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PERLPORT(1)	       Perl Programmers Reference Guide		   PERLPORT(1)

NAME
       perlport - Writing portable Perl

DESCRIPTION
       Perl runs on numerous operating systems.	 While most of them share much
       in common, they also have their own unique features.

       This document is meant to help you to find out what constitutes
       portable Perl code.  That way once you make a decision to write
       portably, you know where the lines are drawn, and you can stay within
       them.

       There is a tradeoff between taking full advantage of one particular
       type of computer and taking advantage of a full range of them.
       Naturally, as you broaden your range and become more diverse, the
       common factors drop, and you are left with an increasingly smaller area
       of common ground in which you can operate to accomplish a particular
       task.  Thus, when you begin attacking a problem, it is important to
       consider under which part of the tradeoff curve you want to operate.
       Specifically, you must decide whether it is important that the task
       that you are coding have the full generality of being portable, or
       whether to just get the job done right now.  This is the hardest choice
       to be made.  The rest is easy, because Perl provides many choices,
       whichever way you want to approach your problem.

       Looking at it another way, writing portable code is usually about
       willfully limiting your available choices.  Naturally, it takes
       discipline and sacrifice to do that.  The product of portability and
       convenience may be a constant.  You have been warned.

       Be aware of two important points:

       Not all Perl programs have to be portable
	   There is no reason you should not use Perl as a language to glue
	   Unix tools together, or to prototype a Macintosh application, or to
	   manage the Windows registry.	 If it makes no sense to aim for
	   portability for one reason or another in a given program, then
	   don't bother.

       Nearly all of Perl already is portable
	   Don't be fooled into thinking that it is hard to create portable
	   Perl code.  It isn't.  Perl tries its level-best to bridge the gaps
	   between what's available on different platforms, and all the means
	   available to use those features.  Thus almost all Perl code runs on
	   any machine without modification.  But there are some significant
	   issues in writing portable code, and this document is entirely
	   about those issues.

       Here's the general rule: When you approach a task commonly done using a
       whole range of platforms, think about writing portable code.  That way,
       you don't sacrifice much by way of the implementation choices you can
       avail yourself of, and at the same time you can give your users lots of
       platform choices.  On the other hand, when you have to take advantage
       of some unique feature of a particular platform, as is often the case
       with systems programming (whether for Unix, Windows, Mac OS, VMS,
       etc.), consider writing platform-specific code.

       When the code will run on only two or three operating systems, you may
       need to consider only the differences of those particular systems.  The
       important thing is to decide where the code will run and to be
       deliberate in your decision.

       The material below is separated into three main sections: main issues
       of portability ("ISSUES"), platform-specific issues ("PLATFORMS"), and
       built-in perl functions that behave differently on various ports
       ("FUNCTION IMPLEMENTATIONS").

       This information should not be considered complete; it includes
       possibly transient information about idiosyncrasies of some of the
       ports, almost all of which are in a state of constant evolution.	 Thus,
       this material should be considered a perpetual work in progress ("<IMG
       SRC="yellow_sign.gif" ALT="Under Construction">").

ISSUES
   Newlines
       In most operating systems, lines in files are terminated by newlines.
       Just what is used as a newline may vary from OS to OS.  Unix
       traditionally uses "\012", one type of DOSish I/O uses "\015\012", and
       Mac OS uses "\015".

       Perl uses "\n" to represent the "logical" newline, where what is
       logical may depend on the platform in use.  In MacPerl, "\n" always
       means "\015".  In DOSish perls, "\n" usually means "\012", but when
       accessing a file in "text" mode, STDIO translates it to (or from)
       "\015\012", depending on whether you're reading or writing.  Unix does
       the same thing on ttys in canonical mode.  "\015\012" is commonly
       referred to as CRLF.

       To trim trailing newlines from text lines use chomp().  With default
       settings that function looks for a trailing "\n" character and thus
       trims in a portable way.

       When dealing with binary files (or text files in binary mode) be sure
       to explicitly set $/ to the appropriate value for your file format
       before using chomp().

       Because of the "text" mode translation, DOSish perls have limitations
       in using "seek" and "tell" on a file accessed in "text" mode.  Stick to
       "seek"-ing to locations you got from "tell" (and no others), and you
       are usually free to use "seek" and "tell" even in "text" mode.  Using
       "seek" or "tell" or other file operations may be non-portable.  If you
       use "binmode" on a file, however, you can usually "seek" and "tell"
       with arbitrary values in safety.

       A common misconception in socket programming is that "\n" eq "\012"
       everywhere.  When using protocols such as common Internet protocols,
       "\012" and "\015" are called for specifically, and the values of the
       logical "\n" and "\r" (carriage return) are not reliable.

	   print SOCKET "Hi there, client!\r\n";      # WRONG
	   print SOCKET "Hi there, client!\015\012";  # RIGHT

       However, using "\015\012" (or "\cM\cJ", or "\x0D\x0A") can be tedious
       and unsightly, as well as confusing to those maintaining the code.  As
       such, the Socket module supplies the Right Thing for those who want it.

	   use Socket qw(:DEFAULT :crlf);
	   print SOCKET "Hi there, client!$CRLF"      # RIGHT

       When reading from a socket, remember that the default input record
       separator $/ is "\n", but robust socket code will recognize as either
       "\012" or "\015\012" as end of line:

	   while (<SOCKET>) {
	       # ...
	   }

       Because both CRLF and LF end in LF, the input record separator can be
       set to LF and any CR stripped later.  Better to write:

	   use Socket qw(:DEFAULT :crlf);
	   local($/) = LF;	# not needed if $/ is already \012

	   while (<SOCKET>) {
	       s/$CR?$LF/\n/;	# not sure if socket uses LF or CRLF, OK
	   #   s/\015?\012/\n/; # same thing
	   }

       This example is preferred over the previous one--even for Unix
       platforms--because now any "\015"'s ("\cM"'s) are stripped out (and
       there was much rejoicing).

       Similarly, functions that return text data--such as a function that
       fetches a web page--should sometimes translate newlines before
       returning the data, if they've not yet been translated to the local
       newline representation.	A single line of code will often suffice:

	   $data =~ s/\015?\012/\n/g;
	   return $data;

       Some of this may be confusing.  Here's a handy reference to the ASCII
       CR and LF characters.  You can print it out and stick it in your
       wallet.

	   LF  eq  \012	 eq  \x0A  eq  \cJ  eq	chr(10)	 eq  ASCII 10
	   CR  eq  \015	 eq  \x0D  eq  \cM  eq	chr(13)	 eq  ASCII 13

		    | Unix | DOS  | Mac	 |
	       ---------------------------
	       \n   |  LF  |  LF  |  CR	 |
	       \r   |  CR  |  CR  |  LF	 |
	       \n * |  LF  | CRLF |  CR	 |
	       \r * |  CR  |  CR  |  LF	 |
	       ---------------------------
	       * text-mode STDIO

       The Unix column assumes that you are not accessing a serial line (like
       a tty) in canonical mode.  If you are, then CR on input becomes "\n",
       and "\n" on output becomes CRLF.

       These are just the most common definitions of "\n" and "\r" in Perl.
       There may well be others.  For example, on an EBCDIC implementation
       such as z/OS (OS/390) or OS/400 (using the ILE, the PASE is ASCII-
       based) the above material is similar to "Unix" but the code numbers
       change:

	   LF  eq  \025	 eq  \x15  eq  \cU  eq	chr(21)	 eq  CP-1047 21
	   LF  eq  \045	 eq  \x25  eq		chr(37)	 eq  CP-0037 37
	   CR  eq  \015	 eq  \x0D  eq  \cM  eq	chr(13)	 eq  CP-1047 13
	   CR  eq  \015	 eq  \x0D  eq  \cM  eq	chr(13)	 eq  CP-0037 13

		    | z/OS | OS/400 |
	       ----------------------
	       \n   |  LF  |  LF    |
	       \r   |  CR  |  CR    |
	       \n * |  LF  |  LF    |
	       \r * |  CR  |  CR    |
	       ----------------------
	       * text-mode STDIO

   Numbers endianness and Width
       Different CPUs store integers and floating point numbers in different
       orders (called endianness) and widths (32-bit and 64-bit being the most
       common today).  This affects your programs when they attempt to
       transfer numbers in binary format from one CPU architecture to another,
       usually either "live" via network connection, or by storing the numbers
       to secondary storage such as a disk file or tape.

       Conflicting storage orders make utter mess out of the numbers.  If a
       little-endian host (Intel, VAX) stores 0x12345678 (305419896 in
       decimal), a big-endian host (Motorola, Sparc, PA) reads it as
       0x78563412 (2018915346 in decimal).  Alpha and MIPS can be either:
       Digital/Compaq used/uses them in little-endian mode; SGI/Cray uses them
       in big-endian mode.  To avoid this problem in network (socket)
       connections use the "pack" and "unpack" formats "n" and "N", the
       "network" orders.  These are guaranteed to be portable.

       As of perl 5.9.2, you can also use the ">" and "<" modifiers to force
       big- or little-endian byte-order.  This is useful if you want to store
       signed integers or 64-bit integers, for example.

       You can explore the endianness of your platform by unpacking a data
       structure packed in native format such as:

	   print unpack("h*", pack("s2", 1, 2)), "\n";
	   # '10002000' on e.g. Intel x86 or Alpha 21064 in little-endian mode
	   # '00100020' on e.g. Motorola 68040

       If you need to distinguish between endian architectures you could use
       either of the variables set like so:

	   $is_big_endian   = unpack("h*", pack("s", 1)) =~ /01/;
	   $is_little_endian = unpack("h*", pack("s", 1)) =~ /^1/;

       Differing widths can cause truncation even between platforms of equal
       endianness.  The platform of shorter width loses the upper parts of the
       number.	There is no good solution for this problem except to avoid
       transferring or storing raw binary numbers.

       One can circumnavigate both these problems in two ways.	Either
       transfer and store numbers always in text format, instead of raw
       binary, or else consider using modules like Data::Dumper (included in
       the standard distribution as of Perl 5.005) and Storable (included as
       of perl 5.8).  Keeping all data as text significantly simplifies
       matters.

       The v-strings are portable only up to v2147483647 (0x7FFFFFFF), that's
       how far EBCDIC, or more precisely UTF-EBCDIC will go.

   Files and Filesystems
       Most platforms these days structure files in a hierarchical fashion.
       So, it is reasonably safe to assume that all platforms support the
       notion of a "path" to uniquely identify a file on the system.  How that
       path is really written, though, differs considerably.

