information back. Look at the Digest
module for more robust algorithms.
If using crypt() on a Unicode string (which poten-
tially has characters with codepoints above 255),
Perl tries to make sense of the situation by trying
to downgrade (a copy of the string) the string back
to an eight-bit byte string before calling crypt()
(on that copy). If that works, good. If not,
crypt() dies with "Wide character in crypt".
dbmclose HASH
[This function has been largely superseded by the
"untie" function.]
Breaks the binding between a DBM file and a hash.
dbmopen HASH,DBNAME,MASK
[This function has been largely superseded by the
"tie" function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or
Berkeley DB file to a hash. HASH is the name of the
hash. (Unlike normal "open", the first argument is
not a filehandle, even though it looks like one).
DBNAME is the name of the database (without the .dir
or .pag extension if any). If the database does not
exist, it is created with protection specified by
MASK (as modified by the "umask"). If your system
supports only the older DBM functions, you may per-
form only one "dbmopen" in your program. In older
versions of Perl, if your system had neither DBM nor
ndbm, calling "dbmopen" produced a fatal error; it
now falls back to sdbm(3).
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If you don't have write access to the DBM file, you
can only read hash variables, not set them. If you
want to test whether you can write, either use file
tests or try setting a dummy hash entry inside an
"eval", which will trap the error.
Note that functions such as "keys" and "values" may
return huge lists when used on large DBM files. You
may prefer to use the "each" function to iterate
over large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also AnyDBM_File for a more general description
of the pros and cons of the various dbm approaches,
as well as DB_File for a particularly rich implemen-
tation.
You can control which DBM library you use by loading
that library before you call dbmopen():
use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";
defined EXPR
defined Returns a Boolean value telling whether EXPR has a
value other than the undefined value "undef". If
EXPR is not present, $_ will be checked.
Many operations return "undef" to indicate failure,
end of file, system error, uninitialized variable,
and other exceptional conditions. This function
allows you to distinguish "undef" from other values.
(A simple Boolean test will not distinguish among
"undef", zero, the empty string, and "0", which are
all equally false.) Note that since "undef" is a
valid scalar, its presence doesn't necessarily indi-
cate an exceptional condition: "pop" returns "undef"
when its argument is an empty array, or when the
element to return happens to be "undef".
You may also use "defined(&func)" to check whether
subroutine &func has ever been defined. The return
value is unaffected by any forward declarations of
&func. Note that a subroutine which is not defined
may still be callable: its package may have an
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"AUTOLOAD" method that makes it spring into
existence the first time that it is called -- see
perlsub.
Use of "defined" on aggregates (hashes and arrays)
is deprecated. It used to report whether memory for
that aggregate has ever been allocated. This
behavior may disappear in future versions of Perl.
You should instead use a simple test for size:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }
When used on a hash element, it tells you whether
the value is defined, not whether the key exists in
the hash. Use "exists" for the latter purpose.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse "defined", and
then are surprised to discover that the number 0 and
"" (the zero-length string) are, in fact, defined
values. For example, if you say
"ab" =~ /a(.*)b/;
The pattern match succeeds, and $1 is defined,
despite the fact that it matched "nothing". It
didn't really fail to match anything. Rather, it
matched something that happened to be zero charac-
ters long. This is all very above-board and honest.
When a function returns an undefined value, it's an
admission that it couldn't give you an honest
answer. So you should use "defined" only when
you're questioning the integrity of what you're try-
ing to do. At other times, a simple comparison to 0
or "" is what you want.
See also "undef", "exists", "ref".
delete EXPR
Given an expression that specifies a hash element,
array element, hash slice, or array slice, deletes
the specified element(s) from the hash or array. In
the case of an array, if the array elements happen
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to be at the end, the size of the array will shrink
to the highest element that tests true for exists()
(or 0 if no such element exists).
Returns a list with the same number of elements as
the number of elements for which deletion was
attempted. Each element of that list consists of
either the value of the element deleted, or the
undefined value. In scalar context, this means that
you get the value of the last element deleted (or
the undefined value if that element did not exist).
%hash = (foo => 11, bar => 22, baz => 33);
$scalar = delete $hash{foo}; # $scalar is 11
$scalar = delete @hash{qw(foo bar)}; # $scalar is 22
@array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
Deleting from %ENV modifies the environment. Delet-
ing from a hash tied to a DBM file deletes the entry
from the DBM file. Deleting from a "tie"d hash or
array may not necessarily return anything.
Deleting an array element effectively returns that
position of the array to its initial, uninitialized
state. Subsequently testing for the same element
with exists() will return false. Also, deleting
array elements in the middle of an array will not
shift the index of the elements after them down.
Use splice() for that. See "exists".
The following (inefficiently) deletes all the values
of %HASH and @ARRAY:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
foreach $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
}
And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];
But both of these are slower than just assigning the
empty list or undefining %HASH or @ARRAY:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
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@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
Note that the EXPR can be arbitrarily complicated as
long as the final operation is a hash element, array
element, hash slice, or array slice lookup:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
delete $ref->[$x][$y][$index];
delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
die LIST
Outside an "eval", prints the value of LIST to
"STDERR" and exits with the current value of $!
(errno). If $! is 0, exits with the value of "($?
>> 8)" (backtick `command` status). If "($? >> 8)"
is 0, exits with 255. Inside an "eval()," the error
message is stuffed into $@ and the "eval" is ter-
minated with the undefined value. This makes "die"
the way to raise an exception.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the last element of LIST does not end in a new-
line, the current script line number and input line
number (if any) are also printed, and a newline is
supplied. Note that the "input line number" (also
known as "chunk") is subject to whatever notion of
"line" happens to be currently in effect, and is
also available as the special variable $.. See "$/"
in perlvar and "$." in perlvar.
Hint: sometimes appending ", stopped" to your mes-
sage will cause it to make better sense when the
string "at foo line 123" is appended. Suppose you
are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also exit(), warn(), and the Carp module.
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If LIST is empty and $@ already contains a value
(typically from a previous eval) that value is
reused after appending "\t...propagated". This is
useful for propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;
If LIST is empty and $@ contains an object reference
that has a "PROPAGATE" method, that method will be
called with additional file and line number parame-
ters. The return value replaces the value in $@.
i.e. as if "$@ = eval { $@->PROPAGATE(__FILE__,
__LINE__) };" were called.
If $@ is empty then the string "Died" is used.
die() can also be called with a reference argument.
If this happens to be trapped within an eval(), $@
contains the reference. This behavior permits a
more elaborate exception handling implementation
using objects that maintain arbitrary state about
the nature of the exception. Such a scheme is some-
times preferable to matching particular string
values of $@ using regular expressions. Here's an
example:
use Scalar::Util 'blessed';
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if ($@) {
if (blessed($@) && $@->isa("Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
Because perl will stringify uncaught exception mes-
sages before displaying them, you may want to over-
load stringification operations on such custom
exception objects. See overload for details about
that.
You can arrange for a callback to be run just before
the "die" does its deed, by setting the
$SIG{__DIE__} hook. The associated handler will be
called with the error text and can change the error
message, if it sees fit, by calling "die" again.
See "$SIG{expr}" in perlvar for details on setting
%SIG entries, and "eval BLOCK" for some examples.
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Although this feature was to be run only right
before your program was to exit, this is not
currently the case--the $SIG{__DIE__} hook is
currently called even inside eval()ed
blocks/strings! If one wants the hook to do nothing
in such situations, put
die @_ if $^S;
as the first line of the handler (see "$^S" in perl-
var). Because this promotes strange action at a
distance, this counterintuitive behavior may be
fixed in a future release.
do BLOCK
Not really a function. Returns the value of the
last command in the sequence of commands indicated
by BLOCK. When modified by the "while" or "until"
loop modifier, executes the BLOCK once before test-
ing the loop condition. (On other statements the
loop modifiers test the conditional first.)
"do BLOCK" does not count as a loop, so the loop
control statements "next", "last", or "redo" cannot
be used to leave or restart the block. See perlsyn
for alternative strategies.
do SUBROUTINE(LIST)
This form of subroutine call is deprecated. See
perlsub.
do EXPR Uses the value of EXPR as a filename and executes
the contents of the file as a Perl script.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient and concise, keeps
track of the current filename for error messages,
searches the @INC directories, and updates %INC if
the file is found. See "Predefined Names" in perl-
var for these variables. It also differs in that
code evaluated with "do FILENAME" cannot see lexi-
cals in the enclosing scope; "eval STRING" does.
It's the same, however, in that it does reparse the
file every time you call it, so you probably don't
want to do this inside a loop.
If "do" cannot read the file, it returns undef and
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sets $! to the error. If "do" can read the file but
cannot compile it, it returns undef and sets an
error message in $@. If the file is successfully
compiled, "do" returns the value of the last expres-
sion evaluated.
Note that inclusion of library modules is better
done with the "use" and "require" operators, which
also do automatic error checking and raise an excep-
tion if there's a problem.
You might like to use "do" to read in a program con-
figuration file. Manual error checking can be done
this way:
# read in config files: system first, then user
for $file ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc")
{
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file" unless $return;
}
}
dump LABEL
dump This function causes an immediate core dump. See
also the -u command-line switch in perlrun, which
does the same thing. Primarily this is so that you
can use the undump program (not supplied) to turn
your core dump into an executable binary after hav-
ing initialized all your variables at the beginning
of the program. When the new binary is executed it
will begin by executing a "goto LABEL" (with all the
restrictions that "goto" suffers). Think of it as a
goto with an intervening core dump and reincarna-
tion. If "LABEL" is omitted, restarts the program
from the top.
WARNING: Any files opened at the time of the dump
will not be open any more when the program is rein-
carnated, with possible resulting confusion on the
part of Perl.
This function is now largely obsolete, partly
because it's very hard to convert a core file into
an executable, and because the real compiler back-
ends for generating portable bytecode and compilable
C code have superseded it. That's why you should
now invoke it as "CORE::dump()", if you don't want
to be warned against a possible typo.
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If you're looking to use dump to speed up your pro-
gram, consider generating bytecode or native C code
as described in perlcc. If you're just trying to
accelerate a CGI script, consider using the
"mod_perl" extension to Apache, or the CPAN module,
CGI::Fast. You might also consider autoloading or
selfloading, which at least make your program appear
to run faster.
each HASH
When called in list context, returns a 2-element
list consisting of the key and value for the next
element of a hash, so that you can iterate over it.
When called in scalar context, returns only the key
for the next element in the hash.
Entries are returned in an apparently random order.
The actual random order is subject to change in
future versions of perl, but it is guaranteed to be
in the same order as either the "keys" or "values"
function would produce on the same (unmodified)
hash. Since Perl 5.8.1 the ordering is different
even between different runs of Perl for security
reasons (see "Algorithmic Complexity Attacks" in
perlsec).
When the hash is entirely read, a null array is
returned in list context (which when assigned pro-
duces a false (0) value), and "undef" in scalar con-
text. The next call to "each" after that will start
iterating again. There is a single iterator for
each hash, shared by all "each", "keys", and
"values" function calls in the program; it can be
reset by reading all the elements from the hash, or
by evaluating "keys HASH" or "values HASH". If you
add or delete elements of a hash while you're
iterating over it, you may get entries skipped or
duplicated, so don't. Exception: It is always safe
to delete the item most recently returned by
"each()", which means that the following code will
work:
while (($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}
The following prints out your environment like the
printenv(1) program, only in a different order:
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while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also "keys", "values" and "sort".
eof FILEHANDLE
eof ()
eof Returns 1 if the next read on FILEHANDLE will return
end of file, or if FILEHANDLE is not open. FILEHAN-
DLE may be an expression whose value gives the real
filehandle. (Note that this function actually reads
a character and then "ungetc"s it, so isn't very
useful in an interactive context.) Do not read from
a terminal file (or call "eof(FILEHANDLE)" on it)
after end-of-file is reached. File types such as
terminals may lose the end-of-file condition if you
do.
An "eof" without an argument uses the last file
read. Using "eof()" with empty parentheses is very
different. It refers to the pseudo file formed from
the files listed on the command line and accessed
via the "<>" operator. Since "<>" isn't explicitly
opened, as a normal filehandle is, an "eof()" before
"<>" has been used will cause @ARGV to be examined
to determine if input is available. Similarly, an
"eof()" after "<>" has returned end-of-file will
assume you are processing another @ARGV list, and if
you haven't set @ARGV, will read input from "STDIN";
see "I/O Operators" in perlop.
In a "while (<>)" loop, "eof" or "eof(ARGV)" can be
used to detect the end of each file, "eof()" will
only detect the end of the last file. Examples:
# reset line numbering on each input file
while (<>) {
next if /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close ARGV if eof; # Not eof()!
}
# insert dashes just before last line of last file
while (<>) {
if (eof()) { # check for end of last file
print "--------------\n";
}
print;
last if eof(); # needed if we're reading from a terminal
}
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Practical hint: you almost never need to use "eof"
in Perl, because the input operators typically
return "undef" when they run out of data, or if
there was an error.
eval EXPR
eval BLOCK
eval In the first form, the return value of EXPR is
parsed and executed as if it were a little Perl pro-
gram. The value of the expression (which is itself
determined within scalar context) is first parsed,
and if there weren't any errors, executed in the
lexical context of the current Perl program, so that
any variable settings or subroutine and format
definitions remain afterwards. Note that the value
is parsed every time the "eval" executes. If EXPR is
omitted, evaluates $_. This form is typically used
to delay parsing and subsequent execution of the
text of EXPR until run time.
In the second form, the code within the BLOCK is
parsed only once--at the same time the code sur-
rounding the "eval" itself was parsed--and executed
within the context of the current Perl program.
This form is typically used to trap exceptions more
efficiently than the first (see below), while also
providing the benefit of checking the code within
BLOCK at compile time.
The final semicolon, if any, may be omitted from the
value of EXPR or within the BLOCK.
In both forms, the value returned is the value of
the last expression evaluated inside the
mini-program; a return statement may be also used,
just as with subroutines. The expression providing
the return value is evaluated in void, scalar, or
list context, depending on the context of the "eval"
itself. See "wantarray" for more on how the evalua-
tion context can be determined.
If there is a syntax error or runtime error, or a
"die" statement is executed, an undefined value is
returned by "eval", and $@ is set to the error mes-
sage. If there was no error, $@ is guaranteed to be
a null string. Beware that using "eval" neither
silences perl from printing warnings to STDERR, nor
does it stuff the text of warning messages into $@.
To do either of those, you have to use the
$SIG{__WARN__} facility, or turn off warnings inside
the BLOCK or EXPR using "no warnings 'all'". See
"warn", perlvar, warnings and perllexwarn.
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Note that, because "eval" traps otherwise-fatal
errors, it is useful for determining whether a par-
ticular feature (such as "socket" or "symlink") is
implemented. It is also Perl's exception trapping
mechanism, where the die operator is used to raise
exceptions.
If the code to be executed doesn't vary, you may use
the eval-BLOCK form to trap run-time errors without
incurring the penalty of recompiling each time. The
error, if any, is still returned in $@. Examples:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
Using the "eval{}" form as an exception trap in
libraries does have some issues. Due to the current
arguably broken state of "__DIE__" hooks, you may
wish not to trigger any "__DIE__" hooks that user
code may have installed. You can use the "local
$SIG{__DIE__}" construct for this purpose, as shown
in this example:
# a very private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
warn $@ if $@;
This is especially significant, given that "__DIE__"
hooks can call "die" again, which has the effect of
changing their error messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} =
sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo lives here" };
print $@ if $@; # prints "bar lives here"
}
Because this promotes action at a distance, this
counterintuitive behavior may be fixed in a future
release.
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With an "eval", you should be especially careful to
remember what's being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$x++"; # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the
code contained in the variable $x. (Although case 2
has misleading double quotes making the reader
wonder what else might be happening (nothing is).)
Cases 3 and 4 likewise behave in the same way: they
run the code '$x', which does nothing but return the
value of $x. (Case 4 is preferred for purely visual
reasons, but it also has the advantage of compiling
at compile-time instead of at run-time.) Case 5 is
a place where normally you would like to use double
quotes, except that in this particular situation,
you can just use symbolic references instead, as in
case 6.
"eval BLOCK" does not count as a loop, so the loop
control statements "next", "last", or "redo" cannot
be used to leave or restart the block.
Note that as a very special case, an "eval ''" exe-
cuted within the "DB" package doesn't see the usual
surrounding lexical scope, but rather the scope of
the first non-DB piece of code that called it. You
don't normally need to worry about this unless you
are writing a Perl debugger.
exec LIST
exec PROGRAM LIST
The "exec" function executes a system command and
never returns-- use "system" instead of "exec" if
you want it to return. It fails and returns false
only if the command does not exist and it is exe-
cuted directly instead of via your system's command
shell (see below).
Since it's a common mistake to use "exec" instead of
"system", Perl warns you if there is a following
statement which isn't "die", "warn", or "exit" (if
"-w" is set - but you always do that). If you
really want to follow an "exec" with some other
statement, you can use one of these styles to avoid
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the warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";
If there is more than one argument in LIST, or if
LIST is an array with more than one value, calls
execvp(3) with the arguments in LIST. If there is
only one scalar argument or an array with one ele-
ment in it, the argument is checked for shell meta-
characters, and if there are any, the entire argu-
ment is passed to the system's command shell for
parsing (this is "/bin/sh -c" on Unix platforms, but
varies on other platforms). If there are no shell
metacharacters in the argument, it is split into
words and passed directly to "execvp", which is more
efficient. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argu-
ment, but want to lie to the program you are execut-
ing about its own name, you can specify the program
you actually want to run as an "indirect object"
(without a comma) in front of the LIST. (This
always forces interpretation of the LIST as a mul-
tivalued list, even if there is only a single scalar
in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system
shell, results will be subject to its quirks and
capabilities. See "`STRING`" in perlop for details.
Using an indirect object with "exec" or "system" is
also more secure. This usage (which also works fine
with system()) forces interpretation of the argu-
ments as a multivalued list, even if the list had
just one argument. That way you're safe from the
shell expanding wildcards or splitting up words with
whitespace in them.
@args = ( "echo surprise" );
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exec @args; # subject to shell escapes
# if @args == 1
exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect
object, ran the echo program, passing it "surprise"
an argument. The second version didn't--it tried to
run a program literally called "echo surprise",
didn't find it, and set $? to a non-zero value indi-
cating failure.
Beginning with v5.6.0, Perl will attempt to flush
all files opened for output before the exec, but
this may not be supported on some platforms (see
perlport). To be safe, you may need to set $|
($AUTOFLUSH in English) or call the "autoflush()"
method of "IO::Handle" on any open handles in order
to avoid lost output.
Note that "exec" will not call your "END" blocks,
nor will it call any "DESTROY" methods in your
objects.
exists EXPR
Given an expression that specifies a hash element or
array element, returns true if the specified element
in the hash or array has ever been initialized, even
if the corresponding value is undefined. The ele-
ment is not autovivified if it doesn't exist.
print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};
print "Exists\n" if exists $array[$index];
print "Defined\n" if defined $array[$index];
print "True\n" if $array[$index];
A hash or array element can be true only if it's
defined, and defined if it exists, but the reverse
doesn't necessarily hold true.
Given an expression that specifies the name of a
subroutine, returns true if the specified subroutine
has ever been declared, even if it is undefined.
Mentioning a subroutine name for exists or defined
does not count as declaring it. Note that a subrou-
tine which does not exist may still be callable: its
package may have an "AUTOLOAD" method that makes it
spring into existence the first time that it is
called -- see perlsub.
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print "Exists\n" if exists &subroutine;
print "Defined\n" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated as
long as the final operation is a hash or array key
lookup or subroutine name:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
if (exists $ref->{A}->{B}->[$ix]) { }
if (exists $hash{A}{B}[$ix]) { }
if (exists &{$ref->{A}{B}{$key}}) { }
Although the deepest nested array or hash will not
spring into existence just because its existence was
tested, any intervening ones will. Thus
"$ref->{"A"}" and "$ref->{"A"}->{"B"}" will spring
into existence due to the existence test for the
$key element above. This happens anywhere the arrow
operator is used, including even:
undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref; # prints HASH(0x80d3d5c)
This surprising autovivification in what does not at
first--or even second--glance appear to be an lvalue
context may be fixed in a future release.
See "Pseudo-hashes: Using an array as a hash" in
perlref for specifics on how exists() acts when used
on a pseudo-hash.
Use of a subroutine call, rather than a subroutine
name, as an argument to exists() is an error.
exists ⊂ # OK
exists &sub(); # Error
exit EXPR
exit Evaluates EXPR and exits immediately with that
value. Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also "die". If EXPR is omitted, exits with 0
status. The only universally recognized values for
EXPR are 0 for success and 1 for error; other values
are subject to interpretation depending on the
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environment in which the Perl program is running.
For example, exiting 69 (EX_UNAVAILABLE) from a
sendmail incoming-mail filter will cause the mailer
to return the item undelivered, but that's not true
everywhere.
Don't use "exit" to abort a subroutine if there's
any chance that someone might want to trap whatever
error happened. Use "die" instead, which can be
trapped by an "eval".
The exit() function does not always exit immedi-
ately. It calls any defined "END" routines first,
but these "END" routines may not themselves abort
the exit. Likewise any object destructors that need
to be called are called before the real exit. If
this is a problem, you can call
"POSIX:_exit($status)" to avoid END and destructor
processing. See perlmod for details.
exp EXPR
exp Returns e (the natural logarithm base) to the power
of EXPR. If EXPR is omitted, gives "exp($_)".
fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably
have to say
use Fcntl;
first to get the correct constant definitions.
Argument processing and value return works just like
"ioctl" below. For example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
or die "can't fcntl F_GETFL: $!";
You don't have to check for "defined" on the return
from "fcntl". Like "ioctl", it maps a 0 return from
the system call into "0 but true" in Perl. This
string is true in boolean context and 0 in numeric
context. It is also exempt from the normal -w warn-
ings on improper numeric conversions.
