mod_perl_traps(3) User Contributed Perl Documentation mod_perl_traps(3)NAMEmod_perl_traps - common/known mod_perl traps
DESCRIPTION
In the CGI environment, the server starts a single external process
(Perl interpreter) per HTTP request which runs single script in that
process space. When the request is over, the process goes away every-
thing is cleaned up and a fresh script is started for the next request.
mod_perl brings Perl inside of the HTTP server not only for speedup of
CGI scripts, but also for access to server functionality that CGI
scripts do not and/or cannot have. Now that we're inside the server,
each process will likely handle more than one Perl script and keep it
"compiled" in memory for longer than a single HTTP request. This new
location and longer lifetime of Perl execution brings with it some com-
mon traps. This document is here to tell you what they are and how to
prevent them. The descriptions here are short, please consult the
mod_perl FAQ for more detail. If you trip over something not docu-
mented here, please send a message to the mod_perl list.
Migrating from CGI
o Be sure to have read cgi_to_mod_perl
o Scripts under Apache::Registry are not run in package main, they
are run in a unique namespace based on the requested uri.
o Apache::Registry scripts cannot contain __END__ or __DATA__ tokens
o Output of "system", "exec" and "open PIPE, "|program"" calls will
not be sent to the browser unless you Perl was configured with
sfio.
o Perl's exit() built-in function cannot be used in mod_perl scripts.
The Apache::exit() function should be used instead. Apache::exit()
automatically overrides the built-in exit() for Apache::Registry
and Apache::PerlRun scripts.
o Your script *will not* run from the command line if your script
makes any direct calls to Apache->methods. See Apache::Fak-
eRequest.
Apache::Registry
undefined subroutine &Apache::Registry::handler
Interaction with certain modules causes the shortcut configuration
to break, if you see this message change your configuration from
this:
<Location /perl>
PerlHandler Apache::Registry
...
</Location>
To this:
PerlModule Apache::Registry
<Location /perl>
PerlHandler Apache::Registry::handler
...
</Location>
Using CGI.pm and CGI::*
o CGI.pm users must have version 2.39 of the package or higher, ear-
lier versions will not work under mod_perl.
o If you use the "SendHeaders()" function, be sure to call
$req_obj->cgi->done when you are done with a request, just as you
would under CGI::MiniSrv.
Perl Modules and Extensions
o Files pulled in via "use" or "require" statements are not automati-
cally reloaded when changed on disk. See the Apache::StatINC or
the Apache::Reload module to add this functionality.
Undefined subroutines
A common trap with required files may result in an error message
similar to this in the error_log:
[Thu Sep 11 11:03:06 1997] Undefined subroutine
&Apache::ROOT::perl::test_2epl::some_function called at
/opt/www/apache/perl/test.pl line 79.
As the above items explains, a file pulled in via "require" will
only happen once per-process (unless %INC is modified). If the
file does not contain a "package" declaration, the file's subrou-
tines and variables will be created in the current package. Under
CGI, this is commonly package "main". However, Apache::Registry
scripts are compiled into a unique package name (base on the uri).
So, if multiple scripts in the same process try to require the same
file, which does not declare a package, only one script will actu-
ally be able to see the subroutines. The solution is to read
perlmodlib, perlmod and related perl documentation and re-work your
required file into a module which exports functions or defines a
method interface. Or something more simple, along these lines:
#required_file.pl
package Test;
sub some_function {...}
...
__END__
Now, have your scripts say:
require "required_file.pl";
Test::some_function();
Undefined subroutine &Foo::Bar::handler called at PerlHandler subrou-
tine `Foo::Bar' line 1.
You mistyped the module name in the 'package' line in your module.
"Use of uninitialized value"
Because of eval context, you may see warnings with useless file-
name/line, example:
Use of uninitialized value at (eval 80) line 12.
Use of uninitialized value at (eval 80) line 43.
Use of uninitialized value at (eval 80) line 44.
To track down where this eval is really happening, try using a
__WARN__ handler to give you a stack trace:
use Carp ();
local $SIG{__WARN__} = \&Carp::cluck;
"Callback called exit"
"Out of memory!"
If something goes really wrong with your code, Perl may die with an
"Out of memory!" message and or "Callback called exit". A common
cause of this are never-ending loops, deep recursion or calling an
undefined subroutine. Here's one way to catch the problem: See
Perl's INSTALL document for this item:
-DPERL_EMERGENCY_SBRK
If PERL_EMERGENCY_SBRK is defined, running out of memory need not
be a fatal error: a memory pool can allocated by assigning to the
special variable $^M. See perlvar(1) for more details.
If you compile with that option and add 'use Apache::Debug level =>
4;' to your PerlScript, it will allocate the $^M emergency pool and
the $SIG{__DIE__} handler will call Carp::confess, giving you a
stack trace which should reveal where the problem is.
See the Apache::Resource module for prevention of spinning httpds.
o If you wish to use a module that is normally linked static with
your Perl, it must be listed in static_ext in Perl's Config.pm to
be linked with httpd during the mod_perl build.
Can't load '$Config{sitearchexp}/auto/Foo/Foo.so' for module Foo...
When starting httpd some people have reported seeing an error along
the lines of:
[Thu Jul 9 17:33:42 1998] [error] Can't load
'/usr/local/ap/lib/perl5/site_perl/sun4-solaris/auto/DBI/DBI.so' for
module DBI: ld.so.1: src/httpd: fatal: relocation error: file
/usr/local/ap/lib/perl5/site_perl/sun4-solaris/auto/DBI/DBI.so: symbol
Perl_sv_undef: referenced symbol not found at
/usr/local/ap/lib/perl5/sun4-solaris/5.00404/DynaLoader.pm line 166.