       Although similar, file path specifications differ between Unix,
       Windows, Mac OS, OS/2, VMS, VOS, RISC OS, and probably others.  Unix,
       for example, is one of the few OSes that has the elegant idea of a
       single root directory.

       DOS, OS/2, VMS, VOS, and Windows can work similarly to Unix with "/" as
       path separator, or in their own idiosyncratic ways (such as having
       several root directories and various "unrooted" device files such NIL:
       and LPT:).

       Mac OS uses ":" as a path separator instead of "/".

       The filesystem may support neither hard links ("link") nor symbolic
       links ("symlink", "readlink", "lstat").

       The filesystem may support neither access timestamp nor change
       timestamp (meaning that about the only portable timestamp is the
       modification timestamp), or one second granularity of any timestamps
       (e.g. the FAT filesystem limits the time granularity to two seconds).

       The "inode change timestamp" (the "-C" filetest) may really be the
       "creation timestamp" (which it is not in UNIX).

       VOS perl can emulate Unix filenames with "/" as path separator.	The
       native pathname characters greater-than, less-than, number-sign, and
       percent-sign are always accepted.

       RISC OS perl can emulate Unix filenames with "/" as path separator, or
       go native and use "." for path separator and ":" to signal filesystems
       and disk names.

       Don't assume UNIX filesystem access semantics: that read, write, and
       execute are all the permissions there are, and even if they exist, that
       their semantics (for example what do r, w, and x mean on a directory)
       are the UNIX ones.  The various UNIX/POSIX compatibility layers usually
       try to make interfaces like chmod() work, but sometimes there simply is
       no good mapping.

       If all this is intimidating, have no (well, maybe only a little) fear.
       There are modules that can help.	 The File::Spec modules provide
       methods to do the Right Thing on whatever platform happens to be
       running the program.

	   use File::Spec::Functions;
	   chdir(updir());	  # go up one directory
	   $file = catfile(curdir(), 'temp', 'file.txt');
	   # on Unix and Win32, './temp/file.txt'
	   # on Mac OS, ':temp:file.txt'
	   # on VMS, '[.temp]file.txt'

       File::Spec is available in the standard distribution as of version
       5.004_05.  File::Spec::Functions is only in File::Spec 0.7 and later,
       and some versions of perl come with version 0.6.	 If File::Spec is not
       updated to 0.7 or later, you must use the object-oriented interface
       from File::Spec (or upgrade File::Spec).

       In general, production code should not have file paths hardcoded.
       Making them user-supplied or read from a configuration file is better,
       keeping in mind that file path syntax varies on different machines.

       This is especially noticeable in scripts like Makefiles and test
       suites, which often assume "/" as a path separator for subdirectories.

       Also of use is File::Basename from the standard distribution, which
       splits a pathname into pieces (base filename, full path to directory,
       and file suffix).

       Even when on a single platform (if you can call Unix a single
       platform), remember not to count on the existence or the contents of
       particular system-specific files or directories, like /etc/passwd,
       /etc/sendmail.conf, /etc/resolv.conf, or even /tmp/.  For example,
       /etc/passwd may exist but not contain the encrypted passwords, because
       the system is using some form of enhanced security.  Or it may not
       contain all the accounts, because the system is using NIS.  If code
       does need to rely on such a file, include a description of the file and
       its format in the code's documentation, then make it easy for the user
       to override the default location of the file.

       Don't assume a text file will end with a newline.  They should, but
       people forget.

       Do not have two files or directories of the same name with different
       case, like test.pl and Test.pl, as many platforms have case-insensitive
       (or at least case-forgiving) filenames.	Also, try not to have non-word
       characters (except for ".") in the names, and keep them to the 8.3
       convention, for maximum portability, onerous a burden though this may
       appear.

       Likewise, when using the AutoSplit module, try to keep your functions
       to 8.3 naming and case-insensitive conventions; or, at the least, make
       it so the resulting files have a unique (case-insensitively) first 8
       characters.

       Whitespace in filenames is tolerated on most systems, but not all, and
       even on systems where it might be tolerated, some utilities might
       become confused by such whitespace.

       Many systems (DOS, VMS ODS-2) cannot have more than one "." in their
       filenames.

       Don't assume ">" won't be the first character of a filename.  Always
       use "<" explicitly to open a file for reading, or even better, use the
       three-arg version of open, unless you want the user to be able to
       specify a pipe open.

	   open(FILE, '<', $existing_file) or die $!;

       If filenames might use strange characters, it is safest to open it with
       "sysopen" instead of "open".  "open" is magic and can translate
       characters like ">", "<", and "|", which may be the wrong thing to do.
       (Sometimes, though, it's the right thing.)  Three-arg open can also
       help protect against this translation in cases where it is undesirable.

       Don't use ":" as a part of a filename since many systems use that for
       their own semantics (Mac OS Classic for separating pathname components,
       many networking schemes and utilities for separating the nodename and
       the pathname, and so on).  For the same reasons, avoid "@", ";" and
       "|".

       Don't assume that in pathnames you can collapse two leading slashes
       "//" into one: some networking and clustering filesystems have special
       semantics for that.  Let the operating system to sort it out.

       The portable filename characters as defined by ANSI C are

	a b c d e f g h i j k l m n o p q r t u v w x y z
	A B C D E F G H I J K L M N O P Q R T U V W X Y Z
	0 1 2 3 4 5 6 7 8 9
	. _ -

       and the "-" shouldn't be the first character.  If you want to be
       hypercorrect, stay case-insensitive and within the 8.3 naming
       convention (all the files and directories have to be unique within one
       directory if their names are lowercased and truncated to eight
       characters before the ".", if any, and to three characters after the
       ".", if any).  (And do not use "."s in directory names.)

   System Interaction
       Not all platforms provide a command line.  These are usually platforms
       that rely primarily on a Graphical User Interface (GUI) for user
       interaction.  A program requiring a command line interface might not
       work everywhere.	 This is probably for the user of the program to deal
       with, so don't stay up late worrying about it.

       Some platforms can't delete or rename files held open by the system,
       this limitation may also apply to changing filesystem metainformation
       like file permissions or owners.	 Remember to "close" files when you
       are done with them.  Don't "unlink" or "rename" an open file.  Don't
       "tie" or "open" a file already tied or opened; "untie" or "close" it
       first.

       Don't open the same file more than once at a time for writing, as some
       operating systems put mandatory locks on such files.

       Don't assume that write/modify permission on a directory gives the
       right to add or delete files/directories in that directory.  That is
       filesystem specific: in some filesystems you need write/modify
       permission also (or even just) in the file/directory itself.  In some
       filesystems (AFS, DFS) the permission to add/delete directory entries
       is a completely separate permission.

       Don't assume that a single "unlink" completely gets rid of the file:
       some filesystems (most notably the ones in VMS) have versioned
       filesystems, and unlink() removes only the most recent one (it doesn't
       remove all the versions because by default the native tools on those
       platforms remove just the most recent version, too).  The portable
       idiom to remove all the versions of a file is

	   1 while unlink "file";

       This will terminate if the file is undeleteable for some reason
       (protected, not there, and so on).

       Don't count on a specific environment variable existing in %ENV.	 Don't
       count on %ENV entries being case-sensitive, or even case-preserving.
       Don't try to clear %ENV by saying "%ENV = ();", or, if you really have
       to, make it conditional on "$^O ne 'VMS'" since in VMS the %ENV table
       is much more than a per-process key-value string table.

       On VMS, some entries in the %ENV hash are dynamically created when
       their key is used on a read if they did not previously exist.  The
       values for $ENV{HOME}, $ENV{TERM}, $ENV{HOME}, and $ENV{USER}, are
       known to be dynamically generated.  The specific names that are
       dynamically generated may vary with the version of the C library on
       VMS, and more may exist than is documented.

       On VMS by default, changes to the %ENV hash are persistent after the
       process exits.  This can cause unintended issues.

       Don't count on signals or %SIG for anything.

       Don't count on filename globbing.  Use "opendir", "readdir", and
       "closedir" instead.

       Don't count on per-program environment variables, or per-program
       current directories.

       Don't count on specific values of $!, neither numeric nor especially
       the strings values-- users may switch their locales causing error
       messages to be translated into their languages.	If you can trust a
       POSIXish environment, you can portably use the symbols defined by the
       Errno module, like ENOENT.  And don't trust on the values of $!	at all
       except immediately after a failed system call.

   Command names versus file pathnames
       Don't assume that the name used to invoke a command or program with
       "system" or "exec" can also be used to test for the existence of the
       file that holds the executable code for that command or program.
       First, many systems have "internal" commands that are built-in to the
       shell or OS and while these commands can be invoked, there is no
       corresponding file.  Second, some operating systems (e.g., Cygwin,
       DJGPP, OS/2, and VOS) have required suffixes for executable files;
       these suffixes are generally permitted on the command name but are not
       required.  Thus, a command like "perl" might exist in a file named
       "perl", "perl.exe", or "perl.pm", depending on the operating system.
       The variable "_exe" in the Config module holds the executable suffix,
       if any.	Third, the VMS port carefully sets up $^X and
       $Config{perlpath} so that no further processing is required.  This is
       just as well, because the matching regular expression used below would
       then have to deal with a possible trailing version number in the VMS
       file name.

       To convert $^X to a file pathname, taking account of the requirements
       of the various operating system possibilities, say:

	 use Config;
	 $thisperl = $^X;
	 if ($^O ne 'VMS')
	    {$thisperl .= $Config{_exe} unless $thisperl =~ m/$Config{_exe}$/i;}

       To convert $Config{perlpath} to a file pathname, say:

	 use Config;
	 $thisperl = $Config{perlpath};
	 if ($^O ne 'VMS')
	    {$thisperl .= $Config{_exe} unless $thisperl =~ m/$Config{_exe}$/i;}

   Networking
       Don't assume that you can reach the public Internet.

       Don't assume that there is only one way to get through firewalls to the
       public Internet.

       Don't assume that you can reach outside world through any other port
       than 80, or some web proxy.  ftp is blocked by many firewalls.

       Don't assume that you can send email by connecting to the local SMTP
       port.

       Don't assume that you can reach yourself or any node by the name
       'localhost'.  The same goes for '127.0.0.1'.  You will have to try
       both.

       Don't assume that the host has only one network card, or that it can't
       bind to many virtual IP addresses.

       Don't assume a particular network device name.

       Don't assume a particular set of ioctl()s will work.

       Don't assume that you can ping hosts and get replies.

       Don't assume that any particular port (service) will respond.