Note that "fcntl" will produce a fatal error if used
on a machine that doesn't implement fcntl(2). See
the Fcntl module or your fcntl(2) manpage to learn
what functions are available on your system.
Here's an example of setting a filehandle named
"REMOTE" to be non-blocking at the system level.
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You'll have to negotiate $| on your own, though.
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
$flags = fcntl(REMOTE, F_GETFL, 0)
or die "Can't get flags for the socket: $!\n";
$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";
fileno FILEHANDLE
Returns the file descriptor for a filehandle, or
undefined if the filehandle is not open. This is
mainly useful for constructing bitmaps for "select"
and low-level POSIX tty-handling operations. If
FILEHANDLE is an expression, the value is taken as
an indirect filehandle, generally its name.
You can use this to find out whether two handles
refer to the same underlying descriptor:
if (fileno(THIS) == fileno(THAT)) {
print "THIS and THAT are dups\n";
}
(Filehandles connected to memory objects via new
features of "open" may return undefined even though
they are open.)
flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHAN-
DLE. Returns true for success, false on failure.
Produces a fatal error if used on a machine that
doesn't implement flock(2), fcntl(2) locking, or
lockf(3). "flock" is Perl's portable file locking
interface, although it locks only entire files, not
records.
Two potentially non-obvious but traditional "flock"
semantics are that it waits indefinitely until the
lock is granted, and that its locks merely advisory.
Such discretionary locks are more flexible, but
offer fewer guarantees. This means that programs
that do not also use "flock" may modify files locked
with "flock". See perlport, your port's specific
documentation, or your system-specific local man-
pages for details. It's best to assume traditional
behavior if you're writing portable programs. (But
if you're not, you should as always feel perfectly
free to write for your own system's idiosyncrasies
(sometimes called "features"). Slavish adherence to
portability concerns shouldn't get in the way of
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your getting your job done.)
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN,
possibly combined with LOCK_NB. These constants are
traditionally valued 1, 2, 8 and 4, but you can use
the symbolic names if you import them from the Fcntl
module, either individually, or as a group using the
':flock' tag. LOCK_SH requests a shared lock,
LOCK_EX requests an exclusive lock, and LOCK_UN
releases a previously requested lock. If LOCK_NB is
bitwise-or'ed with LOCK_SH or LOCK_EX then "flock"
will return immediately rather than blocking waiting
for the lock (check the return status to see if you
got it).
To avoid the possibility of miscoordination, Perl
now flushes FILEHANDLE before locking or unlocking
it.
Note that the emulation built with lockf(3) doesn't
provide shared locks, and it requires that FILEHAN-
DLE be open with write intent. These are the seman-
tics that lockf(3) implements. Most if not all sys-
tems implement lockf(3) in terms of fcntl(2) lock-
ing, though, so the differing semantics shouldn't
bite too many people.
Note that the fcntl(2) emulation of flock(3)
requires that FILEHANDLE be open with read intent to
use LOCK_SH and requires that it be open with write
intent to use LOCK_EX.
Note also that some versions of "flock" cannot lock
things over the network; you would need to use the
more system-specific "fcntl" for that. If you like
you can force Perl to ignore your system's flock(2)
function, and so provide its own fcntl(2)-based emu-
lation, by passing the switch "-Ud_flock" to the
Configure program when you configure perl.
Here's a mailbox appender for BSD systems.
use Fcntl ':flock'; # import LOCK_* constants
sub lock {
flock(MBOX,LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
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sub unlock {
flock(MBOX,LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
lock();
print MBOX $msg,"\n\n";
unlock();
On systems that support a real flock(), locks are
inherited across fork() calls, whereas those that
must resort to the more capricious fcntl() function
lose the locks, making it harder to write servers.
See also DB_File for other flock() examples.
fork Does a fork(2) system call to create a new process
running the same program at the same point. It
returns the child pid to the parent process, 0 to
the child process, or "undef" if the fork is unsuc-
cessful. File descriptors (and sometimes locks on
those descriptors) are shared, while everything else
is copied. On most systems supporting fork(), great
care has gone into making it extremely efficient
(for example, using copy-on-write technology on data
pages), making it the dominant paradigm for multi-
tasking over the last few decades.
Beginning with v5.6.0, Perl will attempt to flush
all files opened for output before forking the child
process, but this may not be supported on some plat-
forms (see perlport). To be safe, you may need to
set $| ($AUTOFLUSH in English) or call the "auto-
flush()" method of "IO::Handle" on any open handles
in order to avoid duplicate output.
If you "fork" without ever waiting on your children,
you will accumulate zombies. On some systems, you
can avoid this by setting $SIG{CHLD} to "IGNORE".
See also perlipc for more examples of forking and
reaping moribund children.
Note that if your forked child inherits system file
descriptors like STDIN and STDOUT that are actually
connected by a pipe or socket, even if you exit,
then the remote server (such as, say, a CGI script
or a backgrounded job launched from a remote shell)
won't think you're done. You should reopen those to
/dev/null if it's any issue.
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format Declare a picture format for use by the "write"
function. For example:
format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;
See perlform for many details and examples.
formline PICTURE,LIST
This is an internal function used by "format"s,
though you may call it, too. It formats (see perl-
form) a list of values according to the contents of
PICTURE, placing the output into the format output
accumulator, $^A (or $ACCUMULATOR in English). Even-
tually, when a "write" is done, the contents of $^A
are written to some filehandle. You could also read
$^A and then set $^A back to "". Note that a format
typically does one "formline" per line of form, but
the "formline" function itself doesn't care how many
newlines are embedded in the PICTURE. This means
that the "~" and "~~" tokens will treat the entire
PICTURE as a single line. You may therefore need to
use multiple formlines to implement a single record
format, just like the format compiler.
Be careful if you put double quotes around the pic-
ture, because an "@" character may be taken to mean
the beginning of an array name. "formline" always
returns true. See perlform for other examples.
getc FILEHANDLE
getc Returns the next character from the input file
attached to FILEHANDLE, or the undefined value at
end of file, or if there was an error (in the latter
case $! is set). If FILEHANDLE is omitted, reads
from STDIN. This is not particularly efficient.
However, it cannot be used by itself to fetch single
characters without waiting for the user to hit
enter. For that, try something more like:
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if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", '-icanon', 'eol', "\001";
}
$key = getc(STDIN);
if ($BSD_STYLE) {
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", 'icanon', 'eol', '^@'; # ASCII null
}
print "\n";
Determination of whether $BSD_STYLE should be set is
left as an exercise to the reader.
The "POSIX::getattr" function can do this more port-
ably on systems purporting POSIX compliance. See
also the "Term::ReadKey" module from your nearest
CPAN site; details on CPAN can be found on "CPAN" in
perlmodlib.
getlogin
This implements the C library function of the same
name, which on most systems returns the current
login from /etc/utmp, if any. If null, use
"getpwuid".
$login = getlogin || getpwuid($<) || "Kilroy";
Do not consider "getlogin" for authentication: it is
not as secure as "getpwuid".
getpeername SOCKET
Returns the packed sockaddr address of other end of
the SOCKET connection.
use Socket;
$hersockaddr = getpeername(SOCK);
($port, $iaddr) = sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($iaddr);
getpgrp PID
Returns the current process group for the specified
PID. Use a PID of 0 to get the current process
group for the current process. Will raise an excep-
tion if used on a machine that doesn't implement
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group of current process. Note that the POSIX ver-
sion of "getpgrp" does not accept a PID argument, so
only "PID==0" is truly portable.
getppid Returns the process id of the parent process.
Note for Linux users: on Linux, the C functions
"getpid()" and "getppid()" return different values
from different threads. In order to be portable,
this behavior is not reflected by the perl-level
function "getppid()", that returns a consistent
value across threads. If you want to call the under-
lying "getppid()", you may use the CPAN module
"Linux::Pid".
getpriority WHICH,WHO
Returns the current priority for a process, a pro-
cess group, or a user. (See getpriority(2).) Will
raise a fatal exception if used on a machine that
doesn't implement getpriority(2).
getpwnam NAME
getgrnam NAME
gethostbyname NAME
getnetbyname NAME
getprotobyname NAME
getpwuid UID
getgrgid GID
getservbyname NAME,PROTO
gethostbyaddr ADDR,ADDRTYPE
getnetbyaddr ADDR,ADDRTYPE
getprotobynumber NUMBER
getservbyport PORT,PROTO
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent STAYOPEN
setnetent STAYOPEN
setprotoent STAYOPEN
setservent STAYOPEN
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
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These routines perform the same functions as their
counterparts in the system library. In list con-
text, the return values from the various get rou-
tines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
(If the entry doesn't exist you get a null list.)
The exact meaning of the $gcos field varies but it
usually contains the real name of the user (as
opposed to the login name) and other information
pertaining to the user. Beware, however, that in
many system users are able to change this informa-
tion and therefore it cannot be trusted and there-
fore the $gcos is tainted (see perlsec). The
$passwd and $shell, user's encrypted password and
login shell, are also tainted, because of the same
reason.
In scalar context, you get the name, unless the
function was a lookup by name, in which case you get
the other thing, whatever it is. (If the entry
doesn't exist you get the undefined value.) For
example:
$uid = getpwnam($name);
$name = getpwuid($num);
$name = getpwent();
$gid = getgrnam($name);
$name = getgrgid($num);
$name = getgrent();
#etc.
In getpw*() the fields $quota, $comment, and $expire
are special cases in the sense that in many systems
they are unsupported. If the $quota is unsupported,
it is an empty scalar. If it is supported, it usu-
ally encodes the disk quota. If the $comment field
is unsupported, it is an empty scalar. If it is
supported it usually encodes some administrative
comment about the user. In some systems the $quota
field may be $change or $age, fields that have to do
with password aging. In some systems the $comment
field may be $class. The $expire field, if present,
encodes the expiration period of the account or the
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password. For the availability and the exact mean-
ing of these fields in your system, please consult
your getpwnam(3) documentation and your pwd.h file.
You can also find out from within Perl what your
$quota and $comment fields mean and whether you have
the $expire field by using the "Config" module and
the values "d_pwquota", "d_pwage", "d_pwchange",
"d_pwcomment", and "d_pwexpire". Shadow password
files are only supported if your vendor has imple-
mented them in the intuitive fashion that calling
the regular C library routines gets the shadow ver-
sions if you're running under privilege or if there
exists the shadow(3) functions as found in System V
(this includes Solaris and Linux.) Those systems
that implement a proprietary shadow password facil-
ity are unlikely to be supported.
The $members value returned by getgr*() is a space
separated list of the login names of the members of
the group.
For the gethost*() functions, if the "h_errno" vari-
able is supported in C, it will be returned to you
via $? if the function call fails. The @addrs value
returned by a successful call is a list of the raw
addresses returned by the corresponding system
library call. In the Internet domain, each address
is four bytes long and you can unpack it by saying
something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
The Socket library makes this slightly easier:
use Socket;
$iaddr = inet_aton("127.1"); # or whatever address
$name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
$straddr = inet_ntoa($iaddr);
If you get tired of remembering which element of the
return list contains which return value, by-name
interfaces are provided in standard modules:
"File::stat", "Net::hostent", "Net::netent",
"Net::protoent", "Net::servent", "Time::gmtime",
"Time::localtime", and "User::grent". These over-
ride the normal built-ins, supplying versions that
return objects with the appropriate names for each
field. For example:
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use File::stat;
use User::pwent;
$is_his = (stat($filename)->uid == pwent($whoever)->uid);
Even though it looks like they're the same method
calls (uid), they aren't, because a "File::stat"
object is different from a "User::pwent" object.
getsockname SOCKET
Returns the packed sockaddr address of this end of
the SOCKET connection, in case you don't know the
address because you have several different IPs that
the connection might have come in on.
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = sockaddr_in($mysockaddr);
printf "Connect to %s [%s]\n",
scalar gethostbyaddr($myaddr, AF_INET),
inet_ntoa($myaddr);
getsockopt SOCKET,LEVEL,OPTNAME
Queries the option named OPTNAME associated with
SOCKET at a given LEVEL. Options may exist at multi-
ple protocol levels depending on the socket type,
but at least the uppermost socket level SOL_SOCKET
(defined in the "Socket" module) will exist. To
query options at another level the protocol number
of the appropriate protocol controlling the option
should be supplied. For example, to indicate that an
option is to be interpreted by the TCP protocol,
LEVEL should be set to the protocol number of TCP,
which you can get using getprotobyname.
The call returns a packed string representing the
requested socket option, or "undef" if there is an
error (the error reason will be in $!). What exactly
is in the packed string depends in the LEVEL and
OPTNAME, consult your system documentation for
details. A very common case however is that the
option is an integer, in which case the result will
be a packed integer which you can decode using
unpack with the "i" (or "I") format.
An example testing if Nagle's algorithm is turned on
on a socket:
use Socket qw(:all);
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defined(my $tcp = getprotobyname("tcp"))
or die "Could not determine the protocol number for tcp";
# my $tcp = IPPROTO_TCP; # Alternative
my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
or die "Could not query TCP_NODELAY socket option: $!";
my $nodelay = unpack("I", $packed);
print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
glob EXPR
glob In list context, returns a (possibly empty) list of
filename expansions on the value of EXPR such as the
standard Unix shell /bin/csh would do. In scalar
context, glob iterates through such filename expan-
sions, returning undef when the list is exhausted.
This is the internal function implementing the
"<*.c>" operator, but you can use it directly. If
EXPR is omitted, $_ is used. The "<*.c>" operator
is discussed in more detail in "I/O Operators" in
perlop.
Beginning with v5.6.0, this operator is implemented
using the standard "File::Glob" extension. See
File::Glob for details.
gmtime EXPR
gmtime Converts a time as returned by the time function to
an 9-element list with the time localized for the
standard Greenwich time zone. Typically used as fol-
lows:
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
gmtime(time);
All list elements are numeric, and come straight out
of the C `struct tm'. $sec, $min, and $hour are the
seconds, minutes, and hours of the specified time.
$mday is the day of the month, and $mon is the month
itself, in the range 0..11 with 0 indicating January
and 11 indicating December. $year is the number of
years since 1900. That is, $year is 123 in year
2023. $wday is the day of the week, with 0 indicat-
ing Sunday and 3 indicating Wednesday. $yday is the
day of the year, in the range 0..364 (or 0..365 in
leap years). $isdst is always 0.
Note that the $year element is not simply the last
two digits of the year. If you assume it is then
you create non-Y2K-compliant programs--and you
wouldn't want to do that, would you?
The proper way to get a complete 4-digit year is
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simply:
$year += 1900;
And to get the last two digits of the year (e.g.,
'01' in 2001) do:
$year = sprintf("%02d", $year % 100);
If EXPR is omitted, "gmtime()" uses the current time
("gmtime(time)").
In scalar context, "gmtime()" returns the ctime(3)
value:
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
If you need local time instead of GMT use the
"localtime" builtin. See also the "timegm" function
provided by the "Time::Local" module, and the
strftime(3) and mktime(3) functions available via
the POSIX module.
This scalar value is not locale dependent (see perl-
locale), but is instead a Perl builtin. To get
somewhat similar but locale dependent date strings,
see the example in "localtime".
See "gmtime" in perlport for portability concerns.
goto LABEL
goto EXPR
goto &NAME
The "goto-LABEL" form finds the statement labeled
with LABEL and resumes execution there. It may not
be used to go into any construct that requires ini-
tialization, such as a subroutine or a "foreach"
loop. It also can't be used to go into a construct
that is optimized away, or to get out of a block or
subroutine given to "sort". It can be used to go
almost anywhere else within the dynamic scope,
including out of subroutines, but it's usually
better to use some other construct such as "last" or
"die". The author of Perl has never felt the need
to use this form of "goto" (in Perl, that is--C is
another matter). (The difference being that C does
not offer named loops combined with loop control.
Perl does, and this replaces most structured uses of
"goto" in other languages.)
The "goto-EXPR" form expects a label name, whose
scope will be resolved dynamically. This allows for
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computed "goto"s per FORTRAN, but isn't necessarily
recommended if you're optimizing for maintainabil-
ity:
goto ("FOO", "BAR", "GLARCH")[$i];
The "goto-&NAME" form is quite different from the
other forms of "goto". In fact, it isn't a goto in
the normal sense at all, and doesn't have the stigma
associated with other gotos. Instead, it exits the
current subroutine (losing any changes set by
local()) and immediately calls in its place the
named subroutine using the current value of @_.
This is used by "AUTOLOAD" subroutines that wish to
load another subroutine and then pretend that the
other subroutine had been called in the first place
(except that any modifications to @_ in the current
subroutine are propagated to the other subroutine.)
After the "goto", not even "caller" will be able to
tell that this routine was called first.
NAME needn't be the name of a subroutine; it can be
a scalar variable containing a code reference, or a
block that evaluates to a code reference.
grep BLOCK LIST
grep EXPR,LIST
This is similar in spirit to, but not the same as,
grep(1) and its relatives. In particular, it is not
limited to using regular expressions.
Evaluates the BLOCK or EXPR for each element of LIST
(locally setting $_ to each element) and returns the
list value consisting of those elements for which
the expression evaluated to true. In scalar con-
text, returns the number of times the expression was
true.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
Note that $_ is an alias to the list value, so it
can be used to modify the elements of the LIST.
While this is useful and supported, it can cause
bizarre results if the elements of LIST are not
variables. Similarly, grep returns aliases into the
original list, much as a for loop's index variable
aliases the list elements. That is, modifying an
element of a list returned by grep (for example, in
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a "foreach", "map" or another "grep") actually modi-
fies the element in the original list. This is usu-
ally something to be avoided when writing clear
code.
See also "map" for a list composed of the results of
the BLOCK or EXPR.
hex EXPR
hex Interprets EXPR as a hex string and returns the
corresponding value. (To convert strings that might
start with either 0, "0x", or "0b", see "oct".) If
EXPR is omitted, uses $_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
Hex strings may only represent integers. Strings
that would cause integer overflow trigger a warning.
Leading whitespace is not stripped, unlike oct(). To
present something as hex, look into "printf",
"sprintf", or "unpack".
import LIST
There is no builtin "import" function. It is just
an ordinary method (subroutine) defined (or inher-
ited) by modules that wish to export names to
another module. The "use" function calls the
"import" method for the package used. See also
"use", perlmod, and Exporter.
index STR,SUBSTR,POSITION
index STR,SUBSTR
The index function searches for one string within
another, but without the wildcard-like behavior of a
full regular-expression pattern match. It returns
the position of the first occurrence of SUBSTR in
STR at or after POSITION. If POSITION is omitted,
starts searching from the beginning of the string.
POSITION before the beginning of the string or after
its end is treated as if it were the beginning or
the end, respectively. POSITION and the return
value are based at 0 (or whatever you've set the $[
variable to--but don't do that). If the substring
is not found, "index" returns one less than the
base, ordinarily "-1".
int EXPR
int Returns the integer portion of EXPR. If EXPR is
omitted, uses $_. You should not use this function
for rounding: one because it truncates towards 0,
and two because machine representations of floating
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point numbers can sometimes produce counterintuitive
results. For example, "int(-6.725/0.025)" produces
-268 rather than the correct -269; that's because
it's really more like -268.99999999999994315658
instead. Usually, the "sprintf", "printf", or the
"POSIX::floor" and "POSIX::ceil" functions will
serve you better than will int().
ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the ioctl(2) function. You'll probably
first have to say
require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
to get the correct function definitions. If
sys/ioctl.ph doesn't exist or doesn't have the
correct definitions you'll have to roll your own,
based on your C header files such as <sys/ioctl.h>.
(There is a Perl script called h2ph that comes with
the Perl kit that may help you in this, but it's
nontrivial.) SCALAR will be read and/or written
depending on the FUNCTION--a pointer to the string
value of SCALAR will be passed as the third argument
of the actual "ioctl" call. (If SCALAR has no
string value but does have a numeric value, that
value will be passed rather than a pointer to the
string value. To guarantee this to be true, add a 0
to the scalar before using it.) The "pack" and
"unpack" functions may be needed to manipulate the
values of structures used by "ioctl".
The return value of "ioctl" (and "fcntl") is as fol-
lows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns true on success and false on
failure, yet you can still easily determine the
actual value returned by the operating system:
$retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;
The special string "0 but true" is exempt from -w
complaints about improper numeric conversions.
join EXPR,LIST
Joins the separate strings of LIST into a single
string with fields separated by the value of EXPR,
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and returns that new string. Example:
$rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
Beware that unlike "split", "join" doesn't take a
pattern as its first argument. Compare "split".
keys HASH
Returns a list consisting of all the keys of the
named hash. (In scalar context, returns the number
of keys.)
The keys are returned in an apparently random order.
The actual random order is subject to change in
future versions of perl, but it is guaranteed to be
the same order as either the "values" or "each"
function produces (given that the hash has not been
modified). Since Perl 5.8.1 the ordering is dif-
ferent even between different runs of Perl for secu-
rity reasons (see "Algorithmic Complexity Attacks"
in perlsec).
As a side effect, calling keys() resets the HASH's
internal iterator (see "each"). In particular, cal-
ling keys() in void context resets the iterator with
no other overhead.
Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
The returned values are copies of the original keys
in the hash, so modifying them will not affect the
original hash. Compare "values".
To sort a hash by value, you'll need to use a "sort"
function. Here's a descending numeric sort of a hash
by its values:
foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}
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As an lvalue "keys" allows you to increase the
number of hash buckets allocated for the given hash.
This can gain you a measure of efficiency if you
know the hash is going to get big. (This is similar
to pre-extending an array by assigning a larger
number to $#array.) If you say
keys %hash = 200;
then %hash will have at least 200 buckets allocated
for it--256 of them, in fact, since it rounds up to
the next power of two. These buckets will be
retained even if you do "%hash = ()", use "undef
%hash" if you want to free the storage while %hash
is still in scope. You can't shrink the number of
buckets allocated for the hash using "keys" in this
way (but you needn't worry about doing this by
accident, as trying has no effect).
See also "each", "values" and "sort".
kill SIGNAL, LIST
Sends a signal to a list of processes. Returns the
number of processes successfully signaled (which is
not necessarily the same as the number actually
killed).