Or similar for the IO module or whatever dynamic module mod_perl
tries to pull in first. The solution is to re-configure, re-build
and re-install Perl and dynamic modules with the following flags
when Configure asks for "additional LD flags":
-Xlinker --export-dynamic
or
-Xlinker -E
This problem is only known to be caused by installing gnu ld under
Solaris.
Other known causes of this problem:
OS distributions that ship with a (broken) binary Perl installa-
tion.
The `perl' program and `libperl.a' library are somehow built with
different binary compatiblity flags.
The solution to these problems is to rebuild Perl and extension
modules from a fresh source tree. Tip for running Perl's Configure
script, use the `"-des"' flags to accepts defaults and `"-D"' flag
to override certain attributes:
% ./Configure -des -Dcc=gcc ... && make test && make install
Read Perl's INSTALL doc for more details.
Clashes with other Apache C modules
mod_auth_dbm
If you are a user of mod_auth_dbm or mod_auth_db, you may need to
edit Perl's "Config" module. When Perl is configured it attempts
to find libraries for ndbm, gdbm, db, etc., for the *DBM*_File mod-
ules. By default, these libraries are linked with Perl and remem-
bered by the Config module. When mod_perl is configured with
apache, the ExtUtils::Embed module returns these libraries to be
linked with httpd so Perl extensions will work under mod_perl.
However, the order in which these libraries are stored in Con-
fig.pm, may confuse "mod_auth_db*". If "mod_auth_db*" does not
work with mod_perl, take a look at this order with the following
command:
% perl -V:libs
If "-lgdbm" or "-ldb" is before "-lndbm", example:
libs='-lnet -lnsl_s -lgdbm -lndbm -ldb -ldld -lm -lc -lndir -lcrypt';
Edit Config.pm and move "-lgdbm" and "-ldb" to the end of the list.
Here's how to find Config.pm:
% perl -MConfig -e 'print "$Config{archlibexp}/Config.pm\n"'
Another solution for building Apache/mod_perl+mod_auth_dbm under
Solaris is to remove the DBM and NDBM "emulation" from libgdbm.a.
Seems Solaris already provides its own DBM and NDBM, and there's no
reason to build GDBM with them (for us anyway).
In our Makefile for GDBM, we changed
OBJS = $(DBM_OF) $(NDBM_OF) $(GDBM_OF)
to
OBJS = $(GDBM_OF)
Rebuild libgdbm, then Apache/mod_perl.
REGULAR EXPRESSIONS
COMPILED REGULAR EXPRESSIONS
When using a regular expression that contains an interpolated Perl
variable, if it is known that the variable (or variables) will not vary
during the execution of the program, a standard optimization technique
consists of adding the "o" modifier to the regexp pattern, to direct
the compiler to build the internal table once, for the entire lifetime
of the script, rather than every time the pattern is executed. Con-
sider:
my $pat = '^foo$'; # likely to be input from an HTML form field
foreach( @list ) {
print if /$pat/o;
}
This is usually a big win in loops over lists, or when using "grep" or
"map".
In long-lived "mod_perl" scripts, however, this can pose a problem if
the variable changes according to the invocation. The first invocation
of a fresh httpd child will compile the table and perform the search
correctly, however, all subsequent uses by the httpd child will con-
tinue to match the original pattern, regardless of the current contents
of the Perl variables the pattern is dependent on. Your script will
appear broken.
There are two solutions to this problem.
The first is to use "eval q//", to force the code to be evaluated each
time. Just make sure that the "eval" block covers the entire loop of
processing, and not just the pattern match itself.
The above code fragment would be rewritten as:
my $pat = '^foo$';
eval q{
foreach( @list ) {
print if /$pat/o;
}
}
Just saying
eval q{ print if /$pat/o; };
is going to be a horribly expensive proposition.
You use this approach if you require more than one pattern match opera-
tor in a given section of code. If the section contains only one opera-
tor (be it an "m//" or "s///"), you can rely on the property of the
null pattern, that reuses the last pattern seen. This leads to the sec-
ond solution, which also eliminates the use of "eval".
The above code fragment becomes:
my $pat = '^foo$';
"something" =~ /$pat/; # dummy match (MUST NOT FAIL!)
foreach( @list ) {
print if //;
}
The only gotcha is that the dummy match that boots the regular expres-
sion engine must absolutely, positively succeed, otherwise the pattern
will not be cached, and the // will match everything. If you can't
count on fixed text to ensure the match succeeds, you have two possi-
bilities.
If you can guaranteee that the pattern variable contains no meta-char-
acters (things like "*", "+", "^", "$"...), you can use the dummy
match:
"$pat" =~ /\Q$pat\E/; # guaranteed if no meta-characters present
If there is a possibility that the pattern can contain meta-characters,
you should search for the pattern or the unsearchable "\377" character
as follows:
"\377" =~ /$pat|^[\377]$/; # guarenteed if meta-characters present
References
The Camel Book, 2nd edition, p. 538 (p. 356 in the 1st edition).
AUTHORS
Doug MacEachern, with contributions from Jens Heunemann <heune-
mann2@janet.de>, David Landgren <david@landgren.net>, Mark Mills
<mark@ntr.net>, Randal Schwartz <merlyn@stonehenge.com> and Ask Bjoern
Hansen <ask@develooper.com>
perl v5.8.8 2007-03-30 mod_perl_traps(3)