       Don't assume that Sys::Hostname (or any other API or command) returns
       either a fully qualified hostname or a non-qualified hostname: it all
       depends on how the system had been configured.  Also remember things
       like DHCP and NAT-- the hostname you get back might not be very useful.

       All the above "don't":s may look daunting, and they are -- but the key
       is to degrade gracefully if one cannot reach the particular network
       service one wants.  Croaking or hanging do not look very professional.

   Interprocess Communication (IPC)
       In general, don't directly access the system in code meant to be
       portable.  That means, no "system", "exec", "fork", "pipe", "``",
       "qx//", "open" with a "|", nor any of the other things that makes being
       a perl hacker worth being.

       Commands that launch external processes are generally supported on most
       platforms (though many of them do not support any type of forking).
       The problem with using them arises from what you invoke them on.
       External tools are often named differently on different platforms, may
       not be available in the same location, might accept different
       arguments, can behave differently, and often present their results in a
       platform-dependent way.	Thus, you should seldom depend on them to
       produce consistent results. (Then again, if you're calling netstat -a,
       you probably don't expect it to run on both Unix and CP/M.)

       One especially common bit of Perl code is opening a pipe to sendmail:

	   open(MAIL, '|/usr/lib/sendmail -t')
	       or die "cannot fork sendmail: $!";

       This is fine for systems programming when sendmail is known to be
       available.  But it is not fine for many non-Unix systems, and even some
       Unix systems that may not have sendmail installed.  If a portable
       solution is needed, see the various distributions on CPAN that deal
       with it.	 Mail::Mailer and Mail::Send in the MailTools distribution are
       commonly used, and provide several mailing methods, including mail,
       sendmail, and direct SMTP (via Net::SMTP) if a mail transfer agent is
       not available.  Mail::Sendmail is a standalone module that provides
       simple, platform-independent mailing.

       The Unix System V IPC ("msg*(), sem*(), shm*()") is not available even
       on all Unix platforms.

       Do not use either the bare result of "pack("N", 10, 20, 30, 40)" or
       bare v-strings (such as "v10.20.30.40") to represent IPv4 addresses:
       both forms just pack the four bytes into network order.	That this
       would be equal to the C language "in_addr" struct (which is what the
       socket code internally uses) is not guaranteed.	To be portable use the
       routines of the Socket extension, such as "inet_aton()", "inet_ntoa()",
       and "sockaddr_in()".

       The rule of thumb for portable code is: Do it all in portable Perl, or
       use a module (that may internally implement it with platform-specific
       code, but expose a common interface).

   External Subroutines (XS)
       XS code can usually be made to work with any platform, but dependent
       libraries, header files, etc., might not be readily available or
       portable, or the XS code itself might be platform-specific, just as
       Perl code might be.  If the libraries and headers are portable, then it
       is normally reasonable to make sure the XS code is portable, too.

       A different type of portability issue arises when writing XS code:
       availability of a C compiler on the end-user's system.  C brings with
       it its own portability issues, and writing XS code will expose you to
       some of those.  Writing purely in Perl is an easier way to achieve
       portability.

   Standard Modules
       In general, the standard modules work across platforms.	Notable
       exceptions are the CPAN module (which currently makes connections to
       external programs that may not be available), platform-specific modules
       (like ExtUtils::MM_VMS), and DBM modules.

       There is no one DBM module available on all platforms.  SDBM_File and
       the others are generally available on all Unix and DOSish ports, but
       not in MacPerl, where only NBDM_File and DB_File are available.

       The good news is that at least some DBM module should be available, and
       AnyDBM_File will use whichever module it can find.  Of course, then the
       code needs to be fairly strict, dropping to the greatest common factor
       (e.g., not exceeding 1K for each record), so that it will work with any
       DBM module.  See AnyDBM_File for more details.

   Time and Date
       The system's notion of time of day and calendar date is controlled in
       widely different ways.  Don't assume the timezone is stored in
       $ENV{TZ}, and even if it is, don't assume that you can control the
       timezone through that variable.	Don't assume anything about the three-
       letter timezone abbreviations (for example that MST would be the
       Mountain Standard Time, it's been known to stand for Moscow Standard
       Time).  If you need to use timezones, express them in some unambiguous
       format like the exact number of minutes offset from UTC, or the POSIX
       timezone format.

       Don't assume that the epoch starts at 00:00:00, January 1, 1970,
       because that is OS- and implementation-specific.	 It is better to store
       a date in an unambiguous representation.	 The ISO 8601 standard defines
       YYYY-MM-DD as the date format, or YYYY-MM-DDTHH-MM-SS (that's a literal
       "T" separating the date from the time).	Please do use the ISO 8601
       instead of making us to guess what date 02/03/04 might be.  ISO 8601
       even sorts nicely as-is.	 A text representation (like "1987-12-18") can
       be easily converted into an OS-specific value using a module like
       Date::Parse.  An array of values, such as those returned by
       "localtime", can be converted to an OS-specific representation using
       Time::Local.

       When calculating specific times, such as for tests in time or date
       modules, it may be appropriate to calculate an offset for the epoch.

	   require Time::Local;
	   $offset = Time::Local::timegm(0, 0, 0, 1, 0, 70);

       The value for $offset in Unix will be 0, but in Mac OS will be some
       large number.  $offset can then be added to a Unix time value to get
       what should be the proper value on any system.

       On Windows (at least), you shouldn't pass a negative value to "gmtime"
       or "localtime".

   Character sets and character encoding
       Assume very little about character sets.

       Assume nothing about numerical values ("ord", "chr") of characters.  Do
       not use explicit code point ranges (like \xHH-\xHH); use for example
       symbolic character classes like "[:print:]".

       Do not assume that the alphabetic characters are encoded contiguously
       (in the numeric sense).	There may be gaps.

       Do not assume anything about the ordering of the characters.  The
       lowercase letters may come before or after the uppercase letters; the
       lowercase and uppercase may be interlaced so that both "a" and "A" come
       before "b"; the accented and other international characters may be
       interlaced so that ae comes before "b".

   Internationalisation
       If you may assume POSIX (a rather large assumption), you may read more
       about the POSIX locale system from perllocale.  The locale system at
       least attempts to make things a little bit more portable, or at least
       more convenient and native-friendly for non-English users.  The system
       affects character sets and encoding, and date and time
       formatting--amongst other things.

       If you really want to be international, you should consider Unicode.
       See perluniintro and perlunicode for more information.

       If you want to use non-ASCII bytes (outside the bytes 0x00..0x7f) in
       the "source code" of your code, to be portable you have to be explicit
       about what bytes they are.  Someone might for example be using your
       code under a UTF-8 locale, in which case random native bytes might be
       illegal ("Malformed UTF-8 ...")	This means that for example embedding
       ISO 8859-1 bytes beyond 0x7f into your strings might cause trouble
       later.  If the bytes are native 8-bit bytes, you can use the "bytes"
       pragma.	If the bytes are in a string (regular expression being a
       curious string), you can often also use the "\xHH" notation instead of
       embedding the bytes as-is.  (If you want to write your code in UTF-8,
       you can use the "utf8".) The "bytes" and "utf8" pragmata are available
       since Perl 5.6.0.

   System Resources
       If your code is destined for systems with severely constrained (or
       missing!) virtual memory systems then you want to be especially mindful
       of avoiding wasteful constructs such as:

	   # NOTE: this is no longer "bad" in perl5.005
	   for (0..10000000) {}			      # bad
	   for (my $x = 0; $x <= 10000000; ++$x) {}   # good

	   @lines = <VERY_LARGE_FILE>;		      # bad

	   while (<FILE>) {$file .= $_}		      # sometimes bad
	   $file = join('', <FILE>);		      # better

       The last two constructs may appear unintuitive to most people.  The
       first repeatedly grows a string, whereas the second allocates a large
       chunk of memory in one go.  On some systems, the second is more
       efficient that the first.

   Security
       Most multi-user platforms provide basic levels of security, usually
       implemented at the filesystem level.  Some, however, do not--
       unfortunately.  Thus the notion of user id, or "home" directory, or
       even the state of being logged-in, may be unrecognizable on many
       platforms.  If you write programs that are security-conscious, it is
       usually best to know what type of system you will be running under so
       that you can write code explicitly for that platform (or class of
       platforms).

       Don't assume the UNIX filesystem access semantics: the operating system
       or the filesystem may be using some ACL systems, which are richer
       languages than the usual rwx.  Even if the rwx exist, their semantics
       might be different.

       (From security viewpoint testing for permissions before attempting to
       do something is silly anyway: if one tries this, there is potential for
       race conditions-- someone or something might change the permissions
       between the permissions check and the actual operation.	Just try the
       operation.)

       Don't assume the UNIX user and group semantics: especially, don't
       expect the $< and $> (or the $( and $)) to work for switching
       identities (or memberships).

       Don't assume set-uid and set-gid semantics. (And even if you do, think
       twice: set-uid and set-gid are a known can of security worms.)

   Style
       For those times when it is necessary to have platform-specific code,
       consider keeping the platform-specific code in one place, making
       porting to other platforms easier.  Use the Config module and the
       special variable $^O to differentiate platforms, as described in
       "PLATFORMS".

       Be careful in the tests you supply with your module or programs.
       Module code may be fully portable, but its tests might not be.  This
       often happens when tests spawn off other processes or call external
       programs to aid in the testing, or when (as noted above) the tests
       assume certain things about the filesystem and paths.  Be careful not
       to depend on a specific output style for errors, such as when checking
       $! after a failed system call.  Using $! for anything else than
       displaying it as output is doubtful (though see the Errno module for
       testing reasonably portably for error value). Some platforms expect a
       certain output format, and Perl on those platforms may have been
       adjusted accordingly.  Most specifically, don't anchor a regex when
       testing an error value.

CPAN Testers
       Modules uploaded to CPAN are tested by a variety of volunteers on
       different platforms.  These CPAN testers are notified by mail of each
       new upload, and reply to the list with PASS, FAIL, NA (not applicable
       to this platform), or UNKNOWN (unknown), along with any relevant
       notations.

       The purpose of the testing is twofold: one, to help developers fix any
       problems in their code that crop up because of lack of testing on other
       platforms; two, to provide users with information about whether a given
       module works on a given platform.

       Also see:

       ·   Mailing list: cpan-testers@perl.org

       ·   Testing results: http://testers.cpan.org/

PLATFORMS
       As of version 5.002, Perl is built with a $^O variable that indicates
       the operating system it was built on.  This was implemented to help
       speed up code that would otherwise have to "use Config" and use the
       value of $Config{osname}.  Of course, to get more detailed information
       about the system, looking into %Config is certainly recommended.