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
If SIGNAL is zero, no signal is sent to the process.
This is a useful way to check that a child process
is alive and hasn't changed its UID. See perlport
for notes on the portability of this construct.
Unlike in the shell, if SIGNAL is negative, it kills
process groups instead of processes. (On System V,
a negative PROCESS number will also kill process
groups, but that's not portable.) That means you
usually want to use positive not negative signals.
You may also use a signal name in quotes.
See "Signals" in perlipc for more details.
last LABEL
last The "last" command is like the "break" statement in
C (as used in loops); it immediately exits the loop
in question. If the LABEL is omitted, the command
refers to the innermost enclosing loop. The "con-
tinue" block, if any, is not executed:
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LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}
"last" cannot be used to exit a block which returns
a value such as "eval {}", "sub {}" or "do {}", and
should not be used to exit a grep() or map() opera-
tion.
Note that a block by itself is semantically identi-
cal to a loop that executes once. Thus "last" can
be used to effect an early exit out of such a block.
See also "continue" for an illustration of how
"last", "next", and "redo" work.
lc EXPR
lc Returns a lowercased version of EXPR. This is the
internal function implementing the "\L" escape in
double-quoted strings. Respects current LC_CTYPE
locale if "use locale" in force. See perllocale and
perlunicode for more details about locale and
Unicode support.
If EXPR is omitted, uses $_.
lcfirst EXPR
lcfirst Returns the value of EXPR with the first character
lowercased. This is the internal function imple-
menting the "\l" escape in double-quoted strings.
Respects current LC_CTYPE locale if "use locale" in
force. See perllocale and perlunicode for more
details about locale and Unicode support.
If EXPR is omitted, uses $_.
length EXPR
length Returns the length in characters of the value of
EXPR. If EXPR is omitted, returns length of $_.
Note that this cannot be used on an entire array or
hash to find out how many elements these have. For
that, use "scalar @array" and "scalar keys %hash"
respectively.
Note the characters: if the EXPR is in Unicode, you
will get the number of characters, not the number of
bytes. To get the length in bytes, use "do { use
bytes; length(EXPR) }", see bytes.
link OLDFILE,NEWFILE
Creates a new filename linked to the old filename.
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Returns true for success, false otherwise.
listen SOCKET,QUEUESIZE
Does the same thing that the listen system call
does. Returns true if it succeeded, false other-
wise. See the example in "Sockets: Client/Server
Communication" in perlipc.
local EXPR
You really probably want to be using "my" instead,
because "local" isn't what most people think of as
"local". See "Private Variables via my()" in perl-
sub for details.
A local modifies the listed variables to be local to
the enclosing block, file, or eval. If more than
one value is listed, the list must be placed in
parentheses. See "Temporary Values via local()" in
perlsub for details, including issues with tied
arrays and hashes.
localtime EXPR
localtime
Converts a time as returned by the time function to
a 9-element list with the time analyzed for the
local time zone. Typically used as follows:
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
All list elements are numeric, and come straight out
of the C `struct tm'. $sec, $min, and $hour are the
seconds, minutes, and hours of the specified time.
$mday is the day of the month, and $mon is the month
itself, in the range 0..11 with 0 indicating January
and 11 indicating December. This makes it easy to
get a month name from a list:
my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
print "$abbr[$mon] $mday";
# $mon=9, $mday=18 gives "Oct 18"
$year is the number of years since 1900, not just
the last two digits of the year. That is, $year is
123 in year 2023. The proper way to get a complete
4-digit year is simply:
$year += 1900;
To get the last two digits of the year (e.g., '01'
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in 2001) do:
$year = sprintf("%02d", $year % 100);
$wday is the day of the week, with 0 indicating Sun-
day and 3 indicating Wednesday. $yday is the day of
the year, in the range 0..364 (or 0..365 in leap
years.)
$isdst is true if the specified time occurs during
Daylight Saving Time, false otherwise.
If EXPR is omitted, "localtime()" uses the current
time ("localtime(time)").
In scalar context, "localtime()" returns the
ctime(3) value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
This scalar value is not locale dependent but is a
Perl builtin. For GMT instead of local time use the
"gmtime" builtin. See also the "Time::Local" module
(to convert the second, minutes, hours, ... back to
the integer value returned by time()), and the POSIX
module's strftime(3) and mktime(3) functions.
To get somewhat similar but locale dependent date
strings, set up your locale environment variables
appropriately (please see perllocale) and try for
example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
# or for GMT formatted appropriately for your locale:
$now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
Note that the %a and %b, the short forms of the day
of the week and the month of the year, may not
necessarily be three characters wide.
See "localtime" in perlport for portability con-
cerns.
lock THING
This function places an advisory lock on a shared
variable, or referenced object contained in THING
until the lock goes out of scope.
lock() is a "weak keyword" : this means that if
you've defined a function by this name (before any
calls to it), that function will be called instead.
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(However, if you've said "use threads", lock() is
always a keyword.) See threads.
log EXPR
log Returns the natural logarithm (base e) of EXPR. If
EXPR is omitted, returns log of $_. To get the log
of another base, use basic algebra: The base-N log
of a number is equal to the natural log of that
number divided by the natural log of N. For exam-
ple:
sub log10 {
my $n = shift;
return log($n)/log(10);
}
See also "exp" for the inverse operation.
lstat EXPR
lstat Does the same thing as the "stat" function (includ-
ing setting the special "_" filehandle) but stats a
symbolic link instead of the file the symbolic link
points to. If symbolic links are unimplemented on
your system, a normal "stat" is done. For much more
detailed information, please see the documentation
for "stat".
If EXPR is omitted, stats $_.
m// The match operator. See perlop.
map BLOCK LIST
map EXPR,LIST
Evaluates the BLOCK or EXPR for each element of LIST
(locally setting $_ to each element) and returns the
list value composed of the results of each such
evaluation. In scalar context, returns the total
number of elements so generated. Evaluates BLOCK or
EXPR in list context, so each element of LIST may
produce zero, one, or more elements in the returned
value.
@chars = map(chr, @nums);
translates a list of numbers to the corresponding
characters. And
%hash = map { getkey($_) => $_ } @array;
is just a funny way to write
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%hash = ();
foreach $_ (@array) {
$hash{getkey($_)} = $_;
}
Note that $_ is an alias to the list value, so it
can be used to modify the elements of the LIST.
While this is useful and supported, it can cause
bizarre results if the elements of LIST are not
variables. Using a regular "foreach" loop for this
purpose would be clearer in most cases. See also
"grep" for an array composed of those items of the
original list for which the BLOCK or EXPR evaluates
to true.
"{" starts both hash references and blocks, so "map
{ ..." could be either the start of map BLOCK LIST
or map EXPR, LIST. Because perl doesn't look ahead
for the closing "}" it has to take a guess at which
its dealing with based what it finds just after the
"{". Usually it gets it right, but if it doesn't it
won't realize something is wrong until it gets to
the "}" and encounters the missing (or unexpected)
comma. The syntax error will be reported close to
the "}" but you'll need to change something near the
"{" such as using a unary "+" to give perl some
help:
%hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
%hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
%hash = map { ("\L$_", 1) } @array # this also works
%hash = map { lc($_), 1 } @array # as does this.
%hash = map +( lc($_), 1 ), @array # this is EXPR and works!
%hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
or to force an anon hash constructor use "+{"
@hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
and you get list of anonymous hashes each with only
1 entry.
mkdir FILENAME,MASK
mkdir FILENAME
Creates the directory specified by FILENAME, with
permissions specified by MASK (as modified by
"umask"). If it succeeds it returns true, otherwise
it returns false and sets $! (errno). If omitted,
MASK defaults to 0777.
In general, it is better to create directories with
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permissive MASK, and let the user modify that with
their "umask", than it is to supply a restrictive
MASK and give the user no way to be more permissive.
The exceptions to this rule are when the file or
directory should be kept private (mail files, for
instance). The perlfunc(1) entry on "umask"
discusses the choice of MASK in more detail.
Note that according to the POSIX 1003.1-1996 the
FILENAME may have any number of trailing slashes.
Some operating and filesystems do not get this
right, so Perl automatically removes all trailing
slashes to keep everyone happy.
msgctl ID,CMD,ARG
Calls the System V IPC function msgctl(2). You'll
probably have to say
use IPC::SysV;
first to get the correct constant definitions. If
CMD is "IPC_STAT", then ARG must be a variable that
will hold the returned "msqid_ds" structure.
Returns like "ioctl": the undefined value for error,
"0 but true" for zero, or the actual return value
otherwise. See also "SysV IPC" in perlipc,
"IPC::SysV", and "IPC::Semaphore" documentation.
msgget KEY,FLAGS
Calls the System V IPC function msgget(2). Returns
the message queue id, or the undefined value if
there is an error. See also "SysV IPC" in perlipc
and "IPC::SysV" and "IPC::Msg" documentation.
msgrcv ID,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive a
message from message queue ID into variable VAR with
a maximum message size of SIZE. Note that when a
message is received, the message type as a native
long integer will be the first thing in VAR, fol-
lowed by the actual message. This packing may be
opened with "unpack("l! a*")". Taints the variable.
Returns true if successful, or false if there is an
error. See also "SysV IPC" in perlipc, "IPC::SysV",
and "IPC::SysV::Msg" documentation.
msgsnd ID,MSG,FLAGS
Calls the System V IPC function msgsnd to send the
message MSG to the message queue ID. MSG must begin
with the native long integer message type, and be
followed by the length of the actual message, and
finally the message itself. This kind of packing
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can be achieved with "pack("l! a*", $type, $mes-
sage)". Returns true if successful, or false if
there is an error. See also "IPC::SysV" and
"IPC::SysV::Msg" documentation.
my EXPR
my TYPE EXPR
my EXPR : ATTRS
my TYPE EXPR : ATTRS
A "my" declares the listed variables to be local
(lexically) to the enclosing block, file, or "eval".
If more than one value is listed, the list must be
placed in parentheses.
The exact semantics and interface of TYPE and ATTRS
are still evolving. TYPE is currently bound to the
use of "fields" pragma, and attributes are handled
using the "attributes" pragma, or starting from Perl
5.8.0 also via the "Attribute::Handlers" module.
See "Private Variables via my()" in perlsub for
details, and fields, attributes, and
Attribute::Handlers.
next LABEL
next The "next" command is like the "continue" statement
in C; it starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
#...
}
Note that if there were a "continue" block on the
above, it would get executed even on discarded
lines. If the LABEL is omitted, the command refers
to the innermost enclosing loop.
"next" cannot be used to exit a block which returns
a value such as "eval {}", "sub {}" or "do {}", and
should not be used to exit a grep() or map() opera-
tion.
Note that a block by itself is semantically identi-
cal to a loop that executes once. Thus "next" will
exit such a block early.
See also "continue" for an illustration of how
"last", "next", and "redo" work.
no Module VERSION LIST
no Module VERSION
no Module LIST
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no Module
See the "use" function, which "no" is the opposite
of.
oct EXPR
oct Interprets EXPR as an octal string and returns the
corresponding value. (If EXPR happens to start off
with "0x", interprets it as a hex string. If EXPR
starts off with "0b", it is interpreted as a binary
string. Leading whitespace is ignored in all three
cases.) The following will handle decimal, binary,
octal, and hex in the standard Perl or C notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_. To go the other way
(produce a number in octal), use sprintf() or
printf():
$perms = (stat("filename"))[2] & 07777;
$oct_perms = sprintf "%lo", $perms;
The oct() function is commonly used when a string
such as 644 needs to be converted into a file mode,
for example. (Although perl will automatically con-
vert strings into numbers as needed, this automatic
conversion assumes base 10.)
open FILEHANDLE,EXPR
open FILEHANDLE,MODE,EXPR
open FILEHANDLE,MODE,EXPR,LIST
open FILEHANDLE,MODE,REFERENCE
open FILEHANDLE
Opens the file whose filename is given by EXPR, and
associates it with FILEHANDLE.
(The following is a comprehensive reference to
open(): for a gentler introduction you may consider
perlopentut.)
If FILEHANDLE is an undefined scalar variable (or
array or hash element) the variable is assigned a
reference to a new anonymous filehandle, otherwise
if FILEHANDLE is an expression, its value is used as
the name of the real filehandle wanted. (This is
considered a symbolic reference, so "use strict
'refs'" should not be in effect.)
If EXPR is omitted, the scalar variable of the same
name as the FILEHANDLE contains the filename. (Note
that lexical variables--those declared with
"my"--will not work for this purpose; so if you're
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using "my", specify EXPR in your call to open.)
If three or more arguments are specified then the
mode of opening and the file name are separate. If
MODE is '<' or nothing, the file is opened for
input. If MODE is '>', the file is truncated and
opened for output, being created if necessary. If
MODE is '>>', the file is opened for appending,
again being created if necessary.
You can put a '+' in front of the '>' or '<' to
indicate that you want both read and write access to
the file; thus '+<' is almost always preferred for
read/write updates--the '+>' mode would clobber the
file first. You can't usually use either read-write
mode for updating textfiles, since they have vari-
able length records. See the -i switch in perlrun
for a better approach. The file is created with
permissions of 0666 modified by the process' "umask"
value.
These various prefixes correspond to the fopen(3)
modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
In the 2-arguments (and 1-argument) form of the call
the mode and filename should be concatenated (in
this order), possibly separated by spaces. It is
possible to omit the mode in these forms if the mode
is '<'.
If the filename begins with '|', the filename is
interpreted as a command to which output is to be
piped, and if the filename ends with a '|', the
filename is interpreted as a command which pipes
output to us. See "Using open() for IPC" in perlipc
for more examples of this. (You are not allowed to
"open" to a command that pipes both in and out, but
see IPC::Open2, IPC::Open3, and "Bidirectional Com-
munication with Another Process" in perlipc for
alternatives.)
For three or more arguments if MODE is '|-', the
filename is interpreted as a command to which output
is to be piped, and if MODE is '-|', the filename is
interpreted as a command which pipes output to us.
In the 2-arguments (and 1-argument) form one should
replace dash ('-') with the command. See "Using
open() for IPC" in perlipc for more examples of
this. (You are not allowed to "open" to a command
that pipes both in and out, but see IPC::Open2,
IPC::Open3, and "Bidirectional Communication" in
perlipc for alternatives.)
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In the three-or-more argument form of pipe opens, if
LIST is specified (extra arguments after the command
name) then LIST becomes arguments to the command
invoked if the platform supports it. The meaning of
"open" with more than three arguments for non-pipe
modes is not yet specified. Experimental "layers"
may give extra LIST arguments meaning.
In the 2-arguments (and 1-argument) form opening '-'
opens STDIN and opening '>-' opens STDOUT.
You may use the three-argument form of open to
specify IO "layers" (sometimes also referred to as
"disciplines") to be applied to the handle that
affect how the input and output are processed (see
open and PerlIO for more details). For example
open(FH, "<:utf8", "file")
will open the UTF-8 encoded file containing Unicode
characters, see perluniintro. Note that if layers
are specified in the three-arg form then default
layers stored in ${^OPEN} (see perlvar; usually set
by the open pragma or the switch -CioD) are ignored.
Open returns nonzero upon success, the undefined
value otherwise. If the "open" involved a pipe, the
return value happens to be the pid of the subpro-
cess.
If you're running Perl on a system that distin-
guishes between text files and binary files, then
you should check out "binmode" for tips for dealing
with this. The key distinction between systems that
need "binmode" and those that don't is their text
file formats. Systems like Unix, Mac OS, and Plan
9, which delimit lines with a single character, and
which encode that character in C as "\n", do not
need "binmode". The rest need it.
When opening a file, it's usually a bad idea to con-
tinue normal execution if the request failed, so
"open" is frequently used in connection with "die".
Even if "die" won't do what you want (say, in a CGI
script, where you want to make a nicely formatted
error message (but there are modules that can help
with that problem)) you should always check the
return value from opening a file. The infrequent
exception is when working with an unopened filehan-
dle is actually what you want to do.
As a special case the 3-arg form with a read/write
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mode and the third argument being "undef":
open(TMP, "+>", undef) or die ...
opens a filehandle to an anonymous temporary file.
Also using "+<" works for symmetry, but you really
should consider writing something to the temporary
file first. You will need to seek() to do the read-
ing.
Since v5.8.0, perl has built using PerlIO by
default. Unless you've changed this (i.e. Configure
-Uuseperlio), you can open file handles to "in
memory" files held in Perl scalars via:
open($fh, '>', \$variable) || ..
Though if you try to re-open "STDOUT" or "STDERR" as
an "in memory" file, you have to close it first:
close STDOUT;
open STDOUT, '>', \$variable or die "Can't open STDOUT: $!";
Examples:
$ARTICLE = 100;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
while (<ARTICLE>) {...
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
# if the open fails, output is discarded
open(DBASE, '+<', 'dbase.mine') # open for update
or die "Can't open 'dbase.mine' for update: $!";
open(DBASE, '+<dbase.mine') # ditto
or die "Can't open 'dbase.mine' for update: $!";
open(ARTICLE, '-|', "caesar <$article") # decrypt article
or die "Can't start caesar: $!";
open(ARTICLE, "caesar <$article |") # ditto
or die "Can't start caesar: $!";
open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
# in memory files
open(MEMORY,'>', \$var)
or die "Can't open memory file: $!";
print MEMORY "foo!\n"; # output will end up in $var
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# process argument list of files along with any includes
foreach $file (@ARGV) {
process($file, 'fh00');
}
sub process {
my($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
local $_;
while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
process($1, $input);
next;
}
#... # whatever
}
}
See perliol for detailed info on PerlIO.
You may also, in the Bourne shell tradition, specify
an EXPR beginning with '>&', in which case the rest
of the string is interpreted as the name of a
filehandle (or file descriptor, if numeric) to be
duped (as dup(2)) and opened. You may use "&" after
">", ">>", "<", "+>", "+>>", and "+<". The mode you
specify should match the mode of the original
filehandle. (Duping a filehandle does not take into
account any existing contents of IO buffers.) If you
use the 3-arg form then you can pass either a
number, the name of a filehandle or the normal
"reference to a glob".
Here is a script that saves, redirects, and restores
"STDOUT" and "STDERR" using various methods:
#!/usr/bin/perl
open my $oldout, ">&STDOUT" or die "Can't dup STDOUT: $!";
open OLDERR, ">&", \*STDERR or die "Can't dup STDERR: $!";
open STDOUT, '>', "foo.out" or die "Can't redirect STDOUT: $!";
open STDERR, ">&STDOUT" or die "Can't dup STDOUT: $!";
select STDERR; $| = 1; # make unbuffered
select STDOUT; $| = 1; # make unbuffered
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print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you specify '<&=X', where "X" is a file descrip-
tor number or a filehandle, then Perl will do an
equivalent of C's "fdopen" of that file descriptor
(and not call dup(2)); this is more parsimonious of
file descriptors. For example:
# open for input, reusing the fileno of $fd
open(FILEHANDLE, "<&=$fd")
or
open(FILEHANDLE, "<&=", $fd)
or
# open for append, using the fileno of OLDFH
open(FH, ">>&=", OLDFH)
or
open(FH, ">>&=OLDFH")
Being parsimonious on filehandles is also useful
(besides being parsimonious) for example when some-
thing is dependent on file descriptors, like for
example locking using flock(). If you do just
"open(A, '>>&B')", the filehandle A will not have
the same file descriptor as B, and therefore
flock(A) will not flock(B), and vice versa. But
with "open(A, '>>&=B')" the filehandles will share
the same file descriptor.
Note that if you are using Perls older than 5.8.0,
Perl will be using the standard C libraries' fdo-
pen() to implement the "=" functionality. On many
UNIX systems fdopen() fails when file descriptors
exceed a certain value, typically 255. For Perls
5.8.0 and later, PerlIO is most often the default.
You can see whether Perl has been compiled with Per-
lIO or not by running "perl -V" and looking for
"useperlio=" line. If "useperlio" is "define", you
have PerlIO, otherwise you don't.
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If you open a pipe on the command '-', i.e., either
'|-' or '-|' with 2-arguments (or 1-argument) form
of open(), then there is an implicit fork done, and
the return value of open is the pid of the child
within the parent process, and 0 within the child
process. (Use "defined($pid)" to determine whether
the open was successful.) The filehandle behaves
normally for the parent, but i/o to that filehandle
is piped from/to the STDOUT/STDIN of the child pro-
cess. In the child process the filehandle isn't
opened--i/o happens from/to the new STDOUT or STDIN.
Typically this is used like the normal piped open
when you want to exercise more control over just how
the pipe command gets executed, such as when you are
running setuid, and don't want to have to scan shell
commands for metacharacters. The following triples
are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
open(FOO, "cat -n '$file'|");
open(FOO, '-|', "cat -n '$file'");
open(FOO, '-|') || exec 'cat', '-n', $file;
open(FOO, '-|', "cat", '-n', $file);
The last example in each block shows the pipe as
"list form", which is not yet supported on all plat-
forms. A good rule of thumb is that if your plat-
form has true "fork()" (in other words, if your
platform is UNIX) you can use the list form.
See "Safe Pipe Opens" in perlipc for more examples
of this.
Beginning with v5.6.0, Perl will attempt to flush
all files opened for output before any operation
that may do a fork, but this may not be supported on
some platforms (see perlport). To be safe, you may
need to set $| ($AUTOFLUSH in English) or call the
"autoflush()" method of "IO::Handle" on any open
handles.
On systems that support a close-on-exec flag on
files, the flag will be set for the newly opened
file descriptor as determined by the value of $^F.
See "$^F" in perlvar.
Closing any piped filehandle causes the parent pro-
cess to wait for the child to finish, and returns
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the status value in $?.
The filename passed to 2-argument (or 1-argument)
form of open() will have leading and trailing whi-
tespace deleted, and the normal redirection charac-
ters honored. This property, known as "magic open",
can often be used to good effect. A user could
specify a filename of "rsh cat file |", or you could
change certain filenames as needed:
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
open(FH, $filename) or die "Can't open $filename: $!";
Use 3-argument form to open a file with arbitrary
weird characters in it,
open(FOO, '<', $file);
otherwise it's necessary to protect any leading and
trailing whitespace:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
(this may not work on some bizarre filesystems).