       %Config cannot always be trusted, however, because it was built at
       compile time.  If perl was built in one place, then transferred
       elsewhere, some values may be wrong.  The values may even have been
       edited after the fact.

   Unix
       Perl works on a bewildering variety of Unix and Unix-like platforms
       (see e.g. most of the files in the hints/ directory in the source code
       kit).  On most of these systems, the value of $^O (hence
       $Config{'osname'}, too) is determined either by lowercasing and
       stripping punctuation from the first field of the string returned by
       typing "uname -a" (or a similar command) at the shell prompt or by
       testing the file system for the presence of uniquely named files such
       as a kernel or header file.  Here, for example, are a few of the more
       popular Unix flavors:

	   uname	 $^O	    $Config{'archname'}
	   --------------------------------------------
	   AIX		 aix	    aix
	   BSD/OS	 bsdos	    i386-bsdos
	   Darwin	 darwin	    darwin
	   dgux		 dgux	    AViiON-dgux
	   DYNIX/ptx	 dynixptx   i386-dynixptx
	   FreeBSD	 freebsd    freebsd-i386
	   Haiku	 haiku	    BePC-haiku
	   Linux	 linux	    arm-linux
	   Linux	 linux	    i386-linux
	   Linux	 linux	    i586-linux
	   Linux	 linux	    ppc-linux
	   HP-UX	 hpux	    PA-RISC1.1
	   IRIX		 irix	    irix
	   Mac OS X	 darwin	    darwin
	   MachTen PPC	 machten    powerpc-machten
	   NeXT 3	 next	    next-fat
	   NeXT 4	 next	    OPENSTEP-Mach
	   openbsd	 openbsd    i386-openbsd
	   OSF1		 dec_osf    alpha-dec_osf
	   reliantunix-n svr4	    RM400-svr4
	   SCO_SV	 sco_sv	    i386-sco_sv
	   SINIX-N	 svr4	    RM400-svr4
	   sn4609	 unicos	    CRAY_C90-unicos
	   sn6521	 unicosmk   t3e-unicosmk
	   sn9617	 unicos	    CRAY_J90-unicos
	   SunOS	 solaris    sun4-solaris
	   SunOS	 solaris    i86pc-solaris
	   SunOS4	 sunos	    sun4-sunos

       Because the value of $Config{archname} may depend on the hardware
       architecture, it can vary more than the value of $^O.

   DOS and Derivatives
       Perl has long been ported to Intel-style microcomputers running under
       systems like PC-DOS, MS-DOS, OS/2, and most Windows platforms you can
       bring yourself to mention (except for Windows CE, if you count that).
       Users familiar with COMMAND.COM or CMD.EXE style shells should be aware
       that each of these file specifications may have subtle differences:

	   $filespec0 = "c:/foo/bar/file.txt";
	   $filespec1 = "c:\\foo\\bar\\file.txt";
	   $filespec2 = 'c:\foo\bar\file.txt';
	   $filespec3 = 'c:\\foo\\bar\\file.txt';

       System calls accept either "/" or "\" as the path separator.  However,
       many command-line utilities of DOS vintage treat "/" as the option
       prefix, so may get confused by filenames containing "/".	 Aside from
       calling any external programs, "/" will work just fine, and probably
       better, as it is more consistent with popular usage, and avoids the
       problem of remembering what to backwhack and what not to.

       The DOS FAT filesystem can accommodate only "8.3" style filenames.
       Under the "case-insensitive, but case-preserving" HPFS (OS/2) and NTFS
       (NT) filesystems you may have to be careful about case returned with
       functions like "readdir" or used with functions like "open" or
       "opendir".

       DOS also treats several filenames as special, such as AUX, PRN, NUL,
       CON, COM1, LPT1, LPT2, etc.  Unfortunately, sometimes these filenames
       won't even work if you include an explicit directory prefix.  It is
       best to avoid such filenames, if you want your code to be portable to
       DOS and its derivatives.	 It's hard to know what these all are,
       unfortunately.

       Users of these operating systems may also wish to make use of scripts
       such as pl2bat.bat or pl2cmd to put wrappers around your scripts.

       Newline ("\n") is translated as "\015\012" by STDIO when reading from
       and writing to files (see "Newlines").  "binmode(FILEHANDLE)" will keep
       "\n" translated as "\012" for that filehandle.  Since it is a no-op on
       other systems, "binmode" should be used for cross-platform code that
       deals with binary data.	That's assuming you realize in advance that
       your data is in binary.	General-purpose programs should often assume
       nothing about their data.

       The $^O variable and the $Config{archname} values for various DOSish
       perls are as follows:

	    OS		  $^O	   $Config{archname}   ID    Version
	    --------------------------------------------------------
	    MS-DOS	  dos	     ?
	    PC-DOS	  dos	     ?
	    OS/2	  os2	     ?
	    Windows 3.1	  ?	     ?		       0      3 01
	    Windows 95	  MSWin32    MSWin32-x86       1      4 00
	    Windows 98	  MSWin32    MSWin32-x86       1      4 10
	    Windows ME	  MSWin32    MSWin32-x86       1      ?
	    Windows NT	  MSWin32    MSWin32-x86       2      4 xx
	    Windows NT	  MSWin32    MSWin32-ALPHA     2      4 xx
	    Windows NT	  MSWin32    MSWin32-ppc       2      4 xx
	    Windows 2000  MSWin32    MSWin32-x86       2      5 00
	    Windows XP	  MSWin32    MSWin32-x86       2      5 01
	    Windows 2003  MSWin32    MSWin32-x86       2      5 02
	    Windows CE	  MSWin32    ?		       3
	    Cygwin	  cygwin     cygwin

       The various MSWin32 Perl's can distinguish the OS they are running on
       via the value of the fifth element of the list returned from
       Win32::GetOSVersion().  For example:

	   if ($^O eq 'MSWin32') {
	       my @os_version_info = Win32::GetOSVersion();
	       print +('3.1','95','NT')[$os_version_info[4]],"\n";
	   }

       There are also Win32::IsWinNT() and Win32::IsWin95(), try "perldoc
       Win32", and as of libwin32 0.19 (not part of the core Perl
       distribution) Win32::GetOSName().  The very portable POSIX::uname()
       will work too:

	   c:\> perl -MPOSIX -we "print join '|', uname"
	   Windows NT|moonru|5.0|Build 2195 (Service Pack 2)|x86

       Also see:

       ·   The djgpp environment for DOS, http://www.delorie.com/djgpp/ and
	   perldos.

       ·   The EMX environment for DOS, OS/2, etc. emx@iaehv.nl,
	   http://www.leo.org/pub/comp/os/os2/leo/gnu/emx+gcc/index.html or
	   ftp://hobbes.nmsu.edu/pub/os2/dev/emx/  Also perlos2.

       ·   Build instructions for Win32 in perlwin32, or under the Cygnus
	   environment in perlcygwin.

       ·   The "Win32::*" modules in Win32.

       ·   The ActiveState Pages, http://www.activestate.com/

       ·   The Cygwin environment for Win32; README.cygwin (installed as
	   perlcygwin), http://www.cygwin.com/

       ·   The U/WIN environment for Win32,
	   http://www.research.att.com/sw/tools/uwin/

       ·   Build instructions for OS/2, perlos2

   Mac OS
       Any module requiring XS compilation is right out for most people,
       because MacPerl is built using non-free (and non-cheap!) compilers.
       Some XS modules that can work with MacPerl are built and distributed in
       binary form on CPAN.

       Directories are specified as:

	   volume:folder:file		   for absolute pathnames
	   volume:folder:		   for absolute pathnames
	   :folder:file			   for relative pathnames
	   :folder:			   for relative pathnames
	   :file			   for relative pathnames
	   file				   for relative pathnames

       Files are stored in the directory in alphabetical order.	 Filenames are
       limited to 31 characters, and may include any character except for null
       and ":", which is reserved as the path separator.

       Instead of "flock", see "FSpSetFLock" and "FSpRstFLock" in the
       Mac::Files module, or "chmod(0444, ...)" and "chmod(0666, ...)".

       In the MacPerl application, you can't run a program from the command
       line; programs that expect @ARGV to be populated can be edited with
       something like the following, which brings up a dialog box asking for
       the command line arguments.

	   if (!@ARGV) {
	       @ARGV = split /\s+/, MacPerl::Ask('Arguments?');
	   }

       A MacPerl script saved as a "droplet" will populate @ARGV with the full
       pathnames of the files dropped onto the script.

       Mac users can run programs under a type of command line interface under
       MPW (Macintosh Programmer's Workshop, a free development environment
       from Apple).  MacPerl was first introduced as an MPW tool, and MPW can
       be used like a shell:

	   perl myscript.plx some arguments

       ToolServer is another app from Apple that provides access to MPW tools
       from MPW and the MacPerl app, which allows MacPerl programs to use
       "system", backticks, and piped "open".

       "Mac OS" is the proper name for the operating system, but the value in
       $^O is "MacOS".	To determine architecture, version, or whether the
       application or MPW tool version is running, check:

	   $is_app    = $MacPerl::Version =~ /App/;
	   $is_tool   = $MacPerl::Version =~ /MPW/;
	   ($version) = $MacPerl::Version =~ /^(\S+)/;
	   $is_ppc    = $MacPerl::Architecture eq 'MacPPC';
	   $is_68k    = $MacPerl::Architecture eq 'Mac68K';

       Mac OS X, based on NeXT's OpenStep OS, runs MacPerl natively, under the
       "Classic" environment.  There is no "Carbon" version of MacPerl to run
       under the primary Mac OS X environment.	Mac OS X and its Open Source
       version, Darwin, both run Unix perl natively.

       Also see:

       ·   MacPerl Development, http://dev.macperl.org/ .

       ·   The MacPerl Pages, http://www.macperl.com/ .

       ·   The MacPerl mailing lists, http://lists.perl.org/ .

       ·   MPW, ftp://ftp.apple.com/developer/Tool_Chest/Core_Mac_OS_Tools/

   VMS
       Perl on VMS is discussed in perlvms in the perl distribution.

       The official name of VMS as of this writing is OpenVMS.

       Perl on VMS can accept either VMS- or Unix-style file specifications as
       in either of the following:

	   $ perl -ne "print if /perl_setup/i" SYS$LOGIN:LOGIN.COM
	   $ perl -ne "print if /perl_setup/i" /sys$login/login.com

       but not a mixture of both as in:

	   $ perl -ne "print if /perl_setup/i" sys$login:/login.com
	   Can't open sys$login:/login.com: file specification syntax error

       Interacting with Perl from the Digital Command Language (DCL) shell
       often requires a different set of quotation marks than Unix shells do.
       For example:

	   $ perl -e "print ""Hello, world.\n"""
	   Hello, world.

       There are several ways to wrap your perl scripts in DCL .COM files, if
       you are so inclined.  For example:

	   $ write sys$output "Hello from DCL!"
	   $ if p1 .eqs. ""
	   $ then perl -x 'f$environment("PROCEDURE")
	   $ else perl -x - 'p1 'p2 'p3 'p4 'p5 'p6 'p7 'p8
	   $ deck/dollars="__END__"
	   #!/usr/bin/perl

	   print "Hello from Perl!\n";

	   __END__
	   $ endif

       Do take care with "$ ASSIGN/nolog/user SYS$COMMAND: SYS$INPUT" if your
       perl-in-DCL script expects to do things like "$read = <STDIN>;".

       The VMS operating system has two filesystems, known as ODS-2 and ODS-5.

       For ODS-2, filenames are in the format "name.extension;version".	 The
       maximum length for filenames is 39 characters, and the maximum length
       for extensions is also 39 characters.  Version is a number from 1 to
       32767.  Valid characters are "/[A-Z0-9$_-]/".

       The ODS-2 filesystem is case-insensitive and does not preserve case.
       Perl simulates this by converting all filenames to lowercase
       internally.

       For ODS-5, filenames may have almost any character in them and can
       include Unicode characters.  Characters that could be misinterpreted by
       the DCL shell or file parsing utilities need to be prefixed with the
       "^" character, or replaced with hexadecimal characters prefixed with
       the "^" character.  Such prefixing is only needed with the pathnames
       are in VMS format in applications.  Programs that can accept the UNIX
       format of pathnames do not need the escape characters.  The maximum
       length for filenames is 255 characters.	The ODS-5 file system can
       handle both a case preserved and a case sensitive mode.

       ODS-5 is only available on the OpenVMS for 64 bit platforms.

       Support for the extended file specifications is being done as optional
       settings to preserve backward compatibility with Perl scripts that
       assume the previous VMS limitations.

       In general routines on VMS that get a UNIX format file specification
       should return it in a UNIX format, and when they get a VMS format
       specification they should return a VMS format unless they are
       documented to do a conversion.

       For routines that generate return a file specification, VMS allows
       setting if the C library which Perl is built on if it will be returned
       in VMS format or in UNIX format.

       With the ODS-2 file system, there is not much difference in syntax of
       filenames without paths for VMS or UNIX.	 With the extended character
       set available with ODS-5 there can be a significant difference.

       Because of this, existing Perl scripts written for VMS were sometimes
       treating VMS and UNIX filenames interchangeably.	 Without the extended
       character set enabled, this behavior will mostly be maintained for
       backwards compatibility.

       When extended characters are enabled with ODS-5, the handling of UNIX
       formatted file specifications is to that of a UNIX system.

       VMS file specifications without extensions have a trailing dot.	An
       equivalent UNIX file specification should not show the trailing dot.

       The result of all of this, is that for VMS, for portable scripts, you
       can not depend on Perl to present the filenames in lowercase, to be
       case sensitive, and that the filenames could be returned in either UNIX
       or VMS format.

       And if a routine returns a file specification, unless it is intended to
       convert it, it should return it in the same format as it found it.

       "readdir" by default has traditionally returned lowercased filenames.
       When the ODS-5 support is enabled, it will return the exact case of the
       filename on the disk.

       Files without extensions have a trailing period on them, so doing a
       "readdir" in the default mode with a file named A.;5 will return a.
       when VMS is (though that file could be opened with "open(FH, 'A')").

       With support for extended file specifications and if "opendir" was
       given a UNIX format directory, a file named A.;5 will return a and
       optionally in the exact case on the disk.  When "opendir" is given a
       VMS format directory, then "readdir" should return a., and again with
       the optionally the exact case.

       RMS had an eight level limit on directory depths from any rooted
       logical (allowing 16 levels overall) prior to VMS 7.2, and even with
       versions of VMS on VAX up through 7.3.  Hence
       "PERL_ROOT:[LIB.2.3.4.5.6.7.8]" is a valid directory specification but
       "PERL_ROOT:[LIB.2.3.4.5.6.7.8.9]" is not.  Makefile.PL authors might
       have to take this into account, but at least they can refer to the
       former as "/PERL_ROOT/lib/2/3/4/5/6/7/8/".

       Pumpkings and module integrators can easily see whether files with too
       many directory levels have snuck into the core by running the following
       in the top-level source directory:

	  $ perl -ne "$_=~s/\s+.*//; print if scalar(split /\//) > 8;" < MANIFEST

       The VMS::Filespec module, which gets installed as part of the build
       process on VMS, is a pure Perl module that can easily be installed on
       non-VMS platforms and can be helpful for conversions to and from RMS
       native formats.	It is also now the only way that you should check to
       see if VMS is in a case sensitive mode.

       What "\n" represents depends on the type of file opened.	 It usually
       represents "\012" but it could also be "\015", "\012", "\015\012",
       "\000", "\040", or nothing depending on the file organization and
       record format.  The VMS::Stdio module provides access to the special
       fopen() requirements of files with unusual attributes on VMS.

       TCP/IP stacks are optional on VMS, so socket routines might not be
       implemented.  UDP sockets may not be supported.

       The TCP/IP library support for all current versions of VMS is
       dynamically loaded if present, so even if the routines are configured,
       they may return a status indicating that they are not implemented.

       The value of $^O on OpenVMS is "VMS".  To determine the architecture
       that you are running on without resorting to loading all of %Config you
       can examine the content of the @INC array like so:

	   if (grep(/VMS_AXP/, @INC)) {
	       print "I'm on Alpha!\n";

	   } elsif (grep(/VMS_VAX/, @INC)) {
	       print "I'm on VAX!\n";

	   } elsif (grep(/VMS_IA64/, @INC)) {
	       print "I'm on IA64!\n";

	   } else {
	       print "I'm not so sure about where $^O is...\n";
	   }

       In general, the significant differences should only be if Perl is
       running on VMS_VAX or one of the 64 bit OpenVMS platforms.

       On VMS, perl determines the UTC offset from the
       "SYS$TIMEZONE_DIFFERENTIAL" logical name.  Although the VMS epoch began
       at 17-NOV-1858 00:00:00.00, calls to "localtime" are adjusted to count
       offsets from 01-JAN-1970 00:00:00.00, just like Unix.

       Also see:

       ·   README.vms (installed as README_vms), perlvms

       ·   vmsperl list, vmsperl-subscribe@perl.org

       ·   vmsperl on the web, http://www.sidhe.org/vmsperl/index.html

   VOS
       Perl on VOS is discussed in README.vos in the perl distribution
       (installed as perlvos).	Perl on VOS can accept either VOS- or Unix-
       style file specifications as in either of the following:

	   $ perl -ne "print if /perl_setup/i" >system>notices
	   $ perl -ne "print if /perl_setup/i" /system/notices

       or even a mixture of both as in:

	   $ perl -ne "print if /perl_setup/i" >system/notices

       Even though VOS allows the slash character to appear in object names,
       because the VOS port of Perl interprets it as a pathname delimiting
       character, VOS files, directories, or links whose names contain a slash
       character cannot be processed.  Such files must be renamed before they
       can be processed by Perl.  Note that VOS limits file names to 32 or
       fewer characters, file names cannot start with a "-" character, or
       contain any character matching "tr/ !%&'()*+;<>?//"

       The value of $^O on VOS is "VOS".  To determine the architecture that
       you are running on without resorting to loading all of %Config you can
       examine the content of the @INC array like so:

	   if ($^O =~ /VOS/) {
	       print "I'm on a Stratus box!\n";
	   } else {
	       print "I'm not on a Stratus box!\n";
	       die;
	   }

       Also see:

       ·   README.vos (installed as perlvos)

       ·   The VOS mailing list.

	   There is no specific mailing list for Perl on VOS.  You can post
	   comments to the comp.sys.stratus newsgroup, or subscribe to the
	   general Stratus mailing list.  Send a letter with "subscribe Info-
	   Stratus" in the message body to majordomo@list.stratagy.com.

       ·   VOS Perl on the web at
	   http://ftp.stratus.com/pub/vos/posix/posix.html

   EBCDIC Platforms
       Recent versions of Perl have been ported to platforms such as OS/400 on
       AS/400 minicomputers as well as OS/390, VM/ESA, and BS2000 for S/390
       Mainframes.  Such computers use EBCDIC character sets internally
       (usually Character Code Set ID 0037 for OS/400 and either 1047 or
       POSIX-BC for S/390 systems).  On the mainframe perl currently works
       under the "Unix system services for OS/390" (formerly known as
       OpenEdition), VM/ESA OpenEdition, or the BS200 POSIX-BC system (BS2000
       is supported in perl 5.6 and greater).  See perlos390 for details.
       Note that for OS/400 there is also a port of Perl 5.8.1/5.9.0 or later
       to the PASE which is ASCII-based (as opposed to ILE which is EBCDIC-
       based), see perlos400.

       As of R2.5 of USS for OS/390 and Version 2.3 of VM/ESA these Unix sub-
       systems do not support the "#!" shebang trick for script invocation.
       Hence, on OS/390 and VM/ESA perl scripts can be executed with a header
       similar to the following simple script:

	   : # use perl
	       eval 'exec /usr/local/bin/perl -S $0 ${1+"$@"}'
		   if 0;
	   #!/usr/local/bin/perl     # just a comment really

	   print "Hello from perl!\n";

       OS/390 will support the "#!" shebang trick in release 2.8 and beyond.
       Calls to "system" and backticks can use POSIX shell syntax on all S/390
       systems.

       On the AS/400, if PERL5 is in your library list, you may need to wrap
       your perl scripts in a CL procedure to invoke them like so:

	   BEGIN
	     CALL PGM(PERL5/PERL) PARM('/QOpenSys/hello.pl')
	   ENDPGM

       This will invoke the perl script hello.pl in the root of the QOpenSys
       file system.  On the AS/400 calls to "system" or backticks must use CL
       syntax.

       On these platforms, bear in mind that the EBCDIC character set may have
       an effect on what happens with some perl functions (such as "chr",
       "pack", "print", "printf", "ord", "sort", "sprintf", "unpack"), as well
       as bit-fiddling with ASCII constants using operators like "^", "&" and
       "|", not to mention dealing with socket interfaces to ASCII computers
       (see "Newlines").