One should conscientiously choose between the magic
and 3-arguments form of open():
open IN, $ARGV[0];
will allow the user to specify an argument of the
form "rsh cat file |", but will not work on a
filename which happens to have a trailing space,
while
open IN, '<', $ARGV[0];
will have exactly the opposite restrictions.
If you want a "real" C "open" (see open(2) on your
system), then you should use the "sysopen" function,
which involves no such magic (but may use subtly
different filemodes than Perl open(), which is
mapped to C fopen()). This is another way to pro-
tect your filenames from interpretation. For exam-
ple:
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use IO::Handle;
sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
or die "sysopen $path: $!";
$oldfh = select(HANDLE); $| = 1; select($oldfh);
print HANDLE "stuff $$\n";
seek(HANDLE, 0, 0);
print "File contains: ", <HANDLE>;
Using the constructor from the "IO::Handle" package
(or one of its subclasses, such as "IO::File" or
"IO::Socket"), you can generate anonymous filehan-
dles that have the scope of whatever variables hold
references to them, and automatically close whenever
and however you leave that scope:
use IO::File;
#...
sub read_myfile_munged {
my $ALL = shift;
my $handle = new IO::File;
open($handle, "myfile") or die "myfile: $!";
$first = <$handle>
or return (); # Automatically closed here.
mung $first or die "mung failed"; # Or here.
return $first, <$handle> if $ALL; # Or here.
$first; # Or here.
}
See "seek" for some details about mixing reading and
writing.
opendir DIRHANDLE,EXPR
Opens a directory named EXPR for processing by
"readdir", "telldir", "seekdir", "rewinddir", and
"closedir". Returns true if successful. DIRHANDLE
may be an expression whose value can be used as an
indirect dirhandle, usually the real dirhandle name.
If DIRHANDLE is an undefined scalar variable (or
array or hash element), the variable is assigned a
reference to a new anonymous dirhandle. DIRHANDLEs
have their own namespace separate from FILEHANDLEs.
ord EXPR
ord Returns the numeric (the native 8-bit encoding, like
ASCII or EBCDIC, or Unicode) value of the first
character of EXPR. If EXPR is omitted, uses $_.
For the reverse, see "chr". See perlunicode and
encoding for more about Unicode.
our EXPR
our EXPR TYPE
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our EXPR : ATTRS
our TYPE EXPR : ATTRS
"our" associates a simple name with a package vari-
able in the current package for use within the
current scope. When "use strict 'vars'" is in
effect, "our" lets you use declared global variables
without qualifying them with package names, within
the lexical scope of the "our" declaration. In this
way "our" differs from "use vars", which is package
scoped.
Unlike "my", which both allocates storage for a
variable and associates a simple name with that
storage for use within the current scope, "our"
associates a simple name with a package variable in
the current package, for use within the current
scope. In other words, "our" has the same scoping
rules as "my", but does not necessarily create a
variable.
If more than one value is listed, the list must be
placed in parentheses.
our $foo;
our($bar, $baz);
An "our" declaration declares a global variable that
will be visible across its entire lexical scope,
even across package boundaries. The package in
which the variable is entered is determined at the
point of the declaration, not at the point of use.
This means the following behavior holds:
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
print $bar; # prints 20, as it refers to $Foo::bar
Multiple "our" declarations with the same name in
the same lexical scope are allowed if they are in
different packages. If they happen to be in the
same package, Perl will emit warnings if you have
asked for them, just like multiple "my" declara-
tions. Unlike a second "my" declaration, which will
bind the name to a fresh variable, a second "our"
declaration in the same package, in the same scope,
is merely redundant.
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use warnings;
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
our $bar = 30; # declares $Bar::bar for rest of lexical scope
print $bar; # prints 30
our $bar; # emits warning but has no other effect
print $bar; # still prints 30
An "our" declaration may also have a list of attri-
butes associated with it.
The exact semantics and interface of TYPE and ATTRS
are still evolving. TYPE is currently bound to the
use of "fields" pragma, and attributes are handled
using the "attributes" pragma, or starting from Perl
5.8.0 also via the "Attribute::Handlers" module.
See "Private Variables via my()" in perlsub for
details, and fields, attributes, and
Attribute::Handlers.
The only currently recognized "our()" attribute is
"unique" which indicates that a single copy of the
global is to be used by all interpreters should the
program happen to be running in a multi-interpreter
environment. (The default behaviour would be for
each interpreter to have its own copy of the glo-
bal.) Examples:
our @EXPORT : unique = qw(foo);
our %EXPORT_TAGS : unique = (bar => [qw(aa bb cc)]);
our $VERSION : unique = "1.00";
Note that this attribute also has the effect of mak-
ing the global readonly when the first new inter-
preter is cloned (for example, when the first new
thread is created).
Multi-interpreter environments can come to being
either through the fork() emulation on Windows plat-
forms, or by embedding perl in a multi-threaded
application. The "unique" attribute does nothing in
all other environments.
Warning: the current implementation of this attri-
bute operates on the typeglob associated with the
variable; this means that "our $x : unique" also has
the effect of "our @x : unique; our %x : unique".
This may be subject to change.
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pack TEMPLATE,LIST
Takes a LIST of values and converts it into a string
using the rules given by the TEMPLATE. The result-
ing string is the concatenation of the converted
values. Typically, each converted value looks like
its machine-level representation. For example, on
32-bit machines a converted integer may be
represented by a sequence of 4 bytes.
The TEMPLATE is a sequence of characters that give
the order and type of values, as follows:
a A string with arbitrary binary data, will be null padded.
A A text (ASCII) string, will be space padded.
Z A null terminated (ASCIZ) string, will be null padded.
b A bit string (ascending bit order inside each byte, like vec()).
B A bit string (descending bit order inside each byte).
h A hex string (low nybble first).
H A hex string (high nybble first).
c A signed char value.
C An unsigned char value. Only does bytes. See U for Unicode.
s A signed short value.
S An unsigned short value.
(This 'short' is _exactly_ 16 bits, which may differ from
what a local C compiler calls 'short'. If you want
native-length shorts, use the '!' suffix.)
i A signed integer value.
I An unsigned integer value.
(This 'integer' is _at_least_ 32 bits wide. Its exact
size depends on what a local C compiler calls 'int',
and may even be larger than the 'long' described in
the next item.)
l A signed long value.
L An unsigned long value.
(This 'long' is _exactly_ 32 bits, which may differ from
what a local C compiler calls 'long'. If you want
native-length longs, use the '!' suffix.)
n An unsigned short in "network" (big-endian) order.
N An unsigned long in "network" (big-endian) order.
v An unsigned short in "VAX" (little-endian) order.
V An unsigned long in "VAX" (little-endian) order.
(These 'shorts' and 'longs' are _exactly_ 16 bits and
_exactly_ 32 bits, respectively.)
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q A signed quad (64-bit) value.
Q An unsigned quad value.
(Quads are available only if your system supports 64-bit
integer values _and_ if Perl has been compiled to support those.
Causes a fatal error otherwise.)
j A signed integer value (a Perl internal integer, IV).
J An unsigned integer value (a Perl internal unsigned integer, UV).
f A single-precision float in the native format.
d A double-precision float in the native format.
F A floating point value in the native native format
(a Perl internal floating point value, NV).
D A long double-precision float in the native format.
(Long doubles are available only if your system supports long
double values _and_ if Perl has been compiled to support those.
Causes a fatal error otherwise.)
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
U A Unicode character number. Encodes to UTF-8 internally
(or UTF-EBCDIC in EBCDIC platforms).
w A BER compressed integer (not an ASN.1 BER, see perlpacktut for
details). Its bytes represent an unsigned integer in base 128,
most significant digit first, with as few digits as possible. Bit
eight (the high bit) is set on each byte except the last.
x A null byte.
X Back up a byte.
@ Null fill to absolute position, counted from the start of
the innermost ()-group.
( Start of a ()-group.
The following rules apply:
* Each letter may optionally be followed by a
number giving a repeat count. With all
types except "a", "A", "Z", "b", "B", "h",
"H", "@", "x", "X" and "P" the pack function
will gobble up that many values from the
LIST. A "*" for the repeat count means to
use however many items are left, except for
"@", "x", "X", where it is equivalent to 0,
and "u", where it is equivalent to 1 (or 45,
what is the same). A numeric repeat count
may optionally be enclosed in brackets, as
in "pack 'C[80]', @arr".
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One can replace the numeric repeat count by
a template enclosed in brackets; then the
packed length of this template in bytes is
used as a count. For example, "x[L]" skips a
long (it skips the number of bytes in a
long); the template "$t X[$t] $t" unpack()s
twice what $t unpacks. If the template in
brackets contains alignment commands (such
as "x![d]"), its packed length is calculated
as if the start of the template has the max-
imal possible alignment.
When used with "Z", "*" results in the addi-
tion of a trailing null byte (so the packed
result will be one longer than the byte
"length" of the item).
The repeat count for "u" is interpreted as
the maximal number of bytes to encode per
line of output, with 0 and 1 replaced by 45.
* The "a", "A", and "Z" types gobble just one
value, but pack it as a string of length
count, padding with nulls or spaces as
necessary. When unpacking, "A" strips
trailing spaces and nulls, "Z" strips every-
thing after the first null, and "a" returns
data verbatim. When packing, "a", and "Z"
are equivalent.
If the value-to-pack is too long, it is
truncated. If too long and an explicit
count is provided, "Z" packs only "$count-1"
bytes, followed by a null byte. Thus "Z"
always packs a trailing null byte under all
circumstances.
* Likewise, the "b" and "B" fields pack a
string that many bits long. Each byte of the
input field of pack() generates 1 bit of the
result. Each result bit is based on the
least-significant bit of the corresponding
input byte, i.e., on "ord($byte)%2". In
particular, bytes "0" and "1" generate bits
0 and 1, as do bytes "\0" and "\1".
Starting from the beginning of the input
string of pack(), each 8-tuple of bytes is
converted to 1 byte of output. With format
"b" the first byte of the 8-tuple determines
the least-significant bit of a byte, and
with format "B" it determines the most-
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significant bit of a byte.
If the length of the input string is not
exactly divisible by 8, the remainder is
packed as if the input string were padded by
null bytes at the end. Similarly, during
unpack()ing the "extra" bits are ignored.
If the input string of pack() is longer than
needed, extra bytes are ignored. A "*" for
the repeat count of pack() means to use all
the bytes of the input field. On
unpack()ing the bits are converted to a
string of "0"s and "1"s.
* The "h" and "H" fields pack a string that
many nybbles (4-bit groups, representable as
hexadecimal digits, 0-9a-f) long.
Each byte of the input field of pack() gen-
erates 4 bits of the result. For non-
alphabetical bytes the result is based on
the 4 least-significant bits of the input
byte, i.e., on "ord($byte)%16". In particu-
lar, bytes "0" and "1" generate nybbles 0
and 1, as do bytes "\0" and "\1". For bytes
"a".."f" and "A".."F" the result is compati-
ble with the usual hexadecimal digits, so
that "a" and "A" both generate the nybble
"0xa==10". The result for bytes "g".."z"
and "G".."Z" is not well-defined.
Starting from the beginning of the input
string of pack(), each pair of bytes is con-
verted to 1 byte of output. With format "h"
the first byte of the pair determines the
least-significant nybble of the output byte,
and with format "H" it determines the most-
significant nybble.
If the length of the input string is not
even, it behaves as if padded by a null byte
at the end. Similarly, during unpack()ing
the "extra" nybbles are ignored.
If the input string of pack() is longer than
needed, extra bytes are ignored. A "*" for
the repeat count of pack() means to use all
the bytes of the input field. On
unpack()ing the bits are converted to a
string of hexadecimal digits.
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* The "p" type packs a pointer to a null-
terminated string. You are responsible for
ensuring the string is not a temporary value
(which can potentially get deallocated
before you get around to using the packed
result). The "P" type packs a pointer to a
structure of the size indicated by the
length. A NULL pointer is created if the
corresponding value for "p" or "P" is
"undef", similarly for unpack().
* The "/" template character allows packing
and unpacking of strings where the packed
structure contains a byte count followed by
the string itself. You write length-
item"/"string-item.
The length-item can be any "pack" template
letter, and describes how the length value
is packed. The ones likely to be of most
use are integer-packing ones like "n" (for
Java strings), "w" (for ASN.1 or SNMP) and
"N" (for Sun XDR).
For "pack", the string-item must, at
present, be "A*", "a*" or "Z*". For "unpack"
the length of the string is obtained from
the length-item, but if you put in the '*'
it will be ignored. For all other codes,
"unpack" applies the length value to the
next item, which must not have a repeat
count.
unpack 'C/a', "\04Gurusamy"; gives 'Guru'
unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
The length-item is not returned explicitly
from "unpack".
Adding a count to the length-item letter is
unlikely to do anything useful, unless that
letter is "A", "a" or "Z". Packing with a
length-item of "a" or "Z" may introduce
"\000" characters, which Perl does not
regard as legal in numeric strings.
* The integer types "s", "S", "l", and "L" may
be immediately followed by a "!" suffix to
signify native shorts or longs--as you can
see from above for example a bare "l" does
mean exactly 32 bits, the native "long" (as
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seen by the local C compiler) may be larger.
This is an issue mainly in 64-bit platforms.
You can see whether using "!" makes any
difference by
print length(pack("s")), " ", length(pack("s!")), "\n";
print length(pack("l")), " ", length(pack("l!")), "\n";
"i!" and "I!" also work but only because of
completeness; they are identical to "i" and
"I".
The actual sizes (in bytes) of native
shorts, ints, longs, and long longs on the
platform where Perl was built are also
available via Config:
use Config;
print $Config{shortsize}, "\n";
print $Config{intsize}, "\n";
print $Config{longsize}, "\n";
print $Config{longlongsize}, "\n";
(The $Config{longlongsize} will be undefined
if your system does not support long longs.)
* The integer formats "s", "S", "i", "I", "l",
"L", "j", and "J" are inherently non-
portable between processors and operating
systems because they obey the native
byteorder and endianness. For example a
4-byte integer 0x12345678 (305419896
decimal) would be ordered natively (arranged
in and handled by the CPU registers) into
bytes as
0x12 0x34 0x56 0x78 # big-endian
0x78 0x56 0x34 0x12 # little-endian
Basically, the Intel and VAX CPUs are
little-endian, while everybody else, for
example Motorola m68k/88k, PPC, Sparc, HP
PA, Power, and Cray are big-endian. Alpha
and MIPS can be either: Digital/Compaq
used/uses them in little-endian mode;
SGI/Cray uses them in big-endian mode.
The names `big-endian' and `little-endian'
are comic references to the classic
"Gulliver's Travels" (via the paper "On Holy
Wars and a Plea for Peace" by Danny Cohen,
USC/ISI IEN 137, April 1, 1980) and the
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egg-eating habits of the Lilliputians.
Some systems may have even weirder byte ord-
ers such as
0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56
You can see your system's preference with
print join(" ", map { sprintf "%#02x", $_ }
unpack("C*",pack("L",0x12345678))), "\n";
The byteorder on the platform where Perl was
built is also available via Config:
use Config;
print $Config{byteorder}, "\n";
Byteorders '1234' and '12345678' are
little-endian, '4321' and '87654321' are
big-endian.
If you want portable packed integers use the
formats "n", "N", "v", and "V", their byte
endianness and size are known. See also
perlport.
* Real numbers (floats and doubles) are in the
native machine format only; due to the mul-
tiplicity of floating formats around, and
the lack of a standard "network" representa-
tion, no facility for interchange has been
made. This means that packed floating point
data written on one machine may not be read-
able on another - even if both use IEEE
floating point arithmetic (as the endian-
ness of the memory representation is not
part of the IEEE spec). See also perlport.
Note that Perl uses doubles internally for
all numeric calculation, and converting from
double into float and thence back to double
again will lose precision (i.e.,
"unpack("f", pack("f", $foo)") will not in
general equal $foo).
* If the pattern begins with a "U", the
resulting string will be treated as
UTF-8-encoded Unicode. You can force UTF-8
encoding on in a string with an initial
"U0", and the bytes that follow will be
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interpreted as Unicode characters. If you
don't want this to happen, you can begin
your pattern with "C0" (or anything else) to
force Perl not to UTF-8 encode your string,
and then follow this with a "U*" somewhere
in your pattern.
* You must yourself do any alignment or pad-
ding by inserting for example enough 'x'es
while packing. There is no way to pack()
and unpack() could know where the bytes are
going to or coming from. Therefore "pack"
(and "unpack") handle their output and input
as flat sequences of bytes.
* A ()-group is a sub-TEMPLATE enclosed in
parentheses. A group may take a repeat
count, both as postfix, and for unpack()
also via the "/" template character. Within
each repetition of a group, positioning with
"@" starts again at 0. Therefore, the result
of
pack( '@1A((@2A)@3A)', 'a', 'b', 'c' )
is the string "\0a\0\0bc".
* "x" and "X" accept "!" modifier. In this
case they act as alignment commands: they
jump forward/back to the closest position
aligned at a multiple of "count" bytes. For
example, to pack() or unpack() C's "struct
{char c; double d; char cc[2]}" one may need
to use the template "C x![d] d C[2]"; this
assumes that doubles must be aligned on the
double's size.
For alignment commands "count" of 0 is
equivalent to "count" of 1; both result in
no-ops.
* A comment in a TEMPLATE starts with "#" and
goes to the end of line. White space may be
used to separate pack codes from each other,
but a "!" modifier and a repeat count must
follow immediately.
* If TEMPLATE requires more arguments to
pack() than actually given, pack() assumes
additional "" arguments. If TEMPLATE
requires fewer arguments to pack() than
actually given, extra arguments are ignored.
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Examples:
$foo = pack("CCCC",65,66,67,68);
# foo eq "ABCD"
$foo = pack("C4",65,66,67,68);
# same thing
$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
# note: the above examples featuring "C" and "c" are true
# only on ASCII and ASCII-derived systems such as ISO Latin 1
# and UTF-8. In EBCDIC the first example would be
# $foo = pack("CCCC",193,194,195,196);
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
$utmp_template = "Z8 Z8 Z16 L";
$utmp = pack($utmp_template, @utmp1);
# a struct utmp (BSDish)
@utmp2 = unpack($utmp_template, $utmp);
# "@utmp1" eq "@utmp2"
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
$foo = pack('sx2l', 12, 34);
# short 12, two zero bytes padding, long 34
$bar = pack('s@4l', 12, 34);
# short 12, zero fill to position 4, long 34
# $foo eq $bar
The same template may generally also be used in
unpack().
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package NAMESPACE
package Declares the compilation unit as being in the given
namespace. The scope of the package declaration is
from the declaration itself through the end of the
enclosing block, file, or eval (the same as the "my"
operator). All further unqualified dynamic identif-
iers will be in this namespace. A package statement
affects only dynamic variables--including those
you've used "local" on--but not lexical variables,
which are created with "my". Typically it would be
the first declaration in a file to be included by
the "require" or "use" operator. You can switch
into a package in more than one place; it merely
influences which symbol table is used by the com-
piler for the rest of that block. You can refer to
variables and filehandles in other packages by pre-
fixing the identifier with the package name and a
double colon: $Package::Variable. If the package
name is null, the "main" package as assumed. That
is, $::sail is equivalent to $main::sail (as well as
to $main'sail, still seen in older code).
If NAMESPACE is omitted, then there is no current
package, and all identifiers must be fully qualified
or lexicals. However, you are strongly advised not
to make use of this feature. Its use can cause unex-
pected behaviour, even crashing some versions of
Perl. It is deprecated, and will be removed from a
future release.
See "Packages" in perlmod for more information about
packages, modules, and classes. See perlsub for
other scoping issues.
pipe READHANDLE,WRITEHANDLE
Opens a pair of connected pipes like the correspond-
ing system call. Note that if you set up a loop of
piped processes, deadlock can occur unless you are
very careful. In addition, note that Perl's pipes
use IO buffering, so you may need to set $| to flush
your WRITEHANDLE after each command, depending on
the application.
See IPC::Open2, IPC::Open3, and "Bidirectional Com-
munication" in perlipc for examples of such things.
On systems that support a close-on-exec flag on
files, the flag will be set for the newly opened
file descriptors as determined by the value of $^F.
See "$^F" in perlvar.
pop ARRAY
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pop Pops and returns the last value of the array, shor-
tening the array by one element. Has an effect
similar to
$ARRAY[$#ARRAY--]
If there are no elements in the array, returns the
undefined value (although this may happen at other
times as well). If ARRAY is omitted, pops the @ARGV
array in the main program, and the @_ array in sub-
routines, just like "shift".
pos SCALAR
pos Returns the offset of where the last "m//g" search
left off for the variable in question ($_ is used
when the variable is not specified). Note that 0 is
a valid match offset. "undef" indicates that the
search position is reset (usually due to match
failure, but can also be because no match has yet
been performed on the scalar). "pos" directly
accesses the location used by the regexp engine to
store the offset, so assigning to "pos" will change
that offset, and so will also influence the "\G"
zero-width assertion in regular expressions. Because
a failed "m//gc" match doesn't reset the offset, the
return from "pos" won't change either in this case.
See perlre and perlop.
print FILEHANDLE LIST
print LIST
print Prints a string or a list of strings. Returns true
if successful. FILEHANDLE may be a scalar variable
name, in which case the variable contains the name
of or a reference to the filehandle, thus introduc-
ing one level of indirection. (NOTE: If FILEHANDLE
is a variable and the next token is a term, it may
be misinterpreted as an operator unless you inter-
pose a "+" or put parentheses around the arguments.)
If FILEHANDLE is omitted, prints by default to stan-
dard output (or to the last selected output
channel--see "select"). If LIST is also omitted,
prints $_ to the currently selected output channel.