       Fortunately, most web servers for the mainframe will correctly
       translate the "\n" in the following statement to its ASCII equivalent
       ("\r" is the same under both Unix and OS/390 & VM/ESA):

	   print "Content-type: text/html\r\n\r\n";

       The values of $^O on some of these platforms includes:

	   uname	 $^O	    $Config{'archname'}
	   --------------------------------------------
	   OS/390	 os390	    os390
	   OS400	 os400	    os400
	   POSIX-BC	 posix-bc   BS2000-posix-bc
	   VM/ESA	 vmesa	    vmesa

       Some simple tricks for determining if you are running on an EBCDIC
       platform could include any of the following (perhaps all):

	   if ("\t" eq "\05")	{ print "EBCDIC may be spoken here!\n"; }

	   if (ord('A') == 193) { print "EBCDIC may be spoken here!\n"; }

	   if (chr(169) eq 'z') { print "EBCDIC may be spoken here!\n"; }

       One thing you may not want to rely on is the EBCDIC encoding of
       punctuation characters since these may differ from code page to code
       page (and once your module or script is rumoured to work with EBCDIC,
       folks will want it to work with all EBCDIC character sets).

       Also see:

       ·   perlos390, README.os390, perlbs2000, README.vmesa, perlebcdic.

       ·   The perl-mvs@perl.org list is for discussion of porting issues as
	   well as general usage issues for all EBCDIC Perls.  Send a message
	   body of "subscribe perl-mvs" to majordomo@perl.org.

       ·   AS/400 Perl information at http://as400.rochester.ibm.com/ as well
	   as on CPAN in the ports/ directory.

   Acorn RISC OS
       Because Acorns use ASCII with newlines ("\n") in text files as "\012"
       like Unix, and because Unix filename emulation is turned on by default,
       most simple scripts will probably work "out of the box".	 The native
       filesystem is modular, and individual filesystems are free to be case-
       sensitive or insensitive, and are usually case-preserving.  Some native
       filesystems have name length limits, which file and directory names are
       silently truncated to fit.  Scripts should be aware that the standard
       filesystem currently has a name length limit of 10 characters, with up
       to 77 items in a directory, but other filesystems may not impose such
       limitations.

       Native filenames are of the form

	   Filesystem#Special_Field::DiskName.$.Directory.Directory.File

       where

	   Special_Field is not usually present, but may contain . and $ .
	   Filesystem =~ m|[A-Za-z0-9_]|
	   DsicName   =~ m|[A-Za-z0-9_/]|
	   $ represents the root directory
	   . is the path separator
	   @ is the current directory (per filesystem but machine global)
	   ^ is the parent directory
	   Directory and File =~ m|[^\0- "\.\$\%\&:\@\\^\|\177]+|

       The default filename translation is roughly "tr|/.|./|;"

       Note that ""ADFS::HardDisk.$.File" ne 'ADFS::HardDisk.$.File'" and that
       the second stage of "$" interpolation in regular expressions will fall
       foul of the $. if scripts are not careful.

       Logical paths specified by system variables containing comma-separated
       search lists are also allowed; hence "System:Modules" is a valid
       filename, and the filesystem will prefix "Modules" with each section of
       "System$Path" until a name is made that points to an object on disk.
       Writing to a new file "System:Modules" would be allowed only if
       "System$Path" contains a single item list.  The filesystem will also
       expand system variables in filenames if enclosed in angle brackets, so
       "<System$Dir>.Modules" would look for the file
       "$ENV{'System$Dir'} . 'Modules'".  The obvious implication of this is
       that fully qualified filenames can start with "<>" and should be
       protected when "open" is used for input.

       Because "." was in use as a directory separator and filenames could not
       be assumed to be unique after 10 characters, Acorn implemented the C
       compiler to strip the trailing ".c" ".h" ".s" and ".o" suffix from
       filenames specified in source code and store the respective files in
       subdirectories named after the suffix.  Hence files are translated:

	   foo.h	   h.foo
	   C:foo.h	   C:h.foo	  (logical path variable)
	   sys/os.h	   sys.h.os	  (C compiler groks Unix-speak)
	   10charname.c	   c.10charname
	   10charname.o	   o.10charname
	   11charname_.c   c.11charname	  (assuming filesystem truncates at 10)

       The Unix emulation library's translation of filenames to native assumes
       that this sort of translation is required, and it allows a user-defined
       list of known suffixes that it will transpose in this fashion.  This
       may seem transparent, but consider that with these rules
       "foo/bar/baz.h" and "foo/bar/h/baz" both map to "foo.bar.h.baz", and
       that "readdir" and "glob" cannot and do not attempt to emulate the
       reverse mapping.	 Other "."'s in filenames are translated to "/".

       As implied above, the environment accessed through %ENV is global, and
       the convention is that program specific environment variables are of
       the form "Program$Name".	 Each filesystem maintains a current
       directory, and the current filesystem's current directory is the global
       current directory.  Consequently, sociable programs don't change the
       current directory but rely on full pathnames, and programs (and
       Makefiles) cannot assume that they can spawn a child process which can
       change the current directory without affecting its parent (and everyone
       else for that matter).

       Because native operating system filehandles are global and are
       currently allocated down from 255, with 0 being a reserved value, the
       Unix emulation library emulates Unix filehandles.  Consequently, you
       can't rely on passing "STDIN", "STDOUT", or "STDERR" to your children.

       The desire of users to express filenames of the form "<Foo$Dir>.Bar" on
       the command line unquoted causes problems, too: "``" command output
       capture has to perform a guessing game.	It assumes that a string
       "<[^<>]+\$[^<>]>" is a reference to an environment variable, whereas
       anything else involving "<" or ">" is redirection, and generally
       manages to be 99% right.	 Of course, the problem remains that scripts
       cannot rely on any Unix tools being available, or that any tools found
       have Unix-like command line arguments.

       Extensions and XS are, in theory, buildable by anyone using free tools.
       In practice, many don't, as users of the Acorn platform are used to
       binary distributions.  MakeMaker does run, but no available make
       currently copes with MakeMaker's makefiles; even if and when this
       should be fixed, the lack of a Unix-like shell will cause problems with
       makefile rules, especially lines of the form "cd sdbm && make all", and
       anything using quoting.

       "RISC OS" is the proper name for the operating system, but the value in
       $^O is "riscos" (because we don't like shouting).

   Other perls
       Perl has been ported to many platforms that do not fit into any of the
       categories listed above.	 Some, such as AmigaOS, Atari MiNT, BeOS, HP
       MPE/iX, QNX, Plan 9, and VOS, have been well-integrated into the
       standard Perl source code kit.  You may need to see the ports/
       directory on CPAN for information, and possibly binaries, for the likes
       of: aos, Atari ST, lynxos, riscos, Novell Netware, Tandem Guardian,
       etc.  (Yes, we know that some of these OSes may fall under the Unix
       category, but we are not a standards body.)

       Some approximate operating system names and their $^O values in the
       "OTHER" category include:

	   OS		 $^O	    $Config{'archname'}
	   ------------------------------------------
	   Amiga DOS	 amigaos    m68k-amigos
	   BeOS		 beos
	   MPE/iX	 mpeix	    PA-RISC1.1

       See also:

       ·   Amiga, README.amiga (installed as perlamiga).

       ·   Atari, README.mint and Guido Flohr's web page
	   http://stud.uni-sb.de/~gufl0000/

       ·   Be OS, README.beos

       ·   HP 300 MPE/iX, README.mpeix and Mark Bixby's web page
	   http://www.bixby.org/mark/perlix.html

       ·   A free perl5-based PERL.NLM for Novell Netware is available in
	   precompiled binary and source code form from http://www.novell.com/
	   as well as from CPAN.

       ·   Plan 9, README.plan9

FUNCTION IMPLEMENTATIONS
       Listed below are functions that are either completely unimplemented or
       else have been implemented differently on various platforms.  Following
       each description will be, in parentheses, a list of platforms that the
       description applies to.

       The list may well be incomplete, or even wrong in some places.  When in
       doubt, consult the platform-specific README files in the Perl source
       distribution, and any other documentation resources accompanying a
       given port.

       Be aware, moreover, that even among Unix-ish systems there are
       variations.

       For many functions, you can also query %Config, exported by default
       from the Config module.	For example, to check whether the platform has
       the "lstat" call, check $Config{d_lstat}.  See Config for a full
       description of available variables.

   Alphabetical Listing of Perl Functions
       -X      "-r", "-w", and "-x" have a limited meaning only; directories
	       and applications are executable, and there are no uid/gid
	       considerations.	"-o" is not supported.	(Mac OS)

	       "-w" only inspects the read-only file attribute
	       (FILE_ATTRIBUTE_READONLY), which determines whether the
	       directory can be deleted, not whether it can be written to.
	       Directories always have read and write access unless denied by
	       discretionary access control lists (DACLs).  (Win32)

	       "-r", "-w", "-x", and "-o" tell whether the file is accessible,
	       which may not reflect UIC-based file protections.  (VMS)

	       "-s" returns the size of the data fork, not the total size of
	       data fork plus resource fork.  (Mac OS).

	       "-s" by name on an open file will return the space reserved on
	       disk, rather than the current extent.  "-s" on an open
	       filehandle returns the current size.  (RISC OS)

	       "-R", "-W", "-X", "-O" are indistinguishable from "-r", "-w",
	       "-x", "-o". (Mac OS, Win32, VMS, RISC OS)

	       "-b", "-c", "-k", "-g", "-p", "-u", "-A" are not implemented.
	       (Mac OS)

	       "-g", "-k", "-l", "-p", "-u", "-A" are not particularly
	       meaningful.  (Win32, VMS, RISC OS)

	       "-d" is true if passed a device spec without an explicit
	       directory.  (VMS)

	       "-T" and "-B" are implemented, but might misclassify Mac text
	       files with foreign characters; this is the case will all
	       platforms, but may affect Mac OS often.	(Mac OS)

	       "-x" (or "-X") determine if a file ends in one of the
	       executable suffixes.  "-S" is meaningless.  (Win32)

	       "-x" (or "-X") determine if a file has an executable file type.
	       (RISC OS)

       atan2   Due to issues with various CPUs, math libraries, compilers, and
	       standards, results for "atan2()" may vary depending on any
	       combination of the above.  Perl attempts to conform to the Open
	       Group/IEEE standards for the results returned from "atan2()",
	       but cannot force the issue if the system Perl is run on does
	       not allow it.  (Tru64, HP-UX 10.20)

	       The current version of the standards for "atan2()" is available
	       at
	       <http://www.opengroup.org/onlinepubs/009695399/functions/atan2.html>.

       binmode Meaningless.  (Mac OS, RISC OS)

	       Reopens file and restores pointer; if function fails,
	       underlying filehandle may be closed, or pointer may be in a
	       different position.  (VMS)

	       The value returned by "tell" may be affected after the call,
	       and the filehandle may be flushed. (Win32)

       chmod   Only limited meaning.  Disabling/enabling write permission is
	       mapped to locking/unlocking the file. (Mac OS)

	       Only good for changing "owner" read-write access, "group", and
	       "other" bits are meaningless. (Win32)

	       Only good for changing "owner" and "other" read-write access.
	       (RISC OS)

	       Access permissions are mapped onto VOS access-control list
	       changes. (VOS)

	       The actual permissions set depend on the value of the "CYGWIN"
	       in the SYSTEM environment settings.  (Cygwin)

       chown   Not implemented. (Mac OS, Win32, Plan 9, RISC OS)

	       Does nothing, but won't fail. (Win32)

	       A little funky, because VOS's notion of ownership is a little
	       funky (VOS).

       chroot  Not implemented. (Mac OS, Win32, VMS, Plan 9, RISC OS, VOS,
	       VM/ESA)

       crypt   May not be available if library or source was not provided when
	       building perl. (Win32)

       dbmclose
	       Not implemented. (VMS, Plan 9, VOS)

       dbmopen Not implemented. (VMS, Plan 9, VOS)

       dump    Not useful. (Mac OS, RISC OS)

	       Not supported. (Cygwin, Win32)

	       Invokes VMS debugger. (VMS)

       exec    Not implemented. (Mac OS)

	       Implemented via Spawn. (VM/ESA)

	       Does not automatically flush output handles on some platforms.
	       (SunOS, Solaris, HP-UX)

       exit    Emulates UNIX exit() (which considers "exit 1" to indicate an
	       error) by mapping the 1 to SS$_ABORT (44).  This behavior may
	       be overridden with the pragma "use vmsish 'exit'".  As with the
	       CRTL's exit() function, "exit 0" is also mapped to an exit
	       status of SS$_NORMAL (1); this mapping cannot be overridden.
	       Any other argument to exit() is used directly as Perl's exit
	       status.	On VMS, unless the future POSIX_EXIT mode is enabled,
	       the exit code should always be a valid VMS exit code and not a
	       generic number.	When the POSIX_EXIT mode is enabled, a generic
	       number will be encoded in a method compatible with the C
	       library _POSIX_EXIT macro so that it can be decoded by other
	       programs, particularly ones written in C, like the GNV package.
	       (VMS)

       fcntl   Not implemented. (Win32) Some functions available based on the
	       version of VMS. (VMS)

       flock   Not implemented (Mac OS, VMS, RISC OS, VOS).

	       Available only on Windows NT (not on Windows 95). (Win32)

       fork    Not implemented. (Mac OS, AmigaOS, RISC OS, VM/ESA, VMS)

	       Emulated using multiple interpreters.  See perlfork.  (Win32)

	       Does not automatically flush output handles on some platforms.
	       (SunOS, Solaris, HP-UX)

       getlogin
	       Not implemented. (Mac OS, RISC OS)

       getpgrp Not implemented. (Mac OS, Win32, VMS, RISC OS)

       getppid Not implemented. (Mac OS, Win32, RISC OS)

       getpriority
	       Not implemented. (Mac OS, Win32, VMS, RISC OS, VOS, VM/ESA)

       getpwnam
	       Not implemented. (Mac OS, Win32)

	       Not useful. (RISC OS)

       getgrnam
	       Not implemented. (Mac OS, Win32, VMS, RISC OS)

       getnetbyname
	       Not implemented. (Mac OS, Win32, Plan 9)

       getpwuid
	       Not implemented. (Mac OS, Win32)

	       Not useful. (RISC OS)

       getgrgid
	       Not implemented. (Mac OS, Win32, VMS, RISC OS)

       getnetbyaddr
	       Not implemented. (Mac OS, Win32, Plan 9)

       getprotobynumber
	       Not implemented. (Mac OS)

       getservbyport
	       Not implemented. (Mac OS)

       getpwent
	       Not implemented. (Mac OS, Win32, VM/ESA)

       getgrent
	       Not implemented. (Mac OS, Win32, VMS, VM/ESA)

       gethostbyname
	       "gethostbyname('localhost')" does not work everywhere: you may
	       have to use "gethostbyname('127.0.0.1')". (Mac OS, Irix 5)

       gethostent
	       Not implemented. (Mac OS, Win32)

       getnetent
	       Not implemented. (Mac OS, Win32, Plan 9)

       getprotoent
	       Not implemented. (Mac OS, Win32, Plan 9)

       getservent
	       Not implemented. (Win32, Plan 9)

       sethostent
	       Not implemented. (Mac OS, Win32, Plan 9, RISC OS)

       setnetent
	       Not implemented. (Mac OS, Win32, Plan 9, RISC OS)

       setprotoent
	       Not implemented. (Mac OS, Win32, Plan 9, RISC OS)

       setservent
	       Not implemented. (Plan 9, Win32, RISC OS)

       endpwent
	       Not implemented. (Mac OS, MPE/iX, VM/ESA, Win32)

       endgrent
	       Not implemented. (Mac OS, MPE/iX, RISC OS, VM/ESA, VMS, Win32)

       endhostent
	       Not implemented. (Mac OS, Win32)

       endnetent
	       Not implemented. (Mac OS, Win32, Plan 9)

       endprotoent
	       Not implemented. (Mac OS, Win32, Plan 9)

       endservent
	       Not implemented. (Plan 9, Win32)

       getsockopt SOCKET,LEVEL,OPTNAME
	       Not implemented. (Plan 9)

       glob    This operator is implemented via the File::Glob extension on
	       most platforms.	See File::Glob for portability information.

       gmtime  Same portability caveats as localtime.

       ioctl FILEHANDLE,FUNCTION,SCALAR
	       Not implemented. (VMS)

	       Available only for socket handles, and it does what the
	       ioctlsocket() call in the Winsock API does. (Win32)

	       Available only for socket handles. (RISC OS)

       kill    "kill(0, LIST)" is implemented for the sake of taint checking;
	       use with other signals is unimplemented. (Mac OS)

	       Not implemented, hence not useful for taint checking. (RISC OS)

	       "kill()" doesn't have the semantics of "raise()", i.e. it
	       doesn't send a signal to the identified process like it does on
	       Unix platforms.	Instead "kill($sig, $pid)" terminates the
	       process identified by $pid, and makes it exit immediately with
	       exit status $sig.  As in Unix, if $sig is 0 and the specified
	       process exists, it returns true without actually terminating
	       it. (Win32)

	       "kill(-9, $pid)" will terminate the process specified by $pid
	       and recursively all child processes owned by it.	 This is
	       different from the Unix semantics, where the signal will be
	       delivered to all processes in the same process group as the
	       process specified by $pid. (Win32)

	       Is not supported for process identification number of 0 or
	       negative numbers. (VMS)

       link    Not implemented. (Mac OS, MPE/iX, RISC OS)

	       Link count not updated because hard links are not quite that
	       hard (They are sort of half-way between hard and soft links).
	       (AmigaOS)

	       Hard links are implemented on Win32 under NTFS only. They are
	       natively supported on Windows 2000 and later.  On Windows NT
	       they are implemented using the Windows POSIX subsystem support
	       and the Perl process will need Administrator or Backup Operator
	       privileges to create hard links.

	       Available on 64 bit OpenVMS 8.2 and later.  (VMS)

       localtime
	       Because Perl currently relies on the native standard C
	       localtime() function, it is only safe to use times between 0
	       and (2**31)-1.  Times outside this range may result in
	       unexpected behavior depending on your operating system's
	       implementation of localtime().

       lstat   Not implemented. (RISC OS)

	       Return values (especially for device and inode) may be bogus.
	       (Win32)

       msgctl
       msgget
       msgsnd
       msgrcv  Not implemented. (Mac OS, Win32, VMS, Plan 9, RISC OS, VOS)

       open    The "|" variants are supported only if ToolServer is installed.
	       (Mac OS)

	       open to "|-" and "-|" are unsupported. (Mac OS, Win32, RISC OS)

	       Opening a process does not automatically flush output handles
	       on some platforms.  (SunOS, Solaris, HP-UX)

       pipe    Very limited functionality. (MiNT)

       readlink
	       Not implemented. (Win32, VMS, RISC OS)

       rename  Can't move directories between directories on different logical
	       volumes. (Win32)

       select  Only implemented on sockets. (Win32, VMS)

	       Only reliable on sockets. (RISC OS)

	       Note that the "select FILEHANDLE" form is generally portable.

       semctl
       semget
       semop   Not implemented. (Mac OS, Win32, VMS, RISC OS, VOS)

       setgrent
	       Not implemented. (Mac OS, MPE/iX, VMS, Win32, RISC OS, VOS)

       setpgrp Not implemented. (Mac OS, Win32, VMS, RISC OS, VOS)

       setpriority
	       Not implemented. (Mac OS, Win32, VMS, RISC OS, VOS)

       setpwent
	       Not implemented. (Mac OS, MPE/iX, Win32, RISC OS, VOS)

       setsockopt
	       Not implemented. (Plan 9)

       shmctl
       shmget
       shmread
       shmwrite
	       Not implemented. (Mac OS, Win32, VMS, RISC OS, VOS)

       sockatmark
	       A relatively recent addition to socket functions, may not be
	       implemented even in UNIX platforms.

       socketpair
	       Not implemented. (RISC OS, VOS, VM/ESA)

	       Available on 64 bit OpenVMS 8.2 and later.  (VMS)

       stat    Platforms that do not have rdev, blksize, or blocks will return
	       these as '', so numeric comparison or manipulation of these
	       fields may cause 'not numeric' warnings.

	       mtime and atime are the same thing, and ctime is creation time
	       instead of inode change time. (Mac OS).

	       ctime not supported on UFS (Mac OS X).

	       ctime is creation time instead of inode change time  (Win32).

	       device and inode are not meaningful.  (Win32)

	       device and inode are not necessarily reliable.  (VMS)

	       mtime, atime and ctime all return the last modification time.
	       Device and inode are not necessarily reliable.  (RISC OS)

	       dev, rdev, blksize, and blocks are not available.  inode is not
	       meaningful and will differ between stat calls on the same file.
	       (os2)

	       some versions of cygwin when doing a stat("foo") and if not
	       finding it may then attempt to stat("foo.exe") (Cygwin)