To set the default output channel to something other
than STDOUT use the select operation. The current
value of $, (if any) is printed between each LIST
item. The current value of "$\" (if any) is printed
after the entire LIST has been printed. Because
print takes a LIST, anything in the LIST is
evaluated in list context, and any subroutine that
you call will have one or more of its expressions
evaluated in list context. Also be careful not to
follow the print keyword with a left parenthesis
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unless you want the corresponding right parenthesis
to terminate the arguments to the print--interpose a
"+" or put parentheses around all the arguments.
Note that if you're storing FILEHANDLEs in an array,
or if you're using any other expression more complex
than a scalar variable to retrieve it, you will have
to use a block returning the filehandle value
instead:
print { $files[$i] } "stuff\n";
print { $OK ? STDOUT : STDERR } "stuff\n";
printf FILEHANDLE FORMAT, LIST
printf FORMAT, LIST
Equivalent to "print FILEHANDLE sprintf(FORMAT,
LIST)", except that "$\" (the output record separa-
tor) is not appended. The first argument of the
list will be interpreted as the "printf" format. See
"sprintf" for an explanation of the format argument.
If "use locale" is in effect, the character used for
the decimal point in formatted real numbers is
affected by the LC_NUMERIC locale. See perllocale.
Don't fall into the trap of using a "printf" when a
simple "print" would do. The "print" is more effi-
cient and less error prone.
prototype FUNCTION
Returns the prototype of a function as a string (or
"undef" if the function has no prototype). FUNCTION
is a reference to, or the name of, the function
whose prototype you want to retrieve.
If FUNCTION is a string starting with "CORE::", the
rest is taken as a name for Perl builtin. If the
builtin is not overridable (such as "qw//") or its
arguments cannot be expressed by a prototype (such
as "system") returns "undef" because the builtin
does not really behave like a Perl function. Other-
wise, the string describing the equivalent prototype
is returned.
push ARRAY,LIST ,
Treats ARRAY as a stack, and pushes the values of
LIST onto the end of ARRAY. The length of ARRAY
increases by the length of LIST. Has the same
effect as
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
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but is more efficient. Returns the number of ele-
ments in the array following the completed "push".
q/STRING/
qq/STRING/
qr/STRING/
qx/STRING/
qw/STRING/
Generalized quotes. See "Regexp Quote-Like Opera-
tors" in perlop.
quotemeta EXPR
quotemeta
Returns the value of EXPR with all non-"word" char-
acters backslashed. (That is, all characters not
matching "/[A-Za-z_0-9]/" will be preceded by a
backslash in the returned string, regardless of any
locale settings.) This is the internal function
implementing the "\Q" escape in double-quoted
strings.
If EXPR is omitted, uses $_.
rand EXPR
rand Returns a random fractional number greater than or
equal to 0 and less than the value of EXPR. (EXPR
should be positive.) If EXPR is omitted, the value
1 is used. Currently EXPR with the value 0 is also
special-cased as 1 - this has not been documented
before perl 5.8.0 and is subject to change in future
versions of perl. Automatically calls "srand"
unless "srand" has already been called. See also
"srand".
Apply "int()" to the value returned by "rand()" if
you want random integers instead of random frac-
tional numbers. For example,
int(rand(10))
returns a random integer between 0 and 9, inclusive.
(Note: If your rand function consistently returns
numbers that are too large or too small, then your
version of Perl was probably compiled with the wrong
number of RANDBITS.)
read FILEHANDLE,SCALAR,LENGTH,OFFSET
read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH characters of data into
variable SCALAR from the specified FILEHANDLE.
Returns the number of characters actually read, 0 at
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end of file, or undef if there was an error (in the
latter case $! is also set). SCALAR will be grown
or shrunk so that the last character actually read
is the last character of the scalar after the read.
An OFFSET may be specified to place the read data at
some place in the string other than the beginning.
A negative OFFSET specifies placement at that many
characters counting backwards from the end of the
string. A positive OFFSET greater than the length
of SCALAR results in the string being padded to the
required size with "\0" bytes before the result of
the read is appended.
The call is actually implemented in terms of either
Perl's or system's fread() call. To get a true
read(2) system call, see "sysread".
Note the characters: depending on the status of the
filehandle, either (8-bit) bytes or characters are
read. By default all filehandles operate on bytes,
but for example if the filehandle has been opened
with the ":utf8" I/O layer (see "open", and the
"open" pragma, open), the I/O will operate on UTF-8
encoded Unicode characters, not bytes. Similarly
for the ":encoding" pragma: in that case pretty much
any characters can be read.
readdir DIRHANDLE
Returns the next directory entry for a directory
opened by "opendir". If used in list context,
returns all the rest of the entries in the direc-
tory. If there are no more entries, returns an
undefined value in scalar context or a null list in
list context.
If you're planning to filetest the return values out
of a "readdir", you'd better prepend the directory
in question. Otherwise, because we didn't "chdir"
there, it would have been testing the wrong file.
opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
@dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
closedir DIR;
readline EXPR
Reads from the filehandle whose typeglob is con-
tained in EXPR. In scalar context, each call reads
and returns the next line, until end-of-file is
reached, whereupon the subsequent call returns
undef. In list context, reads until end-of-file is
reached and returns a list of lines. Note that the
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notion of "line" used here is however you may have
defined it with $/ or $INPUT_RECORD_SEPARATOR). See
"$/" in perlvar.
When $/ is set to "undef", when readline() is in
scalar context (i.e. file slurp mode), and when an
empty file is read, it returns '' the first time,
followed by "undef" subsequently.
This is the internal function implementing the
"<EXPR>" operator, but you can use it directly. The
"<EXPR>" operator is discussed in more detail in
"I/O Operators" in perlop.
$line = <STDIN>;
$line = readline(*STDIN); # same thing
If readline encounters an operating system error, $!
will be set with the corresponding error message.
It can be helpful to check $! when you are reading
from filehandles you don't trust, such as a tty or a
socket. The following example uses the operator
form of "readline", and takes the necessary steps to
ensure that "readline" was successful.
for (;;) {
undef $!;
unless (defined( $line = <> )) {
die $! if $!;
last; # reached EOF
}
# ...
}
readlink EXPR
readlink
Returns the value of a symbolic link, if symbolic
links are implemented. If not, gives a fatal error.
If there is some system error, returns the undefined
value and sets $! (errno). If EXPR is omitted, uses
$_.
readpipe EXPR
EXPR is executed as a system command. The collected
standard output of the command is returned. In
scalar context, it comes back as a single (poten-
tially multi-line) string. In list context, returns
a list of lines (however you've defined lines with
$/ or $INPUT_RECORD_SEPARATOR). This is the internal
function implementing the "qx/EXPR/" operator, but
you can use it directly. The "qx/EXPR/" operator is
discussed in more detail in "I/O Operators" in
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perlop.
recv SOCKET,SCALAR,LENGTH,FLAGS
Receives a message on a socket. Attempts to receive
LENGTH characters of data into variable SCALAR from
the specified SOCKET filehandle. SCALAR will be
grown or shrunk to the length actually read. Takes
the same flags as the system call of the same name.
Returns the address of the sender if SOCKET's proto-
col supports this; returns an empty string other-
wise. If there's an error, returns the undefined
value. This call is actually implemented in terms of
recvfrom(2) system call. See "UDP: Message Passing"
in perlipc for examples.
Note the characters: depending on the status of the
socket, either (8-bit) bytes or characters are
received. By default all sockets operate on bytes,
but for example if the socket has been changed using
binmode() to operate with the ":utf8" I/O layer (see
the "open" pragma, open), the I/O will operate on
UTF-8 encoded Unicode characters, not bytes. Simi-
larly for the ":encoding" pragma: in that case
pretty much any characters can be read.
redo LABEL
redo The "redo" command restarts the loop block without
evaluating the conditional again. The "continue"
block, if any, is not executed. If the LABEL is
omitted, the command refers to the innermost enclos-
ing loop. Programs that want to lie to themselves
about what was just input normally use this command:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
LINE: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front\{|;
redo LINE;
}
}
}
print;
}
"redo" cannot be used to retry a block which returns
a value such as "eval {}", "sub {}" or "do {}", and
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should not be used to exit a grep() or map() opera-
tion.
Note that a block by itself is semantically identi-
cal to a loop that executes once. Thus "redo"
inside such a block will effectively turn it into a
looping construct.
See also "continue" for an illustration of how
"last", "next", and "redo" work.
ref EXPR
ref Returns a non-empty string if EXPR is a reference,
the empty string otherwise. If EXPR is not speci-
fied, $_ will be used. The value returned depends
on the type of thing the reference is a reference
to. Builtin types include:
SCALAR
ARRAY
HASH
CODE
REF
GLOB
LVALUE
If the referenced object has been blessed into a
package, then that package name is returned instead.
You can think of "ref" as a "typeof" operator.
if (ref($r) eq "HASH") {
print "r is a reference to a hash.\n";
}
unless (ref($r)) {
print "r is not a reference at all.\n";
}
See also perlref.
rename OLDNAME,NEWNAME
Changes the name of a file; an existing file NEWNAME
will be clobbered. Returns true for success, false
otherwise.
Behavior of this function varies wildly depending on
your system implementation. For example, it will
usually not work across file system boundaries, even
though the system mv command sometimes compensates
for this. Other restrictions include whether it
works on directories, open files, or pre-existing
files. Check perlport and either the rename(2) man-
page or equivalent system documentation for details.
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require VERSION
require EXPR
require Demands a version of Perl specified by VERSION, or
demands some semantics specified by EXPR or by $_ if
EXPR is not supplied.
VERSION may be either a numeric argument such as
5.006, which will be compared to $], or a literal of
the form v5.6.1, which will be compared to $^V (aka
$PERL_VERSION). A fatal error is produced at run
time if VERSION is greater than the version of the
current Perl interpreter. Compare with "use", which
can do a similar check at compile time.
Specifying VERSION as a literal of the form v5.6.1
should generally be avoided, because it leads to
misleading error messages under earlier versions of
Perl that do not support this syntax. The
equivalent numeric version should be used instead.
require v5.6.1; # run time version check
require 5.6.1; # ditto
require 5.006_001; # ditto; preferred for backwards compatibility
Otherwise, "require" demands that a library file be
included if it hasn't already been included. The
file is included via the do-FILE mechanism, which is
essentially just a variety of "eval". Has semantics
similar to the following subroutine:
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sub require {
my ($filename) = @_;
if (exists $INC{$filename}) {
return 1 if $INC{$filename};
die "Compilation failed in require";
}
my ($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$INC{$filename} = $realfilename;
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
if ($@) {
$INC{$filename} = undef;
die $@;
} elsif (!$result) {
delete $INC{$filename};
die "$filename did not return true value";
} else {
return $result;
}
}
Note that the file will not be included twice under
the same specified name.
The file must return true as the last statement to
indicate successful execution of any initialization
code, so it's customary to end such a file with "1;"
unless you're sure it'll return true otherwise. But
it's better just to put the "1;", in case you add
more statements.
If EXPR is a bareword, the require assumes a ".pm"
extension and replaces "::" with "/" in the filename
for you, to make it easy to load standard modules.
This form of loading of modules does not risk alter-
ing your namespace.
In other words, if you try this:
require Foo::Bar; # a splendid bareword
The require function will actually look for the
"Foo/Bar.pm" file in the directories specified in
the @INC array.
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But if you try this:
$class = 'Foo::Bar';
require $class; # $class is not a bareword
#or
require "Foo::Bar"; # not a bareword because of the ""
The require function will look for the "Foo::Bar"
file in the @INC array and will complain about not
finding "Foo::Bar" there. In this case you can do:
eval "require $class";
Now that you understand how "require" looks for
files in the case of a bareword argument, there is a
little extra functionality going on behind the
scenes. Before "require" looks for a ".pm" exten-
sion, it will first look for a filename with a
".pmc" extension. A file with this extension is
assumed to be Perl bytecode generated by
B::Bytecode. If this file is found, and its modifi-
cation time is newer than a coinciding ".pm" non-
compiled file, it will be loaded in place of that
non-compiled file ending in a ".pm" extension.
You can also insert hooks into the import facility,
by putting directly Perl code into the @INC array.
There are three forms of hooks: subroutine refer-
ences, array references and blessed objects.
Subroutine references are the simplest case. When
the inclusion system walks through @INC and
encounters a subroutine, this subroutine gets called
with two parameters, the first being a reference to
itself, and the second the name of the file to be
included (e.g. "Foo/Bar.pm"). The subroutine should
return "undef" or a filehandle, from which the file
to include will be read. If "undef" is returned,
"require" will look at the remaining elements of
@INC.
If the hook is an array reference, its first element
must be a subroutine reference. This subroutine is
called as above, but the first parameter is the
array reference. This enables to pass indirectly
some arguments to the subroutine.
In other words, you can write:
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push @INC, \&my_sub;
sub my_sub {
my ($coderef, $filename) = @_; # $coderef is \&my_sub
...
}
or:
push @INC, [ \&my_sub, $x, $y, ... ];
sub my_sub {
my ($arrayref, $filename) = @_;
# Retrieve $x, $y, ...
my @parameters = @$arrayref[1..$#$arrayref];
...
}
If the hook is an object, it must provide an INC
method that will be called as above, the first
parameter being the object itself. (Note that you
must fully qualify the sub's name, as it is always
forced into package "main".) Here is a typical code
layout:
# In Foo.pm
package Foo;
sub new { ... }
sub Foo::INC {
my ($self, $filename) = @_;
...
}
# In the main program
push @INC, new Foo(...);
Note that these hooks are also permitted to set the
%INC entry corresponding to the files they have
loaded. See "%INC" in perlvar.
For a yet-more-powerful import facility, see "use"
and perlmod.
reset EXPR
reset Generally used in a "continue" block at the end of a
loop to clear variables and reset "??" searches so
that they work again. The expression is interpreted
as a list of single characters (hyphens allowed for
ranges). All variables and arrays beginning with
one of those letters are reset to their pristine
state. If the expression is omitted, one-match
searches ("?pattern?") are reset to match again.
Resets only variables or searches in the current
package. Always returns 1. Examples:
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reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?one-time? searches
Resetting "A-Z" is not recommended because you'll
wipe out your @ARGV and @INC arrays and your %ENV
hash. Resets only package variables--lexical vari-
ables are unaffected, but they clean themselves up
on scope exit anyway, so you'll probably want to use
them instead. See "my".
return EXPR
return Returns from a subroutine, "eval", or "do FILE" with
the value given in EXPR. Evaluation of EXPR may be
in list, scalar, or void context, depending on how
the return value will be used, and the context may
vary from one execution to the next (see "wantar-
ray"). If no EXPR is given, returns an empty list
in list context, the undefined value in scalar con-
text, and (of course) nothing at all in a void con-
text.
(Note that in the absence of an explicit "return", a
subroutine, eval, or do FILE will automatically
return the value of the last expression evaluated.)
reverse LIST
In list context, returns a list value consisting of
the elements of LIST in the opposite order. In
scalar context, concatenates the elements of LIST
and returns a string value with all characters in
the opposite order.
print reverse <>; # line tac, last line first
undef $/; # for efficiency of <>
print scalar reverse <>; # character tac, last line tsrif
Used without arguments in scalar context, reverse()
reverses $_.
This operator is also handy for inverting a hash,
although there are some caveats. If a value is
duplicated in the original hash, only one of those
can be represented as a key in the inverted hash.
Also, this has to unwind one hash and build a whole
new one, which may take some time on a large hash,
such as from a DBM file.
%by_name = reverse %by_address; # Invert the hash
rewinddir DIRHANDLE
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Sets the current position to the beginning of the
directory for the "readdir" routine on DIRHANDLE.
rindex STR,SUBSTR,POSITION
rindex STR,SUBSTR
Works just like index() except that it returns the
position of the last occurrence of SUBSTR in STR.
If POSITION is specified, returns the last
occurrence beginning at or before that position.
rmdir FILENAME
rmdir Deletes the directory specified by FILENAME if that
directory is empty. If it succeeds it returns true,
otherwise it returns false and sets $! (errno). If
FILENAME is omitted, uses $_.
s/// The substitution operator. See perlop.
scalar EXPR
Forces EXPR to be interpreted in scalar context and
returns the value of EXPR.
@counts = ( scalar @a, scalar @b, scalar @c );
There is no equivalent operator to force an expres-
sion to be interpolated in list context because in
practice, this is never needed. If you really
wanted to do so, however, you could use the con-
struction "@{[ (some expression) ]}", but usually a
simple "(some expression)" suffices.
Because "scalar" is unary operator, if you acciden-
tally use for EXPR a parenthesized list, this
behaves as a scalar comma expression, evaluating all
but the last element in void context and returning
the final element evaluated in scalar context. This
is seldom what you want.
The following single statement:
print uc(scalar(&foo,$bar)),$baz;
is the moral equivalent of these two:
&foo;
print(uc($bar),$baz);
See perlop for more details on unary operators and
the comma operator.
seek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's position, just like the "fseek"
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call of "stdio". FILEHANDLE may be an expression
whose value gives the name of the filehandle. The
values for WHENCE are 0 to set the new position in
bytes to POSITION, 1 to set it to the current posi-
tion plus POSITION, and 2 to set it to EOF plus
POSITION (typically negative). For WHENCE you may
use the constants "SEEK_SET", "SEEK_CUR", and
"SEEK_END" (start of the file, current position, end
of the file) from the Fcntl module. Returns 1 upon
success, 0 otherwise.
Note the in bytes: even if the filehandle has been
set to operate on characters (for example by using
the ":utf8" open layer), tell() will return byte
offsets, not character offsets (because implementing
that would render seek() and tell() rather slow).
If you want to position file for "sysread" or
"syswrite", don't use "seek"--buffering makes its
effect on the file's system position unpredictable
and non-portable. Use "sysseek" instead.
Due to the rules and rigors of ANSI C, on some sys-
tems you have to do a seek whenever you switch
between reading and writing. Amongst other things,
this may have the effect of calling stdio's clear-
err(3). A WHENCE of 1 ("SEEK_CUR") is useful for not
moving the file position:
seek(TEST,0,1);
This is also useful for applications emulating "tail
-f". Once you hit EOF on your read, and then sleep
for a while, you might have to stick in a seek() to
reset things. The "seek" doesn't change the current
position, but it does clear the end-of-file condi-
tion on the handle, so that the next "<FILE>" makes
Perl try again to read something. We hope.
If that doesn't work (some IO implementations are
particularly cantankerous), then you may need some-
thing more like this:
for (;;) {
for ($curpos = tell(FILE); $_ = <FILE>;
$curpos = tell(FILE)) {
# search for some stuff and put it into files
}
sleep($for_a_while);
seek(FILE, $curpos, 0);
}
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seekdir DIRHANDLE,POS
Sets the current position for the "readdir" routine
on DIRHANDLE. POS must be a value returned by
"telldir". "seekdir" also has the same caveats
about possible directory compaction as the
corresponding system library routine.
select FILEHANDLE
select Returns the currently selected filehandle. Sets the
current default filehandle for output, if FILEHANDLE
is supplied. This has two effects: first, a "write"
or a "print" without a filehandle will default to
this FILEHANDLE. Second, references to variables
related to output will refer to this output channel.
For example, if you have to set the top of form for-
mat for more than one output channel, you might do
the following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives
the name of the actual filehandle. Thus:
$oldfh = select(STDERR); $| = 1; select($oldfh);
Some programmers may prefer to think of filehandles
as objects with methods, preferring to write the
last example as:
use IO::Handle;
STDERR->autoflush(1);
select RBITS,WBITS,EBITS,TIMEOUT
This calls the select(2) system call with the bit
masks specified, which can be constructed using
"fileno" and "vec", along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles you might
wish to write a subroutine:
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sub fhbits {
my(@fhlist) = split(' ',$_[0]);
my($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready just do
this
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
Most systems do not bother to return anything useful
in $timeleft, so calling select() in scalar context
just returns $nfound.
Any of the bit masks can also be undef. The
timeout, if specified, is in seconds, which may be
fractional. Note: not all implementations are capa-
ble of returning the $timeleft. If not, they always
return $timeleft equal to the supplied $timeout.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
Note that whether "select" gets restarted after sig-
nals (say, SIGALRM) is implementation-dependent.
See also perlport for notes on the portability of
"select".
On error, "select" behaves like the select(2) system
call : it returns -1 and sets $!.
Note: on some Unixes, the select(2) system call may
report a socket file descriptor as "ready for read-
ing", when actually no data is available, thus a
subsequent read blocks. It can be avoided using
always the O_NONBLOCK flag on the socket. See
select(2) and fcntl(2) for further details.
WARNING: One should not attempt to mix buffered I/O
(like "read" or <FH>) with "select", except as per-
mitted by POSIX, and even then only on POSIX
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systems. You have to use "sysread" instead.
semctl ID,SEMNUM,CMD,ARG
Calls the System V IPC function "semctl". You'll
probably have to say
use IPC::SysV;
first to get the correct constant definitions. If
CMD is IPC_STAT or GETALL, then ARG must be a vari-
able that will hold the returned semid_ds structure
or semaphore value array. Returns like "ioctl": the
undefined value for error, ""0 but true"" for zero,
or the actual return value otherwise. The ARG must
consist of a vector of native short integers, which
may be created with "pack("s!",(0)x$nsem)". See also
"SysV IPC" in perlipc, "IPC::SysV", "IPC::Semaphore"
documentation.
semget KEY,NSEMS,FLAGS
Calls the System V IPC function semget. Returns the
semaphore id, or the undefined value if there is an
error. See also "SysV IPC" in perlipc, "IPC::SysV",
"IPC::SysV::Semaphore" documentation.
semop KEY,OPSTRING
Calls the System V IPC function semop to perform
semaphore operations such as signalling and waiting.
OPSTRING must be a packed array of semop structures.
Each semop structure can be generated with
"pack("s!3", $semnum, $semop, $semflag)". The
length of OPSTRING implies the number of semaphore
operations. Returns true if successful, or false if
there is an error. As an example, the following
code waits on semaphore $semnum of semaphore id
$semid:
$semop = pack("s!3", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace "-1" with 1. See
also "SysV IPC" in perlipc, "IPC::SysV", and
"IPC::SysV::Semaphore" documentation.
send SOCKET,MSG,FLAGS,TO
send SOCKET,MSG,FLAGS
Sends a message on a socket. Attempts to send the
scalar MSG to the SOCKET filehandle. Takes the same
flags as the system call of the same name. On
unconnected sockets you must specify a destination
to send TO, in which case it does a C "sendto".
Returns the number of characters sent, or the
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undefined value if there is an error. The C system
call sendmsg(2) is currently unimplemented. See
"UDP: Message Passing" in perlipc for examples.
Note the characters: depending on the status of the
socket, either (8-bit) bytes or characters are sent.
By default all sockets operate on bytes, but for
example if the socket has been changed using bin-
mode() to operate with the ":utf8" I/O layer (see
"open", or the "open" pragma, open), the I/O will
operate on UTF-8 encoded Unicode characters, not
bytes. Similarly for the ":encoding" pragma: in
that case pretty much any characters can be sent.
setpgrp PID,PGRP
Sets the current process group for the specified
PID, 0 for the current process. Will produce a
fatal error if used on a machine that doesn't imple-
ment POSIX setpgid(2) or BSD setpgrp(2). If the
arguments are omitted, it defaults to "0,0". Note
that the BSD 4.2 version of "setpgrp" does not
accept any arguments, so only "setpgrp(0,0)" is
portable. See also "POSIX::setsid()".
setpriority WHICH,WHO,PRIORITY
Sets the current priority for a process, a process
group, or a user. (See setpriority(2).) Will pro-
duce a fatal error if used on a machine that doesn't
implement setpriority(2).
setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
Sets the socket option requested. Returns undefined
if there is an error. Use integer constants pro-
vided by the "Socket" module for LEVEL and OPNAME.
Values for LEVEL can also be obtained from getproto-
byname. OPTVAL might either be a packed string or
an integer. An integer OPTVAL is shorthand for
pack("i", OPTVAL).
An example disabling the Nagle's algorithm for a
socket:
use Socket qw(IPPROTO_TCP TCP_NODELAY);
setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
shift ARRAY
shift Shifts the first value of the array off and returns
it, shortening the array by 1 and moving everything
down. If there are no elements in the array,
returns the undefined value. If ARRAY is omitted,
shifts the @_ array within the lexical scope of sub-
routines and formats, and the @ARGV array at file
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scopes or within the lexical scopes established by
the "eval ''", "BEGIN {}", "INIT {}", "CHECK {}",
and "END {}" constructs.
See also "unshift", "push", and "pop". "shift" and
"unshift" do the same thing to the left end of an
array that "pop" and "push" do to the right end.
shmctl ID,CMD,ARG
Calls the System V IPC function shmctl. You'll
probably have to say
use IPC::SysV;
first to get the correct constant definitions. If
CMD is "IPC_STAT", then ARG must be a variable that
will hold the returned "shmid_ds" structure.
Returns like ioctl: the undefined value for error,
"0 but true" for zero, or the actual return value
otherwise. See also "SysV IPC" in perlipc and
"IPC::SysV" documentation.
shmget KEY,SIZE,FLAGS
Calls the System V IPC function shmget. Returns the
shared memory segment id, or the undefined value if
there is an error. See also "SysV IPC" in perlipc
and "IPC::SysV" documentation.
shmread ID,VAR,POS,SIZE
shmwrite ID,STRING,POS,SIZE
Reads or writes the System V shared memory segment
ID starting at position POS for size SIZE by attach-
ing to it, copying in/out, and detaching from it.
When reading, VAR must be a variable that will hold
the data read. When writing, if STRING is too long,
only SIZE bytes are used; if STRING is too short,
nulls are written to fill out SIZE bytes. Return
true if successful, or false if there is an error.
shmread() taints the variable. See also "SysV IPC"
in perlipc, "IPC::SysV" documentation, and the
"IPC::Shareable" module from CPAN.
shutdown SOCKET,HOW
Shuts down a socket connection in the manner indi-
cated by HOW, which has the same interpretation as
in the system call of the same name.
shutdown(SOCKET, 0); # I/we have stopped reading data
shutdown(SOCKET, 1); # I/we have stopped writing data
shutdown(SOCKET, 2); # I/we have stopped using this socket
This is useful with sockets when you want to tell
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the other side you're done writing but not done
reading, or vice versa. It's also a more insistent
form of close because it also disables the file
descriptor in any forked copies in other processes.
sin EXPR
sin Returns the sine of EXPR (expressed in radians). If
EXPR is omitted, returns sine of $_.
For the inverse sine operation, you may use the
"Math::Trig::asin" function, or use this relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
sleep EXPR
sleep Causes the script to sleep for EXPR seconds, or for-
ever if no EXPR. May be interrupted if the process
receives a signal such as "SIGALRM". Returns the
number of seconds actually slept. You probably can-
not mix "alarm" and "sleep" calls, because "sleep"
is often implemented using "alarm".
On some older systems, it may sleep up to a full
second less than what you requested, depending on
how it counts seconds. Most modern systems always
sleep the full amount. They may appear to sleep
longer than that, however, because your process
might not be scheduled right away in a busy multi-
tasking system.
For delays of finer granularity than one second, you
may use Perl's "syscall" interface to access setiti-
mer(2) if your system supports it, or else see
"select" above. The Time::HiRes module (from CPAN,
and starting from Perl 5.8 part of the standard dis-
tribution) may also help.
See also the POSIX module's "pause" function.
socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches it
to filehandle SOCKET. DOMAIN, TYPE, and PROTOCOL
are specified the same as for the system call of the
same name. You should "use Socket" first to get the
proper definitions imported. See the examples in
"Sockets: Client/Server Communication" in perlipc.
On systems that support a close-on-exec flag on
files, the flag will be set for the newly opened
file descriptor, as determined by the value of $^F.
See "$^F" in perlvar.
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socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
Creates an unnamed pair of sockets in the specified
domain, of the specified type. DOMAIN, TYPE, and
PROTOCOL are specified the same as for the system
call of the same name. If unimplemented, yields a
fatal error. Returns true if successful.
On systems that support a close-on-exec flag on
files, the flag will be set for the newly opened
file descriptors, as determined by the value of $^F.
See "$^F" in perlvar.
Some systems defined "pipe" in terms of "socket-
pair", in which a call to "pipe(Rdr, Wtr)" is essen-
tially:
use Socket;
socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown(Rdr, 1); # no more writing for reader
shutdown(Wtr, 0); # no more reading for writer
See perlipc for an example of socketpair use. Perl
5.8 and later will emulate socketpair using IP sock-
ets to localhost if your system implements sockets
but not socketpair.
sort SUBNAME LIST
sort BLOCK LIST
sort LIST
In list context, this sorts the LIST and returns the
sorted list value. In scalar context, the behaviour
of "sort()" is undefined.
If SUBNAME or BLOCK is omitted, "sort"s in standard
string comparison order. If SUBNAME is specified,
it gives the name of a subroutine that returns an
integer less than, equal to, or greater than 0,
depending on how the elements of the list are to be
ordered. (The "<=>" and "cmp" operators are
extremely useful in such routines.) SUBNAME may be a
scalar variable name (unsubscripted), in which case
the value provides the name of (or a reference to)
the actual subroutine to use. In place of a SUB-
NAME, you can provide a BLOCK as an anonymous, in-
line sort subroutine.
If the subroutine's prototype is "($$)", the ele-
ments to be compared are passed by reference in @_,
as for a normal subroutine. This is slower than
unprototyped subroutines, where the elements to be
compared are passed into the subroutine as the pack-
age global variables $a and $b (see example below).
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Note that in the latter case, it is usually
counter-productive to declare $a and $b as lexicals.
In either case, the subroutine may not be recursive.
The values to be compared are always passed by
reference and should not be modified.
You also cannot exit out of the sort block or sub-
routine using any of the loop control operators
described in perlsyn or with "goto".
When "use locale" is in effect, "sort LIST" sorts
LIST according to the current collation locale. See
perllocale.
sort() returns aliases into the original list, much
as a for loop's index variable aliases the list ele-
ments. That is, modifying an element of a list
returned by sort() (for example, in a "foreach",
"map" or "grep") actually modifies the element in
the original list. This is usually something to be
avoided when writing clear code.
Perl 5.6 and earlier used a quicksort algorithm to
implement sort. That algorithm was not stable, and
could go quadratic. (A stable sort preserves the
input order of elements that compare equal.
Although quicksort's run time is O(NlogN) when aver-
aged over all arrays of length N, the time can be
O(N**2), quadratic behavior, for some inputs.) In
5.7, the quicksort implementation was replaced with
a stable mergesort algorithm whose worst-case
behavior is O(NlogN). But benchmarks indicated that
for some inputs, on some platforms, the original
quicksort was faster. 5.8 has a sort pragma for
limited control of the sort. Its rather blunt con-
trol of the underlying algorithm may not persist
into future Perls, but the ability to characterize
the input or output in implementation independent
ways quite probably will. See sort.
Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# now case-insensitively
@articles = sort {uc($a) cmp uc($b)} @files;
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# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# this sorts the %age hash by value instead of key
# using an in-line function
@eldest = sort { $age{$b} <=> $age{$a} } keys %age;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
sub backwards { $b cmp $a }
@harry = qw(dog cat x Cain Abel);
@george = qw(gone chased yz Punished Axed);
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise
@new = sort {
($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
||
uc($a) cmp uc($b)
} @old;
# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
@nums = @caps = ();
for (@old) {
push @nums, /=(\d+)/;
push @caps, uc($_);
}
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@new = @old[ sort {
$nums[$b] <=> $nums[$a]
||
$caps[$a] cmp $caps[$b]
} 0..$#old
];
# same thing, but without any temps
@new = map { $_->[0] }
sort { $b->[1] <=> $a->[1]
||
$a->[2] cmp $b->[2]
} map { [$_, /=(\d+)/, uc($_)] } @old;
# using a prototype allows you to use any comparison subroutine
# as a sort subroutine (including other package's subroutines)
package other;
sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
package main;
@new = sort other::backwards @old;
# guarantee stability, regardless of algorithm
use sort 'stable';
@new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
# force use of mergesort (not portable outside Perl 5.8)
use sort '_mergesort'; # note discouraging _
@new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
If you're using strict, you must not declare $a and
$b as lexicals. They are package globals. That
means if you're in the "main" package and type
@articles = sort {$b <=> $a} @files;
then $a and $b are $main::a and $main::b (or $::a
and $::b), but if you're in the "FooPack" package,
it's the same as typing
@articles = sort {$FooPack::b <=> $FooPack::a} @files;
The comparison function is required to behave. If
it returns inconsistent results (sometimes saying
$x[1] is less than $x[2] and sometimes saying the
opposite, for example) the results are not
well-defined.
Because "<=>" returns "undef" when either operand is
"NaN" (not-a-number), and because "sort" will
trigger a fatal error unless the result of a com-
parison is defined, when sorting with a comparison
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function like "$a <=> $b", be careful about lists
that might contain a "NaN". The following example
takes advantage of the fact that "NaN != NaN" to
eliminate any "NaN"s from the input.
@result = sort { $a <=> $b } grep { $_ == $_ } @input;
splice ARRAY,OFFSET,LENGTH,LIST
splice ARRAY,OFFSET,LENGTH
splice ARRAY,OFFSET
splice ARRAY
Removes the elements designated by OFFSET and LENGTH
from an array, and replaces them with the elements
of LIST, if any. In list context, returns the ele-
ments removed from the array. In scalar context,
returns the last element removed, or "undef" if no
elements are removed. The array grows or shrinks as
necessary. If OFFSET is negative then it starts that
far from the end of the array. If LENGTH is omitted,
removes everything from OFFSET onward. If LENGTH is
negative, removes the elements from OFFSET onward
except for -LENGTH elements at the end of the array.
If both OFFSET and LENGTH are omitted, removes
everything. If OFFSET is past the end of the array,
perl issues a warning, and splices at the end of the
array.
The following equivalences hold (assuming "$[ == 0
and $#a >= $i" )
push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$i] = $y splice(@a,$i,1,$y)
Example, assuming array lengths are passed before
arrays:
sub aeq { # compare two list values
my(@a) = splice(@_,0,shift);
my(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
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split /PATTERN/
split Splits the string EXPR into a list of strings and
returns that list. By default, empty leading fields
are preserved, and empty trailing ones are deleted.
(If all fields are empty, they are considered to be
trailing.)
In scalar context, returns the number of fields
found and splits into the @_ array. Use of split in
scalar context is deprecated, however, because it
clobbers your subroutine arguments.
If EXPR is omitted, splits the $_ string. If PAT-
TERN is also omitted, splits on whitespace (after
skipping any leading whitespace). Anything matching
PATTERN is taken to be a delimiter separating the
fields. (Note that the delimiter may be longer than
one character.)
If LIMIT is specified and positive, it represents
the maximum number of fields the EXPR will be split
into, though the actual number of fields returned
depends on the number of times PATTERN matches
within EXPR. If LIMIT is unspecified or zero,
trailing null fields are stripped (which potential
users of "pop" would do well to remember). If LIMIT
is negative, it is treated as if an arbitrarily
large LIMIT had been specified. Note that splitting
an EXPR that evaluates to the empty string always
returns the empty list, regardless of the LIMIT
specified.
A pattern matching the null string (not to be con-
fused with a null pattern "//", which is just one
member of the set of patterns matching a null
string) will split the value of EXPR into separate
characters at each point it matches that way. For
example:
print join(':', split(/ */, 'hi there'));
produces the output 'h:i:t:h:e:r:e'.
As a special case for "split", using the empty pat-
tern "//" specifically matches only the null string,
and is not be confused with the regular use of "//"
to mean "the last successful pattern match". So,
for "split", the following:
print join(':', split(//, 'hi there'));
produces the output 'h:i: :t:h:e:r:e'.
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Empty leading (or trailing) fields are produced when
there are positive width matches at the beginning
(or end) of the string; a zero-width match at the
beginning (or end) of the string does not produce an
empty field. For example:
print join(':', split(/(?=\w)/, 'hi there!'));
produces the output 'h:i :t:h:e:r:e!'.
The LIMIT parameter can be used to split a line par-
tially
($login, $passwd, $remainder) = split(/:/, $_, 3);
When assigning to a list, if LIMIT is omitted, or
zero, Perl supplies a LIMIT one larger than the
number of variables in the list, to avoid unneces-
sary work. For the list above LIMIT would have been
4 by default. In time critical applications it
behooves you not to split into more fields than you
really need.
If the PATTERN contains parentheses, additional list
elements are created from each matching substring in
the delimiter.
split(/([,-])/, "1-10,20", 3);
produces the list value
(1, '-', 10, ',', 20)
If you had the entire header of a normal Unix email
message in $header, you could split it up into
fields and their values this way:
$header =~ s/\n\s+/ /g; # fix continuation lines
%hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
The pattern "/PATTERN/" may be replaced with an
expression to specify patterns that vary at runtime.
(To do runtime compilation only once, use
"/$variable/o".)
As a special case, specifying a PATTERN of space
(' ') will split on white space just as "split" with
no arguments does. Thus, "split(' ')" can be used
to emulate awk's default behavior, whereas
"split(/ /)" will give you as many null initial
fields as there are leading spaces. A "split" on
"/\s+/" is like a "split(' ')" except that any
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leading whitespace produces a null first field. A
"split" with no arguments really does a
"split(' ', $_)" internally.
A PATTERN of "/^/" is treated as if it were "/^/m",
since it isn't much use otherwise.
Example:
open(PASSWD, '/etc/passwd');
while (<PASSWD>) {
chomp;
($login, $passwd, $uid, $gid,
$gcos, $home, $shell) = split(/:/);
#...
}
As with regular pattern matching, any capturing
parentheses that are not matched in a "split()" will
be set to "undef" when returned:
@fields = split /(A)|B/, "1A2B3";
# @fields is (1, 'A', 2, undef, 3)
sprintf FORMAT, LIST
Returns a string formatted by the usual "printf"
conventions of the C library function "sprintf".
See below for more details and see sprintf(3) or
printf(3) on your system for an explanation of the
general principles.
For example:
# Format number with up to 8 leading zeroes
$result = sprintf("%08d", $number);
# Round number to 3 digits after decimal point
$rounded = sprintf("%.3f", $number);
Perl does its own "sprintf" formatting--it emulates
the C function "sprintf", but it doesn't use it
(except for floating-point numbers, and even then
only the standard modifiers are allowed). As a
result, any non-standard extensions in your local
"sprintf" are not available from Perl.
Unlike "printf", "sprintf" does not do what you
probably mean when you pass it an array as your
first argument. The array is given scalar context,
and instead of using the 0th element of the array as
the format, Perl will use the count of elements in
the array as the format, which is almost never
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useful.
Perl's "sprintf" permits the following universally-
known conversions:
%% a percent sign
%c a character with the given number
%s a string
%d a signed integer, in decimal
%u an unsigned integer, in decimal
%o an unsigned integer, in octal
%x an unsigned integer, in hexadecimal
%e a floating-point number, in scientific notation
%f a floating-point number, in fixed decimal notation
%g a floating-point number, in %e or %f notation
In addition, Perl permits the following widely-
supported conversions:
%X like %x, but using upper-case letters
%E like %e, but using an upper-case "E"
%G like %g, but with an upper-case "E" (if applicable)
%b an unsigned integer, in binary
%p a pointer (outputs the Perl value's address in hexadecimal)
%n special: *stores* the number of characters output so far
into the next variable in the parameter list
Finally, for backward (and we do mean "backward")
compatibility, Perl permits these unnecessary but
widely-supported conversions:
%i a synonym for %d
%D a synonym for %ld
%U a synonym for %lu
%O a synonym for %lo
%F a synonym for %f
Note that the number of exponent digits in the
scientific notation produced by %e, %E, %g and %G
for numbers with the modulus of the exponent less
than 100 is system-dependent: it may be three or
less (zero-padded as necessary). In other words,
1.23 times ten to the 99th may be either "1.23e99"
or "1.23e099".
Between the "%" and the format letter, you may
specify a number of additional attributes control-
ling the interpretation of the format. In order,
these are:
format parameter index
An explicit format parameter index, such as
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"2$". By default sprintf will format the next
unused argument in the list, but this allows you
to take the arguments out of order, e.g.:
printf '%2$d %1$d', 12, 34; # prints "34 12"
printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
flags
one or more of:
space prefix positive number with a space
+ prefix positive number with a plus
sign
- left-justify within the field
0 use zeros, not spaces, to right-
justify
# prefix non-zero octal with "0", non-
zero hex with "0x",
non-zero binary with "0b"
For example:
printf '<% d>', 12; # prints "< 12>"
printf '<%+d>', 12; # prints "<+12>"
printf '<%6s>', 12; # prints "< 12>"
printf '<%-6s>', 12; # prints "<12 >"
printf '<%06s>', 12; # prints "<000012>"
printf '<%#x>', 12; # prints "<0xc>"
vector flag
This flag tells perl to interpret the supplied
string as a vector of integers, one for each
character in the string. Perl applies the format
to each integer in turn, then joins the result-
ing strings with a separator (a dot "." by
default). This can be useful for displaying
ordinal values of characters in arbitrary
strings:
printf "%vd", "AB\x{100}"; # prints "65.66.256"
printf "version is v%vd\n", $^V; # Perl's version
Put an asterisk "*" before the "v" to override
the string to use to separate the numbers:
printf "address is %*vX\n", ":", $addr; # IPv6 address
printf "bits are %0*v8b\n", " ", $bits; # random bitstring
You can also explicitly specify the argument
number to use for the join string using e.g.
"*2$v":
printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
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(minimum) width
Arguments are usually formatted to be only as
wide as required to display the given value. You
can override the width by putting a number here,
or get the width from the next argument (with
"*") or from a specified argument (with e.g.
"*2$"):
printf '<%s>', "a"; # prints "<a>"
printf '<%6s>', "a"; # prints "< a>"
printf '<%*s>', 6, "a"; # prints "< a>"
printf '<%*2$s>', "a", 6; # prints "< a>"
printf '<%2s>', "long"; # prints "<long>" (does not truncate)
If a field width obtained through "*" is nega-
tive, it has the same effect as the "-" flag:
left-justification.
precision, or maximum width
You can specify a precision (for numeric conver-
sions) or a maximum width (for string conver-
sions) by specifying a "." followed by a number.
For floating point formats, with the exception
of 'g' and 'G', this specifies the number of
decimal places to show (the default being 6),
e.g.:
# these examples are subject to system-specific variation
printf '<%f>', 1; # prints "<1.000000>"
printf '<%.1f>', 1; # prints "<1.0>"
printf '<%.0f>', 1; # prints "<1>"
printf '<%e>', 10; # prints "<1.000000e+01>"
printf '<%.1e>', 10; # prints "<1.0e+01>"
For 'g' and 'G', this specifies the maximum
number of digits to show, including prior to the
decimal point as well as after it, e.g.:
# these examples are subject to system-specific variation
printf '<%g>', 1; # prints "<1>"
printf '<%.10g>', 1; # prints "<1>"
printf '<%g>', 100; # prints "<100>"
printf '<%.1g>', 100; # prints "<1e+02>"
printf '<%.2g>', 100.01; # prints "<1e+02>"
printf '<%.5g>', 100.01; # prints "<100.01>"
printf '<%.4g>', 100.01; # prints "<100>"
For integer conversions, specifying a precision
implies that the output of the number itself
should be zero-padded to this width:
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printf '<%.6x>', 1; # prints "<000001>"
printf '<%#.6x>', 1; # prints "<0x000001>"
printf '<%-10.6x>', 1; # prints "<000001 >"
For string conversions, specifying a precision
truncates the string to fit in the specified
width:
printf '<%.5s>', "truncated"; # prints "<trunc>"
printf '<%10.5s>', "truncated"; # prints "< trunc>"
You can also get the precision from the next
argument using ".*":
printf '<%.6x>', 1; # prints "<000001>"
printf '<%.*x>', 6, 1; # prints "<000001>"
You cannot currently get the precision from a
specified number, but it is intended that this
will be possible in the future using e.g.