	       On Win32 stat() needs to open the file to determine the link
	       count and update attributes that may have been changed through
	       hard links.  Setting ${^WIN32_SLOPPY_STAT} to a true value
	       speeds up stat() by not performing this operation. (Win32)

       symlink Not implemented. (Win32, RISC OS)

	       Implemented on 64 bit VMS 8.3.  VMS requires the symbolic link
	       to be in Unix syntax if it is intended to resolve to a valid
	       path.

       syscall Not implemented. (Mac OS, Win32, VMS, RISC OS, VOS, VM/ESA)

       sysopen The traditional "0", "1", and "2" MODEs are implemented with
	       different numeric values on some systems.  The flags exported
	       by "Fcntl" (O_RDONLY, O_WRONLY, O_RDWR) should work everywhere
	       though.	(Mac OS, OS/390, VM/ESA)

       system  Only implemented if ToolServer is installed. (Mac OS)

	       As an optimization, may not call the command shell specified in
	       $ENV{PERL5SHELL}.  "system(1, @args)" spawns an external
	       process and immediately returns its process designator, without
	       waiting for it to terminate.  Return value may be used
	       subsequently in "wait" or "waitpid".  Failure to spawn() a
	       subprocess is indicated by setting $? to "255 << 8".  $? is set
	       in a way compatible with Unix (i.e. the exitstatus of the
	       subprocess is obtained by "$? >> 8", as described in the
	       documentation).	(Win32)

	       There is no shell to process metacharacters, and the native
	       standard is to pass a command line terminated by "\n" "\r" or
	       "\0" to the spawned program.  Redirection such as "> foo" is
	       performed (if at all) by the run time library of the spawned
	       program.	 "system" list will call the Unix emulation library's
	       "exec" emulation, which attempts to provide emulation of the
	       stdin, stdout, stderr in force in the parent, providing the
	       child program uses a compatible version of the emulation
	       library.	 scalar will call the native command line direct and
	       no such emulation of a child Unix program will exists.  Mileage
	       will vary.  (RISC OS)

	       Far from being POSIX compliant.	Because there may be no
	       underlying /bin/sh tries to work around the problem by forking
	       and execing the first token in its argument string.  Handles
	       basic redirection ("<" or ">") on its own behalf. (MiNT)

	       Does not automatically flush output handles on some platforms.
	       (SunOS, Solaris, HP-UX)

	       The return value is POSIX-like (shifted up by 8 bits), which
	       only allows room for a made-up value derived from the severity
	       bits of the native 32-bit condition code (unless overridden by
	       "use vmsish 'status'").	If the native condition code is one
	       that has a POSIX value encoded, the POSIX value will be decoded
	       to extract the expected exit value.  For more details see "$?"
	       in perlvms. (VMS)

       times   Only the first entry returned is nonzero. (Mac OS)

	       "cumulative" times will be bogus.  On anything other than
	       Windows NT or Windows 2000, "system" time will be bogus, and
	       "user" time is actually the time returned by the clock()
	       function in the C runtime library. (Win32)

	       Not useful. (RISC OS)

       truncate
	       Not implemented. (Older versions of VMS)

	       Truncation to same-or-shorter lengths only. (VOS)

	       If a FILEHANDLE is supplied, it must be writable and opened in
	       append mode (i.e., use "open(FH, '>>filename')" or
	       "sysopen(FH,...,O_APPEND|O_RDWR)".  If a filename is supplied,
	       it should not be held open elsewhere. (Win32)

       umask   Returns undef where unavailable, as of version 5.005.

	       "umask" works but the correct permissions are set only when the
	       file is finally closed. (AmigaOS)

       utime   Only the modification time is updated. (BeOS, Mac OS, VMS,
	       RISC OS)

	       May not behave as expected.  Behavior depends on the C runtime
	       library's implementation of utime(), and the filesystem being
	       used.  The FAT filesystem typically does not support an "access
	       time" field, and it may limit timestamps to a granularity of
	       two seconds. (Win32)

       wait
       waitpid Not implemented. (Mac OS)

	       Can only be applied to process handles returned for processes
	       spawned using "system(1, ...)" or pseudo processes created with
	       "fork()". (Win32)

	       Not useful. (RISC OS)

Supported Platforms
       As of July 2002 (the Perl release 5.8.0), the following platforms are
       able to build Perl from the standard source code distribution available
       at http://www.cpan.org/src/index.html

	       AIX
	       BeOS
	       BSD/OS	       (BSDi)
	       Cygwin
	       DG/UX
	       DOS DJGPP       1)
	       DYNIX/ptx
	       EPOC R5
	       FreeBSD
	       HI-UXMPP	       (Hitachi) (5.8.0 worked but we didn't know it)
	       HP-UX
	       IRIX
	       Linux
	       Mac OS Classic
	       Mac OS X	       (Darwin)
	       MPE/iX
	       NetBSD
	       NetWare
	       NonStop-UX
	       ReliantUNIX     (formerly SINIX)
	       OpenBSD
	       OpenVMS	       (formerly VMS)
	       Open UNIX       (Unixware) (since Perl 5.8.1/5.9.0)
	       OS/2
	       OS/400	       (using the PASE) (since Perl 5.8.1/5.9.0)
	       PowerUX
	       POSIX-BC	       (formerly BS2000)
	       QNX
	       Solaris
	       SunOS 4
	       SUPER-UX	       (NEC)
	       Tru64 UNIX      (formerly DEC OSF/1, Digital UNIX)
	       UNICOS
	       UNICOS/mk
	       UTS
	       VOS
	       Win95/98/ME/2K/XP 2)
	       WinCE
	       z/OS	       (formerly OS/390)
	       VM/ESA

	       1) in DOS mode either the DOS or OS/2 ports can be used
	       2) compilers: Borland, MinGW (GCC), VC6

       The following platforms worked with the previous releases (5.6 and
       5.7), but we did not manage either to fix or to test these in time for
       the 5.8.0 release.  There is a very good chance that many of these will
       work fine with the 5.8.0.

	       BSD/OS
	       DomainOS
	       Hurd
	       LynxOS
	       MachTen
	       PowerMAX
	       SCO SV
	       SVR4
	       Unixware
	       Windows 3.1

       Known to be broken for 5.8.0 (but 5.6.1 and 5.7.2 can be used):

	       AmigaOS

       The following platforms have been known to build Perl from source in
       the past (5.005_03 and earlier), but we haven't been able to verify
       their status for the current release, either because the
       hardware/software platforms are rare or because we don't have an active
       champion on these platforms--or both.  They used to work, though, so go
       ahead and try compiling them, and let perlbug@perl.org of any trouble.

	       3b1
	       A/UX
	       ConvexOS
	       CX/UX
	       DC/OSx
	       DDE SMES
	       DOS EMX
	       Dynix
	       EP/IX
	       ESIX
	       FPS
	       GENIX
	       Greenhills
	       ISC
	       MachTen 68k
	       MiNT
	       MPC
	       NEWS-OS
	       NextSTEP
	       OpenSTEP
	       Opus
	       Plan 9
	       RISC/os
	       SCO ODT/OSR
	       Stellar
	       SVR2
	       TI1500
	       TitanOS
	       Ultrix
	       Unisys Dynix

       The following platforms have their own source code distributions and
       binaries available via http://www.cpan.org/ports/

				       Perl release

	       OS/400 (ILE)	       5.005_02
	       Tandem Guardian	       5.004

       The following platforms have only binaries available via
       http://www.cpan.org/ports/index.html :

				       Perl release

	       Acorn RISCOS	       5.005_02
	       AOS		       5.002
	       LynxOS		       5.004_02

       Although we do suggest that you always build your own Perl from the
       source code, both for maximal configurability and for security, in case
       you are in a hurry you can check http://www.cpan.org/ports/index.html
       for binary distributions.

SEE ALSO
       perlaix, perlamiga, perlapollo, perlbeos, perlbs2000, perlce,
       perlcygwin, perldgux, perldos, perlepoc, perlebcdic, perlfreebsd,
       perlhurd, perlhpux, perlirix, perlmachten, perlmacos, perlmacosx,
       perlmint, perlmpeix, perlnetware, perlos2, perlos390, perlos400,
       perlplan9, perlqnx, perlsolaris, perltru64, perlunicode, perlvmesa,
       perlvms, perlvos, perlwin32, and Win32.

AUTHORS / CONTRIBUTORS
       Abigail <abigail@foad.org>, Charles Bailey <bailey@newman.upenn.edu>,
       Graham Barr <gbarr@pobox.com>, Tom Christiansen <tchrist@perl.com>,
       Nicholas Clark <nick@ccl4.org>, Thomas Dorner <Thomas.Dorner@start.de>,
       Andy Dougherty <doughera@lafayette.edu>, Dominic Dunlop
       <domo@computer.org>, Neale Ferguson <neale@vma.tabnsw.com.au>, David J.
       Fiander <davidf@mks.com>, Paul Green <Paul.Green@stratus.com>, M.J.T.
       Guy <mjtg@cam.ac.uk>, Jarkko Hietaniemi <jhi@iki.fi>, Luther Huffman
       <lutherh@stratcom.com>, Nick Ing-Simmons <nick@ing-simmons.net>,
       Andreas J. Koenig <a.koenig@mind.de>, Markus Laker
       <mlaker@contax.co.uk>, Andrew M. Langmead <aml@world.std.com>, Larry
       Moore <ljmoore@freespace.net>, Paul Moore
       <Paul.Moore@uk.origin-it.com>, Chris Nandor <pudge@pobox.com>, Matthias
       Neeracher <neeracher@mac.com>, Philip Newton <pne@cpan.org>, Gary Ng
       <71564.1743@CompuServe.COM>, Tom Phoenix <rootbeer@teleport.com>, Andre
       Pirard <A.Pirard@ulg.ac.be>, Peter Prymmer <pvhp@forte.com>, Hugo van
       der Sanden <hv@crypt0.demon.co.uk>, Gurusamy Sarathy
       <gsar@activestate.com>, Paul J. Schinder <schinder@pobox.com>, Michael
       G Schwern <schwern@pobox.com>, Dan Sugalski <dan@sidhe.org>, Nathan
       Torkington <gnat@frii.com>.  John Malmberg <wb8tyw@qsl.net>

perl v5.10.1			  2009-05-14			   PERLPORT(1)
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