".*2$":
printf '<%.*2$x>', 1, 6; # INVALID, but in future will print "<000001>"
size
For numeric conversions, you can specify the
size to interpret the number as using "l", "h",
"V", "q", "L", or "ll". For integer conversions
("d u o x X b i D U O"), numbers are usually
assumed to be whatever the default integer size
is on your platform (usually 32 or 64 bits), but
you can override this to use instead one of the
standard C types, as supported by the compiler
used to build Perl:
l interpret integer as C type "long" or "unsigned long"
h interpret integer as C type "short" or "unsigned short"
q, L or ll interpret integer as C type "long long", "unsigned long long".
or "quads" (typically 64-bit integers)
The last will produce errors if Perl does not
understand "quads" in your installation. (This
requires that either the platform natively sup-
ports quads or Perl was specifically compiled to
support quads.) You can find out whether your
Perl supports quads via Config:
use Config;
($Config{use64bitint} eq 'define' || $Config{longsize} >= 8) &&
print "quads\n";
For floating point conversions ("e f g E F G"),
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numbers are usually assumed to be the default
floating point size on your platform (double or
long double), but you can force 'long double'
with "q", "L", or "ll" if your platform supports
them. You can find out whether your Perl sup-
ports long doubles via Config:
use Config;
$Config{d_longdbl} eq 'define' && print "long doubles\n";
You can find out whether Perl considers 'long
double' to be the default floating point size to
use on your platform via Config:
use Config;
($Config{uselongdouble} eq 'define') &&
print "long doubles by default\n";
It can also be the case that long doubles and
doubles are the same thing:
use Config;
($Config{doublesize} == $Config{longdblsize}) &&
print "doubles are long doubles\n";
The size specifier "V" has no effect for Perl
code, but it is supported for compatibility with
XS code; it means 'use the standard size for a
Perl integer (or floating-point number)', which
is already the default for Perl code.
order of arguments
Normally, sprintf takes the next unused argument
as the value to format for each format specifi-
cation. If the format specification uses "*" to
require additional arguments, these are consumed
from the argument list in the order in which
they appear in the format specification before
the value to format. Where an argument is speci-
fied using an explicit index, this does not
affect the normal order for the arguments (even
when the explicitly specified index would have
been the next argument in any case).
So:
printf '<%*.*s>', $a, $b, $c;
would use $a for the width, $b for the precision
and $c as the value to format, while:
print '<%*1$.*s>', $a, $b;
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would use $a for the width and the precision,
and $b as the value to format.
Here are some more examples - beware that when
using an explicit index, the "$" may need to be
escaped:
printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
If "use locale" is in effect, the character used for
the decimal point in formatted real numbers is
affected by the LC_NUMERIC locale. See perllocale.
sqrt EXPR
sqrt Return the square root of EXPR. If EXPR is omitted,
returns square root of $_. Only works on non-
negative operands, unless you've loaded the standard
Math::Complex module.
use Math::Complex;
print sqrt(-2); # prints 1.4142135623731i
srand EXPR
srand Sets the random number seed for the "rand" operator.
The point of the function is to "seed" the "rand"
function so that "rand" can produce a different
sequence each time you run your program.
If srand() is not called explicitly, it is called
implicitly at the first use of the "rand" operator.
However, this was not the case in versions of Perl
before 5.004, so if your script will run under older
Perl versions, it should call "srand".
Most programs won't even call srand() at all, except
those that need a cryptographically-strong starting
point rather than the generally acceptable default,
which is based on time of day, process ID, and
memory allocation, or the /dev/urandom device, if
available.
You can call srand($seed) with the same $seed to
reproduce the same sequence from rand(), but this is
usually reserved for generating predictable results
for testing or debugging. Otherwise, don't call
srand() more than once in your program.
Do not call srand() (i.e. without an argument) more
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than once in a script. The internal state of the
random number generator should contain more entropy
than can be provided by any seed, so calling srand()
again actually loses randomness.
Most implementations of "srand" take an integer and
will silently truncate decimal numbers. This means
"srand(42)" will usually produce the same results as
"srand(42.1)". To be safe, always pass "srand" an
integer.
In versions of Perl prior to 5.004 the default seed
was just the current "time". This isn't a particu-
larly good seed, so many old programs supply their
own seed value (often "time ^ $$" or "time ^ ($$ +
($$ << 15))"), but that isn't necessary any more.
For cryptographic purposes, however, you need some-
thing much more random than the default seed.
Checksumming the compressed output of one or more
rapidly changing operating system status programs is
the usual method. For example:
srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
If you're particularly concerned with this, see the
"Math::TrulyRandom" module in CPAN.
Frequently called programs (like CGI scripts) that
simply use
time ^ $$
for a seed can fall prey to the mathematical pro-
perty that
a^b == (a+1)^(b+1)
one-third of the time. So don't do that.
stat FILEHANDLE
stat EXPR
stat Returns a 13-element list giving the status info for
a file, either the file opened via FILEHANDLE, or
named by EXPR. If EXPR is omitted, it stats $_.
Returns a null list if the stat fails. Typically
used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
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Not all fields are supported on all filesystem
types. Here are the meanings of the fields:
0 dev device number of filesystem
1 ino inode number
2 mode file mode (type and permissions)
3 nlink number of (hard) links to the file
4 uid numeric user ID of file's owner
5 gid numeric group ID of file's owner
6 rdev the device identifier (special files only)
7 size total size of file, in bytes
8 atime last access time in seconds since the epoch
9 mtime last modify time in seconds since the epoch
10 ctime inode change time in seconds since the epoch (*)
11 blksize preferred block size for file system I/O
12 blocks actual number of blocks allocated
(The epoch was at 00:00 January 1, 1970 GMT.)
(*) Not all fields are supported on all filesystem
types. Notably, the ctime field is non-portable. In
particular, you cannot expect it to be a "creation
time", see "Files and Filesystems" in perlport for
details.
If "stat" is passed the special filehandle consist-
ing of an underline, no stat is done, but the
current contents of the stat structure from the last
"stat", "lstat", or filetest are returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This works on machines only for which the device
number is negative under NFS.)
Because the mode contains both the file type and its
permissions, you should mask off the file type por-
tion and (s)printf using a "%o" if you want to see
the real permissions.
$mode = (stat($filename))[2];
printf "Permissions are %04o\n", $mode & 07777;
In scalar context, "stat" returns a boolean value
indicating success or failure, and, if successful,
sets the information associated with the special
filehandle "_".
The File::stat module provides a convenient, by-name
access mechanism:
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use File::stat;
$sb = stat($filename);
printf "File is %s, size is %s, perm %04o, mtime %s\n",
$filename, $sb->size, $sb->mode & 07777,
scalar localtime $sb->mtime;
You can import symbolic mode constants ("S_IF*") and
functions ("S_IS*") from the Fcntl module:
use Fcntl ':mode';
$mode = (stat($filename))[2];
$user_rwx = ($mode & S_IRWXU) >> 6;
$group_read = ($mode & S_IRGRP) >> 3;
$other_execute = $mode & S_IXOTH;
printf "Permissions are %04o\n", S_IMODE($mode), "\n";
$is_setuid = $mode & S_ISUID;
$is_setgid = S_ISDIR($mode);
You could write the last two using the "-u" and "-d"
operators. The commonly available "S_IF*" constants
are
# Permissions: read, write, execute, for user, group, others.
S_IRWXU S_IRUSR S_IWUSR S_IXUSR
S_IRWXG S_IRGRP S_IWGRP S_IXGRP
S_IRWXO S_IROTH S_IWOTH S_IXOTH
# Setuid/Setgid/Stickiness/SaveText.
# Note that the exact meaning of these is system dependent.
S_ISUID S_ISGID S_ISVTX S_ISTXT
# File types. Not necessarily all are available on your system.
S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
# The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
S_IREAD S_IWRITE S_IEXEC
and the "S_IF*" functions are
S_IMODE($mode) the part of $mode containing the permission bits
and the setuid/setgid/sticky bits
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S_IFMT($mode) the part of $mode containing the file type
which can be bit-anded with e.g. S_IFREG
or with the following functions
# The operators -f, -d, -l, -b, -c, -p, and -S.
S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
# No direct -X operator counterpart, but for the first one
# the -g operator is often equivalent. The ENFMT stands for
# record flocking enforcement, a platform-dependent feature.
S_ISENFMT($mode) S_ISWHT($mode)
See your native chmod(2) and stat(2) documentation
for more details about the "S_*" constants. To get
status info for a symbolic link instead of the tar-
get file behind the link, use the "lstat" function.
study SCALAR
study Takes extra time to study SCALAR ($_ if unspecified)
in anticipation of doing many pattern matches on the
string before it is next modified. This may or may
not save time, depending on the nature and number of
patterns you are searching on, and on the distribu-
tion of character frequencies in the string to be
searched--you probably want to compare run times
with and without it to see which runs faster. Those
loops that scan for many short constant strings
(including the constant parts of more complex pat-
terns) will benefit most. You may have only one
"study" active at a time--if you study a different
scalar the first is "unstudied". (The way "study"
works is this: a linked list of every character in
the string to be searched is made, so we know, for
example, where all the 'k' characters are. From
each search string, the rarest character is
selected, based on some static frequency tables con-
structed from some C programs and English text.
Only those places that contain this "rarest" charac-
ter are examined.)
For example, here is a loop that inserts index pro-
ducing entries before any line containing a certain
pattern:
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while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
# ...
print;
}
In searching for "/\bfoo\b/", only those locations
in $_ that contain "f" will be looked at, because
"f" is rarer than "o". In general, this is a big
win except in pathological cases. The only question
is whether it saves you more time than it took to
build the linked list in the first place.
Note that if you have to look for strings that you
don't know till runtime, you can build an entire
loop as a string and "eval" that to avoid recompil-
ing all your patterns all the time. Together with
undefining $/ to input entire files as one record,
this can be very fast, often faster than specialized
programs like fgrep(1). The following scans a list
of files (@files) for a list of words (@words), and
prints out the names of those files that contain a
match:
$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delimiter
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
sub NAME BLOCK
sub NAME (PROTO) BLOCK
sub NAME : ATTRS BLOCK
sub NAME (PROTO) : ATTRS BLOCK
This is subroutine definition, not a real function
per se. Without a BLOCK it's just a forward declara-
tion. Without a NAME, it's an anonymous function
declaration, and does actually return a value: the
CODE ref of the closure you just created.
See perlsub and perlref for details about subrou-
tines and references, and attributes and
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Attribute::Handlers for more information about
attributes.
substr EXPR,OFFSET,LENGTH,REPLACEMENT
substr EXPR,OFFSET,LENGTH
substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it.
First character is at offset 0, or whatever you've
set $[ to (but don't do that). If OFFSET is negative
(or more precisely, less than $[), starts that far
from the end of the string. If LENGTH is omitted,
returns everything to the end of the string. If
LENGTH is negative, leaves that many characters off
the end of the string.
You can use the substr() function as an lvalue, in
which case EXPR must itself be an lvalue. If you
assign something shorter than LENGTH, the string
will shrink, and if you assign something longer than
LENGTH, the string will grow to accommodate it. To
keep the string the same length you may need to pad
or chop your value using "sprintf".
If OFFSET and LENGTH specify a substring that is
partly outside the string, only the part within the
string is returned. If the substring is beyond
either end of the string, substr() returns the unde-
fined value and produces a warning. When used as an
lvalue, specifying a substring that is entirely out-
side the string is a fatal error. Here's an example
showing the behavior for boundary cases:
my $name = 'fred';
substr($name, 4) = 'dy'; # $name is now 'freddy'
my $null = substr $name, 6, 2; # returns '' (no warning)
my $oops = substr $name, 7; # returns undef, with warning
substr($name, 7) = 'gap'; # fatal error
An alternative to using substr() as an lvalue is to
specify the replacement string as the 4th argument.
This allows you to replace parts of the EXPR and
return what was there before in one operation, just
as you can with splice().
symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the
old filename. Returns 1 for success, 0 otherwise.
On systems that don't support symbolic links, pro-
duces a fatal error at run time. To check for that,
use eval:
$symlink_exists = eval { symlink("",""); 1 };
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syscall NUMBER, LIST
Calls the system call specified as the first element
of the list, passing the remaining elements as argu-
ments to the system call. If unimplemented, pro-
duces a fatal error. The arguments are interpreted
as follows: if a given argument is numeric, the
argument is passed as an int. If not, the pointer
to the string value is passed. You are responsible
to make sure a string is pre-extended long enough to
receive any result that might be written into a
string. You can't use a string literal (or other
read-only string) as an argument to "syscall"
because Perl has to assume that any string pointer
might be written through. If your integer arguments
are not literals and have never been interpreted in
a numeric context, you may need to add 0 to them to
force them to look like numbers. This emulates the
"syswrite" function (or vice versa):
require 'syscall.ph'; # may need to run h2ph
$s = "hi there\n";
syscall(&SYS_write, fileno(STDOUT), $s, length $s);
Note that Perl supports passing of up to only 14
arguments to your system call, which in practice
should usually suffice.
Syscall returns whatever value returned by the sys-
tem call it calls. If the system call fails, "sys-
call" returns "-1" and sets $! (errno). Note that
some system calls can legitimately return "-1". The
proper way to handle such calls is to assign "$!=0;"
before the call and check the value of $! if syscall
returns "-1".
There's a problem with "syscall(&SYS_pipe)": it
returns the file number of the read end of the pipe
it creates. There is no way to retrieve the file
number of the other end. You can avoid this problem
by using "pipe" instead.
sysopen FILEHANDLE,FILENAME,MODE
sysopen FILEHANDLE,FILENAME,MODE,PERMS
Opens the file whose filename is given by FILENAME,
and associates it with FILEHANDLE. If FILEHANDLE is
an expression, its value is used as the name of the
real filehandle wanted. This function calls the
underlying operating system's "open" function with
the parameters FILENAME, MODE, PERMS.
The possible values and flag bits of the MODE param-
eter are system-dependent; they are available via
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the standard module "Fcntl". See the documentation
of your operating system's "open" to see which
values and flag bits are available. You may combine
several flags using the "|"-operator.
Some of the most common values are "O_RDONLY" for
opening the file in read-only mode, "O_WRONLY" for
opening the file in write-only mode, and "O_RDWR"
for opening the file in read-write mode.
For historical reasons, some values work on almost
every system supported by perl: zero means
read-only, one means write-only, and two means
read/write. We know that these values do not work
under OS/390 & VM/ESA Unix and on the Macintosh; you
probably don't want to use them in new code.
If the file named by FILENAME does not exist and the
"open" call creates it (typically because MODE
includes the "O_CREAT" flag), then the value of
PERMS specifies the permissions of the newly created
file. If you omit the PERMS argument to "sysopen",
Perl uses the octal value 0666. These permission
values need to be in octal, and are modified by your
process's current "umask".
In many systems the "O_EXCL" flag is available for
opening files in exclusive mode. This is not lock-
ing: exclusiveness means here that if the file
already exists, sysopen() fails. "O_EXCL" may not
work on network filesystems, and has no effect
unless the "O_CREAT" flag is set as well. Setting
"O_CREAT|O_EXCL" prevents the file from being opened
if it is a symbolic link. It does not protect
against symbolic links in the file's path.
Sometimes you may want to truncate an already-
existing file. This can be done using the "O_TRUNC"
flag. The behavior of "O_TRUNC" with "O_RDONLY" is
undefined.
You should seldom if ever use 0644 as argument to
"sysopen", because that takes away the user's option
to have a more permissive umask. Better to omit it.
See the perlfunc(1) entry on "umask" for more on
this.
Note that "sysopen" depends on the fdopen() C
library function. On many UNIX systems, fdopen() is
known to fail when file descriptors exceed a certain
value, typically 255. If you need more file descrip-
tors than that, consider rebuilding Perl to use the
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"sfio" library, or perhaps using the POSIX::open()
function.
See perlopentut for a kinder, gentler explanation of
opening files.
sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
sysread FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable
SCALAR from the specified FILEHANDLE, using the sys-
tem call read(2). It bypasses buffered IO, so mix-
ing this with other kinds of reads, "print",
"write", "seek", "tell", or "eof" can cause confu-
sion because the perlio or stdio layers usually
buffers data. Returns the number of bytes actually
read, 0 at end of file, or undef if there was an
error (in the latter case $! is also set). SCALAR
will be grown or shrunk so that the last byte actu-
ally read is the last byte of the scalar after the
read.
An OFFSET may be specified to place the read data at
some place in the string other than the beginning.
A negative OFFSET specifies placement at that many
characters counting backwards from the end of the
string. A positive OFFSET greater than the length
of SCALAR results in the string being padded to the
required size with "\0" bytes before the result of
the read is appended.
There is no syseof() function, which is ok, since
eof() doesn't work very well on device files (like
ttys) anyway. Use sysread() and check for a return
value for 0 to decide whether you're done.
Note that if the filehandle has been marked as
":utf8" Unicode characters are read instead of bytes
(the LENGTH, OFFSET, and the return value of sys-
read() are in Unicode characters). The ":encod-
ing(...)" layer implicitly introduces the ":utf8"
layer. See "binmode", "open", and the "open" pragma,
open.
sysseek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's system position in bytes using the
system call lseek(2). FILEHANDLE may be an expres-
sion whose value gives the name of the filehandle.
The values for WHENCE are 0 to set the new position
to POSITION, 1 to set the it to the current position
plus POSITION, and 2 to set it to EOF plus POSITION
(typically negative).
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Note the in bytes: even if the filehandle has been
set to operate on characters (for example by using
the ":utf8" I/O layer), tell() will return byte
offsets, not character offsets (because implementing
that would render sysseek() very slow).
sysseek() bypasses normal buffered IO, so mixing
this with reads (other than "sysread", for example
"<>" or read()) "print", "write", "seek", "tell", or
"eof" may cause confusion.
For WHENCE, you may also use the constants
"SEEK_SET", "SEEK_CUR", and "SEEK_END" (start of the
file, current position, end of the file) from the
Fcntl module. Use of the constants is also more
portable than relying on 0, 1, and 2. For example
to define a "systell" function:
use Fcntl 'SEEK_CUR';
sub systell { sysseek($_[0], 0, SEEK_CUR) }
Returns the new position, or the undefined value on
failure. A position of zero is returned as the
string "0 but true"; thus "sysseek" returns true on
success and false on failure, yet you can still
easily determine the new position.
system LIST
system PROGRAM LIST
Does exactly the same thing as "exec LIST", except
that a fork is done first, and the parent process
waits for the child process to complete. Note that
argument processing varies depending on the number
of arguments. If there is more than one argument in
LIST, or if LIST is an array with more than one
value, starts the program given by the first element
of the list with arguments given by the rest of the
list. If there is only one scalar argument, the
argument is checked for shell metacharacters, and if
there are any, the entire argument is passed to the
system's command shell for parsing (this is "/bin/sh
-c" on Unix platforms, but varies on other plat-
forms). If there are no shell metacharacters in the
argument, it is split into words and passed directly
to "execvp", which is more efficient.
Beginning with v5.6.0, Perl will attempt to flush
all files opened for output before any operation
that may do a fork, but this may not be supported on
some platforms (see perlport). To be safe, you may
need to set $| ($AUTOFLUSH in English) or call the
"autoflush()" method of "IO::Handle" on any open
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handles.
The return value is the exit status of the program
as returned by the "wait" call. To get the actual
exit value, shift right by eight (see below). See
also "exec". This is not what you want to use to
capture the output from a command, for that you
should use merely backticks or "qx//", as described
in "`STRING`" in perlop. Return value of -1 indi-
cates a failure to start the program or an error of
the wait(2) system call (inspect $! for the reason).
Like "exec", "system" allows you to lie to a program
about its name if you use the "system PROGRAM LIST"
syntax. Again, see "exec".
Since "SIGINT" and "SIGQUIT" are ignored during the
execution of "system", if you expect your program to
terminate on receipt of these signals you will need
to arrange to do so yourself based on the return
value.
@args = ("command", "arg1", "arg2");
system(@args) == 0
or die "system @args failed: $?"
You can check all the failure possibilities by
inspecting $? like this:
if ($? == -1) {
print "failed to execute: $!\n";
}
elsif ($? & 127) {
printf "child died with signal %d, %s coredump\n",
($? & 127), ($? & 128) ? 'with' : 'without';
}
else {
printf "child exited with value %d\n", $? >> 8;
}
or more portably by using the W*() calls of the
POSIX extension; see perlport for more information.
When the arguments get executed via the system
shell, results and return codes will be subject to
its quirks and capabilities. See "`STRING`" in per-
lop and "exec" for details.
syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
syswrite FILEHANDLE,SCALAR,LENGTH
syswrite FILEHANDLE,SCALAR
Attempts to write LENGTH bytes of data from variable
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SCALAR to the specified FILEHANDLE, using the system
call write(2). If LENGTH is not specified, writes
whole SCALAR. It bypasses buffered IO, so mixing
this with reads (other than sysread()), "print",
"write", "seek", "tell", or "eof" may cause confu-
sion because the perlio and stdio layers usually
buffers data. Returns the number of bytes actually
written, or "undef" if there was an error (in this
case the errno variable $! is also set). If the
LENGTH is greater than the available data in the
SCALAR after the OFFSET, only as much data as is
available will be written.
An OFFSET may be specified to write the data from
some part of the string other than the beginning. A
negative OFFSET specifies writing that many charac-
ters counting backwards from the end of the string.
In the case the SCALAR is empty you can use OFFSET
but only zero offset.
Note that if the filehandle has been marked as
":utf8", Unicode characters are written instead of
bytes (the LENGTH, OFFSET, and the return value of
syswrite() are in UTF-8 encoded Unicode characters).
The ":encoding(...)" layer implicitly introduces the
":utf8" layer. See "binmode", "open", and the "open"
pragma, open.
tell FILEHANDLE
tell Returns the current position in bytes for FILEHAN-
DLE, or -1 on error. FILEHANDLE may be an expres-
sion whose value gives the name of the actual
filehandle. If FILEHANDLE is omitted, assumes the
file last read.
Note the in bytes: even if the filehandle has been
set to operate on characters (for example by using
the ":utf8" open layer), tell() will return byte
offsets, not character offsets (because that would
render seek() and tell() rather slow).
The return value of tell() for the standard streams
like the STDIN depends on the operating system: it
may return -1 or something else. tell() on pipes,
fifos, and sockets usually returns -1.
There is no "systell" function. Use "sysseek(FH, 0,
1)" for that.
Do not use tell() (or other buffered I/O operations)
on a file handle that has been manipulated by sys-
read(), syswrite() or sysseek(). Those functions
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ignore the buffering, while tell() does not.
telldir DIRHANDLE
Returns the current position of the "readdir" rou-
tines on DIRHANDLE. Value may be given to "seekdir"
to access a particular location in a directory.
"telldir" has the same caveats about possible direc-
tory compaction as the corresponding system library
routine.
tie VARIABLE,CLASSNAME,LIST
This function binds a variable to a package class
that will provide the implementation for the vari-
able. VARIABLE is the name of the variable to be
enchanted. CLASSNAME is the name of a class imple-
menting objects of correct type. Any additional
arguments are passed to the "new" method of the
class (meaning "TIESCALAR", "TIEHANDLE", "TIEARRAY",
or "TIEHASH"). Typically these are arguments such
as might be passed to the "dbm_open()" function of
C. The object returned by the "new" method is also
returned by the "tie" function, which would be use-
ful if you want to access other methods in
CLASSNAME.
Note that functions such as "keys" and "values" may
return huge lists when used on large objects, like
DBM files. You may prefer to use the "each" func-
tion to iterate over such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
A class implementing a hash should have the follow-
ing methods:
TIEHASH classname, LIST
FETCH this, key
STORE this, key, value
DELETE this, key
CLEAR this
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
SCALAR this
DESTROY this
UNTIE this
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A class implementing an ordinary array should have
the following methods:
TIEARRAY classname, LIST
FETCH this, key
STORE this, key, value
FETCHSIZE this
STORESIZE this, count
CLEAR this
PUSH this, LIST
POP this
SHIFT this
UNSHIFT this, LIST
SPLICE this, offset, length, LIST
EXTEND this, count
DESTROY this
UNTIE this
A class implementing a file handle should have the
following methods:
TIEHANDLE classname, LIST
READ this, scalar, length, offset
READLINE this
GETC this
WRITE this, scalar, length, offset
PRINT this, LIST
PRINTF this, format, LIST
BINMODE this
EOF this
FILENO this
SEEK this, position, whence
TELL this
OPEN this, mode, LIST
CLOSE this
DESTROY this
UNTIE this
A class implementing a scalar should have the fol-
lowing methods:
TIESCALAR classname, LIST
FETCH this,
STORE this, value
DESTROY this
UNTIE this
Not all methods indicated above need be implemented.
See perltie, Tie::Hash, Tie::Array, Tie::Scalar, and
Tie::Handle.
Unlike "dbmopen", the "tie" function will not use or
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require a module for you--you need to do that expli-
citly yourself. See DB_File or the Config module
for interesting "tie" implementations.
For further details see perltie, "tied VARIABLE".
tied VARIABLE
Returns a reference to the object underlying VARI-
ABLE (the same value that was originally returned by
the "tie" call that bound the variable to a pack-
age.) Returns the undefined value if VARIABLE isn't
tied to a package.
time Returns the number of non-leap seconds since what-
ever time the system considers to be the epoch,
suitable for feeding to "gmtime" and "localtime". On
most systems the epoch is 00:00:00 UTC, January 1,
1970; a prominent exception being Mac OS Classic
which uses 00:00:00, January 1, 1904 in the current
local time zone for its epoch.
For measuring time in better granularity than one
second, you may use either the Time::HiRes module
(from CPAN, and starting from Perl 5.8 part of the
standard distribution), or if you have gettimeof-
day(2), you may be able to use the "syscall" inter-
face of Perl. See perlfaq8 for details.
times Returns a four-element list giving the user and sys-
tem times, in seconds, for this process and the
children of this process.
($user,$system,$cuser,$csystem) = times;
In scalar context, "times" returns $user.
tr/// The transliteration operator. Same as "y///". See
perlop.
truncate FILEHANDLE,LENGTH
truncate EXPR,LENGTH
Truncates the file opened on FILEHANDLE, or named by
EXPR, to the specified length. Produces a fatal
error if truncate isn't implemented on your system.
Returns true if successful, the undefined value oth-
erwise.
The behavior is undefined if LENGTH is greater than
the length of the file.
uc EXPR
uc Returns an uppercased version of EXPR. This is the
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internal function implementing the "\U" escape in
double-quoted strings. Respects current LC_CTYPE
locale if "use locale" in force. See perllocale and
perlunicode for more details about locale and
Unicode support. It does not attempt to do titlecase
mapping on initial letters. See "ucfirst" for that.
If EXPR is omitted, uses $_.
ucfirst EXPR
ucfirst Returns the value of EXPR with the first character
in uppercase (titlecase in Unicode). This is the
internal function implementing the "\u" escape in
double-quoted strings. Respects current LC_CTYPE
locale if "use locale" in force. See perllocale and
perlunicode for more details about locale and
Unicode support.
If EXPR is omitted, uses $_.
umask EXPR
umask Sets the umask for the process to EXPR and returns
the previous value. If EXPR is omitted, merely
returns the current umask.
The Unix permission "rwxr-x---" is represented as
three sets of three bits, or three octal digits:
0750 (the leading 0 indicates octal and isn't one of
the digits). The "umask" value is such a number
representing disabled permissions bits. The permis-
sion (or "mode") values you pass "mkdir" or "syso-
pen" are modified by your umask, so even if you tell
"sysopen" to create a file with permissions 0777, if
your umask is 0022 then the file will actually be
created with permissions 0755. If your "umask" were
0027 (group can't write; others can't read, write,
or execute), then passing "sysopen" 0666 would
create a file with mode 0640 ("0666 &~ 027" is
0640).
Here's some advice: supply a creation mode of 0666
for regular files (in "sysopen") and one of 0777 for
directories (in "mkdir") and executable files. This
gives users the freedom of choice: if they want pro-
tected files, they might choose process umasks of
022, 027, or even the particularly antisocial mask
of 077. Programs should rarely if ever make policy
decisions better left to the user. The exception to
this is when writing files that should be kept
private: mail files, web browser cookies, .rhosts
files, and so on.
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If umask(2) is not implemented on your system and
you are trying to restrict access for yourself
(i.e., (EXPR & 0700) > 0), produces a fatal error at
run time. If umask(2) is not implemented and you
are not trying to restrict access for yourself,
returns "undef".
Remember that a umask is a number, usually given in
octal; it is not a string of octal digits. See also
"oct", if all you have is a string.
undef EXPR
undef Undefines the value of EXPR, which must be an
lvalue. Use only on a scalar value, an array (using
"@"), a hash (using "%"), a subroutine (using "&"),
or a typeglob (using "*"). (Saying "undef
$hash{$key}" will probably not do what you expect on
most predefined variables or DBM list values, so
don't do that; see delete.) Always returns the
undefined value. You can omit the EXPR, in which
case nothing is undefined, but you still get an
undefined value that you could, for instance, return
from a subroutine, assign to a variable or pass as a
parameter. Examples:
undef $foo;
undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
($a, $b, undef, $c) = &foo; # Ignore third value returned
Note that this is a unary operator, not a list
operator.
unlink LIST
unlink Deletes a list of files. Returns the number of
files successfully deleted.
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink <*.bak>;
Note: "unlink" will not attempt to delete direc-
tories unless you are superuser and the -U flag is
supplied to Perl. Even if these conditions are met,
be warned that unlinking a directory can inflict
damage on your filesystem. Finally, using "unlink"
on directories is not supported on many operating
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systems. Use "rmdir" instead.
If LIST is omitted, uses $_.
unpack TEMPLATE,EXPR
"unpack" does the reverse of "pack": it takes a
string and expands it out into a list of values. (In
scalar context, it returns merely the first value
produced.)
The string is broken into chunks described by the
TEMPLATE. Each chunk is converted separately to a
value. Typically, either the string is a result of
"pack", or the bytes of the string represent a C
structure of some kind.
The TEMPLATE has the same format as in the "pack"
function. Here's a subroutine that does substring:
sub substr {
my($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ordinal { unpack("c",$_[0]); } # same as ord()
In addition to fields allowed in pack(), you may
prefix a field with a %<number> to indicate that you
want a <number>-bit checksum of the items instead of
the items themselves. Default is a 16-bit checksum.
Checksum is calculated by summing numeric values of
expanded values (for string fields the sum of
"ord($char)" is taken, for bit fields the sum of
zeroes and ones).
For example, the following computes the same number
as the System V sum program:
$checksum = do {
local $/; # slurp!
unpack("%32C*",<>) % 65535;
};
The following efficiently counts the number of set
bits in a bit vector:
$setbits = unpack("%32b*", $selectmask);
The "p" and "P" formats should be used with care.
Since Perl has no way of checking whether the value
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passed to "unpack()" corresponds to a valid memory
location, passing a pointer value that's not known
to be valid is likely to have disastrous conse-
quences.
If there are more pack codes or if the repeat count
of a field or a group is larger than what the
remainder of the input string allows, the result is
not well defined: in some cases, the repeat count is
decreased, or "unpack()" will produce null strings
or zeroes, or terminate with an error. If the input
string is longer than one described by the TEMPLATE,
the rest is ignored.
See "pack" for more examples and notes.
untie VARIABLE
Breaks the binding between a variable and a package.
(See "tie".) Has no effect if the variable is not
tied.
unshift ARRAY,LIST
Does the opposite of a "shift". Or the opposite of
a "push", depending on how you look at it. Prepends
list to the front of the array, and returns the new
number of elements in the array.
unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element at
a time, so the prepended elements stay in the same
order. Use "reverse" to do the reverse.
use Module VERSION LIST
use Module VERSION
use Module LIST
use Module
use VERSION
Imports some semantics into the current package from
the named module, generally by aliasing certain sub-
routine or variable names into your package. It is
exactly equivalent to
BEGIN { require Module; import Module LIST; }
except that Module must be a bareword.
VERSION may be either a numeric argument such as
5.006, which will be compared to $], or a literal of
the form v5.6.1, which will be compared to $^V (aka
$PERL_VERSION. A fatal error is produced if VERSION
is greater than the version of the current Perl
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interpreter; Perl will not attempt to parse the rest
of the file. Compare with "require", which can do a
similar check at run time.
Specifying VERSION as a literal of the form v5.6.1
should generally be avoided, because it leads to
misleading error messages under earlier versions of
Perl that do not support this syntax. The
equivalent numeric version should be used instead.
use v5.6.1; # compile time version check
use 5.6.1; # ditto
use 5.006_001; # ditto; preferred for backwards compatibility
This is often useful if you need to check the
current Perl version before "use"ing library modules
that have changed in incompatible ways from older
versions of Perl. (We try not to do this more than
we have to.)
The "BEGIN" forces the "require" and "import" to
happen at compile time. The "require" makes sure
the module is loaded into memory if it hasn't been
yet. The "import" is not a builtin--it's just an
ordinary static method call into the "Module" pack-
age to tell the module to import the list of
features back into the current package. The module
can implement its "import" method any way it likes,
though most modules just choose to derive their
"import" method via inheritance from the "Exporter"
class that is defined in the "Exporter" module. See
Exporter. If no "import" method can be found then
the call is skipped.
If you do not want to call the package's "import"
method (for instance, to stop your namespace from
being altered), explicitly supply the empty list:
use Module ();
That is exactly equivalent to
BEGIN { require Module }
If the VERSION argument is present between Module
and LIST, then the "use" will call the VERSION
method in class Module with the given version as an
argument. The default VERSION method, inherited
from the UNIVERSAL class, croaks if the given ver-
sion is larger than the value of the variable
$Module::VERSION.
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Again, there is a distinction between omitting LIST
("import" called with no arguments) and an explicit
empty LIST "()" ("import" not called). Note that
there is no comma after VERSION!
Because this is a wide-open interface, pragmas (com-
piler directives) are also implemented this way.
Currently implemented pragmas are:
use constant;
use diagnostics;
use integer;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
use warnings qw(all);
use sort qw(stable _quicksort _mergesort);
Some of these pseudo-modules import semantics into
the current block scope (like "strict" or "integer",
unlike ordinary modules, which import symbols into
the current package (which are effective through the
end of the file).
There's a corresponding "no" command that unimports
meanings imported by "use", i.e., it calls "unimport
Module LIST" instead of "import".
no integer;
no strict 'refs';
no warnings;
See perlmodlib for a list of standard modules and
pragmas. See perlrun for the "-M" and "-m"
command-line options to perl that give "use" func-
tionality from the command-line.
utime LIST
Changes the access and modification times on each
file of a list of files. The first two elements of
the list must be the NUMERICAL access and modifica-
tion times, in that order. Returns the number of
files successfully changed. The inode change time
of each file is set to the current time. For exam-
ple, this code has the same effect as the Unix
touch(1) command when the files already exist and
belong to the user running the program:
#!/usr/bin/perl
$atime = $mtime = time;
utime $atime, $mtime, @ARGV;
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Since perl 5.7.2, if the first two elements of the
list are "undef", then the utime(2) function in the
C library will be called with a null second argu-
ment. On most systems, this will set the file's
access and modification times to the current time
(i.e. equivalent to the example above) and will even
work on other users' files where you have write per-
mission:
utime undef, undef, @ARGV;
Under NFS this will use the time of the NFS server,
not the time of the local machine. If there is a
time synchronization problem, the NFS server and
local machine will have different times. The Unix
touch(1) command will in fact normally use this form
instead of the one shown in the first example.
Note that only passing one of the first two elements
as "undef" will be equivalent of passing it as 0 and
will not have the same effect as described when they
are both "undef". This case will also trigger an
uninitialized warning.
values HASH
Returns a list consisting of all the values of the
named hash. (In a scalar context, returns the number
of values.)
The values are returned in an apparently random
order. The actual random order is subject to change
in future versions of perl, but it is guaranteed to
be the same order as either the "keys" or "each"
function would produce on the same (unmodified)
hash. Since Perl 5.8.1 the ordering is different
even between different runs of Perl for security
reasons (see "Algorithmic Complexity Attacks" in
perlsec).
As a side effect, calling values() resets the HASH's
internal iterator, see "each". (In particular, cal-
ling values() in void context resets the iterator
with no other overhead.)
Note that the values are not copied, which means
modifying them will modify the contents of the hash:
for (values %hash) { s/foo/bar/g } # modifies %hash values
for (@hash{keys %hash}) { s/foo/bar/g } # same
See also "keys", "each", and "sort".
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vec EXPR,OFFSET,BITS
Treats the string in EXPR as a bit vector made up of
elements of width BITS, and returns the value of the
element specified by OFFSET as an unsigned integer.
BITS therefore specifies the number of bits that are
reserved for each element in the bit vector. This
must be a power of two from 1 to 32 (or 64, if your
platform supports that).
If BITS is 8, "elements" coincide with bytes of the
input string.
If BITS is 16 or more, bytes of the input string are
grouped into chunks of size BITS/8, and each group
is converted to a number as with pack()/unpack()
with big-endian formats "n"/"N" (and analogously for
BITS==64). See "pack" for details.
If bits is 4 or less, the string is broken into
bytes, then the bits of each byte are broken into
8/BITS groups. Bits of a byte are numbered in a
little-endian-ish way, as in 0x01, 0x02, 0x04, 0x08,
0x10, 0x20, 0x40, 0x80. For example, breaking the
single input byte "chr(0x36)" into two groups gives
a list "(0x6, 0x3)"; breaking it into 4 groups gives
"(0x2, 0x1, 0x3, 0x0)".
"vec" may also be assigned to, in which case
parentheses are needed to give the expression the
correct precedence as in
vec($image, $max_x * $x + $y, 8) = 3;
If the selected element is outside the string, the
value 0 is returned. If an element off the end of
the string is written to, Perl will first extend the
string with sufficiently many zero bytes. It is an
error to try to write off the beginning of the
string (i.e. negative OFFSET).
The string should not contain any character with the
value > 255 (which can only happen if you're using
UTF-8 encoding). If it does, it will be treated as
something that is not UTF-8 encoded. When the "vec"
was assigned to, other parts of your program will
also no longer consider the string to be UTF-8
encoded. In other words, if you do have such char-
acters in your string, vec() will operate on the
actual byte string, and not the conceptual character
string.
Strings created with "vec" can also be manipulated
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with the logical operators "|", "&", "^", and "~".
These operators will assume a bit vector operation
is desired when both operands are strings. See "Bit-
wise String Operators" in perlop.
The following code will build up an ASCII string
saying 'PerlPerlPerl'. The comments show the string
after each step. Note that this code works in the
same way on big-endian or little-endian machines.
my $foo = '';
vec($foo, 0, 32) = 0x5065726C; # 'Perl'
# $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
vec($foo, 2, 16) = 0x5065; # 'PerlPe'
vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
vec($foo, 21, 4) = 7; # 'PerlPerlPer'
# 'r' is "\x72"
vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
# 'l' is "\x6c"
To transform a bit vector into a string or list of
0's and 1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used
in place of the "*".
Here is an example to illustrate how the bits actu-
ally fall in place:
#!/usr/bin/perl -wl
print <<'EOT';
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
EOT
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for $w (0..3) {
$width = 2**$w;
for ($shift=0; $shift < $width; ++$shift) {
for ($off=0; $off < 32/$width; ++$off) {
$str = pack("B*", "0"x32);
$bits = (1<<$shift);
vec($str, $off, $width) = $bits;
$res = unpack("b*",$str);
$val = unpack("V", $str);
write;
}
}
}
format STDOUT =
vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
$off, $width, $bits, $val, $res
.
__END__
Regardless of the machine architecture on which it
is run, the above example should print the following
table:
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0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
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vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
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vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
wait Behaves like the wait(2) system call on your system:
it waits for a child process to terminate and
returns the pid of the deceased process, or "-1" if
there are no child processes. The status is
returned in $?. Note that a return value of "-1"
could mean that child processes are being automati-
cally reaped, as described in perlipc.
waitpid PID,FLAGS
Waits for a particular child process to terminate
and returns the pid of the deceased process, or "-1"
if there is no such child process. On some systems,
a value of 0 indicates that there are processes
still running. The status is returned in $?. If you
say
use POSIX ":sys_wait_h";
#...
do {
$kid = waitpid(-1, WNOHANG);
} until $kid > 0;
then you can do a non-blocking wait for all pending
zombie processes. Non-blocking wait is available on
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machines supporting either the waitpid(2) or
wait4(2) system calls. However, waiting for a par-
ticular pid with FLAGS of 0 is implemented every-
where. (Perl emulates the system call by remember-
ing the status values of processes that have exited
but have not been harvested by the Perl script yet.)
Note that on some systems, a return value of "-1"
could mean that child processes are being automati-
cally reaped. See perlipc for details, and for
other examples.
wantarray
Returns true if the context of the currently execut-
ing subroutine or "eval" is looking for a list
value. Returns false if the context is looking for
a scalar. Returns the undefined value if the con-
text is looking for no value (void context).
return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";
"wantarray()"'s result is unspecified in the top
level of a file, in a "BEGIN", "CHECK", "INIT" or
"END" block, or in a "DESTROY" method.
This function should have been named wantlist()
instead.
warn LIST
Produces a message on STDERR just like "die", but
doesn't exit or throw an exception.
If LIST is empty and $@ already contains a value
(typically from a previous eval) that value is used
after appending "\t...caught" to $@. This is useful
for staying almost, but not entirely similar to
"die".
If $@ is empty then the string "Warning: Something's
wrong" is used.
No message is printed if there is a $SIG{__WARN__}
handler installed. It is the handler's responsibil-
ity to deal with the message as it sees fit (like,
for instance, converting it into a "die"). Most
handlers must therefore make arrangements to actu-
ally display the warnings that they are not prepared
to deal with, by calling "warn" again in the
handler. Note that this is quite safe and will not
produce an endless loop, since "__WARN__" hooks are
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not called from inside one.
You will find this behavior is slightly different
from that of $SIG{__DIE__} handlers (which don't
suppress the error text, but can instead call "die"
again to change it).
Using a "__WARN__" handler provides a powerful way
to silence all warnings (even the so-called manda-
tory ones). An example:
# wipe out *all* compile-time warnings
BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20; # no warning about duplicate my $foo,
# but hey, you asked for it!
# no compile-time or run-time warnings before here
$DOWARN = 1;
# run-time warnings enabled after here
warn "\$foo is alive and $foo!"; # does show up
See perlvar for details on setting %SIG entries, and
for more examples. See the Carp module for other
kinds of warnings using its carp() and cluck() func-
tions.
write FILEHANDLE
write EXPR
write Writes a formatted record (possibly multi-line) to
the specified FILEHANDLE, using the format associ-
ated with that file. By default the format for a
file is the one having the same name as the filehan-
dle, but the format for the current output channel
(see the "select" function) may be set explicitly by
assigning the name of the format to the $~ variable.
Top of form processing is handled automatically: if
there is insufficient room on the current page for
the formatted record, the page is advanced by writ-
ing a form feed, a special top-of-page format is
used to format the new page header, and then the
record is written. By default the top-of-page format
is the name of the filehandle with "_TOP" appended,
but it may be dynamically set to the format of your
choice by assigning the name to the $^ variable
while the filehandle is selected. The number of
lines remaining on the current page is in variable
"$-", which can be set to 0 to force a new page.
If FILEHANDLE is unspecified, output goes to the
current default output channel, which starts out as
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STDOUT but may be changed by the "select" operator.
If the FILEHANDLE is an EXPR, then the expression is
evaluated and the resulting string is used to look
up the name of the FILEHANDLE at run time. For more
on formats, see perlform.
Note that write is not the opposite of "read".
Unfortunately.
y/// The transliteration operator. Same as "tr///". See
perlop.
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