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Mod Perl Icon Mod Perl Icon Warnings and Errors Troubleshooting Index

Table of Contents:

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The Writing Apache Modules with Perl and C book can be purchased online from O'Reilly and Amazon.com.
Your corrections of the technical and grammatical errors are very welcome. You are encouraged to help me improve this guide. If you have something to contribute please send it directly to me.
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General Advice

Perl's warnings mode is immensely helpful in detecting possible problems. Make sure you always turn on warnings while you are developing code. See The Importance of Warnings.

Enabling use diagnostics; generally helps you to determine the source of the problem and how to solve it. See diagnostics pragma for more information.

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Building and Installation

See make Troubleshooting and make test Troubleshooting

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Configuration and Startup

This section talks about errors reported when you attempt to start the server.

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libexec/libperl.so: open failed: No such file or directory

If when you run the server you get the following error:

  libexec/libperl.so: open failed: No such file or directory

it probably means that Perl was compiled with a shared library. mod_perl does detect this and links the Apache executable to the Perl shared library (libperl.so).

First of all make sure you have Perl installed on the machine, and that you have libperl.so in <perlroot>/<version>/<architecture>/CORE. For example in /usr/local/lib/perl5/5.00503/sun4-solaris/CORE.

Then make sure that directory is included in the environment variable LD_LIBRARY_PRELOAD. Under normal circumstances, Apache should have the path configured at compile time, but this way you can override the library path.

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install_driver(Oracle) failed: Can't load '.../DBD/Oracle/Oracle.so' for module DBD::Oracle

  install_driver(Oracle) failed: Can't load
  for module DBD::Oracle:
  libclntsh.so.8.0: cannot open shared object file: 
  No such file or directory at
  /usr/lib/perl5/5.00503/i386-linux/DynaLoader.pm line 169. 
  at (eval 27) line 3
  Perhaps a required shared
  library or dll isn't installed where expected at 
  /usr/local/apache/perl/tmp.pl line 11

On BSD style filesystems LD_LIBRARY_PATH is not searched for setuid programs (a.k.a., Apache). This isn't a problem for CGI script since they don't do a setuid (and are forked off), but Apache does, and mod_perl is in Apache. Therefore the first solution is to explicitly load the library from the system wide ldconfig configuration file:

  # echo $ORACLE_HOME/lib >> /etc/ld.so.conf
  # ldconfig

Another solution to this problem is to modify the resulting Makefile ( after running perl Makefile.PL) as follows:

1. search for the line LD_RUN_PATH=

2. replace it with LD_RUN_PATH=(my_oracle_home)/lib

(my_oracle_home) is, of course, the home path to your oracle installation. In particular, the file libclntsh.so.8.0 should exist in that directory. (If you use CPAN, the build directory for DBD::Oracle should be in ~/.cpan/build/DBD-Oracle-1.06/ if you're logged in as root.)

Then, just type make install, and all should go well.

FYI, setting LD_RUN_PATH has the effect of hard-coding the path to (my_oracle_home)/lib in the resulting Oracle.so file generated by the DBD::Oracle so that at run-time, it doesn't have to go searching through LD_LIBRARY_PATH or the default directories used by ld.

For more information see the ld manpage and an essay on LD_LIBRARY_PATH: http://www.visi.com/~barr/ldpath.html

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Invalid command 'PerlHandler'...

  Syntax error on line 393 of /etc/httpd/conf/httpd.conf: Invalid
  command 'PerlHandler', perhaps mis-spelled or defined by a module
  not included in the server configuration [FAILED]

This can happen when you have a mod_perl enabled Apache compiled with DSO (generally it's an installed RPM or other binary package) but the mod_perl module isn't loaded. In this case you have to tell Apache to load mod_perl by adding:

  AddModule mod_perl.c

in your httpd.conf.

This can also happen when you try to run a non-mod_perl Apache server using the configuration from a mod_perl server.

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RegistryLoader: Translation of uri [...] to filename failed

  RegistryLoader: Translation of uri [/home/httpd/perl/test.pl] to filename 
                  failed [tried: /home/httpd/docs/home/httpd/perl/test.pl]

This error shows up when Apache::RegistryLoader fails to translate the URI into the corresponding filesystem path. Most failures happen when one passes a file path instead of URI. (A reminder: /home/httpd/perl/test.pl is a file path, while /perl/test.pl is a URI). In most cases all you have to do is to pass something that Apache::RegistryLoader expects to get - the URI, but there are more complex cases. Apache::RegistryLoader's man page shows how to handle these cases as well (look for the trans() sub).

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"Apache.pm failed to load!"

If your server startup fails with:

  Apache.pm failed to load!

try adding this to httpd.conf:

  PerlModule Apache

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Code Parsing and Compilation

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Value of $x will not stay shared at - line 5

my() Scoped Variable in Nested Subroutines.

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Value of $x may be unavailable at - line 5.

my() Scoped Variable in Nested Subroutines.

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Can't locate loadable object for module XXX

There is no object built for this module. e.g. when you see:

  Can't locate loadable object for module Apache::Util in @INC...

make sure to give mod_perl's Makefile.PL PERL_UTIL_API=1, EVERYTHING=1 or DYNAMIC=1 parameters to enable and build all the components of Apache::Util.

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Can't locate object method "get_handlers"...

  Can't locate object method "get_handlers" via package "Apache"

You need to rebuild your mod_perl with stacked handlers, i.e. PERL_STACKED_HANDLERS=1 or more simply EVERYTHING=1.

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Missing right bracket at line ...

Most often you will find that you really do have a syntax error. However the other reason might be that a script running under Apache::Registry is using __DATA__ or __END__ tokens. Learn why.

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Can't load '.../auto/DBI/DBI.so' for module DBI

Check that all your modules are compiled with the same Perl that is compiled into mod_perl. Perl 5.005 and 5.004 are not binary compatible by default.

Other known causes of this problem:

OS distributions that ship with a broken binary Perl installation.

The `perl' program and `libperl.a' library are somehow built with different binary compatibility flags.

The solution to these problems is to rebuild Perl and any 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 document for more details.

Solaris OS specific:

``Can't load DBI'' or similar error 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


  -Xlinker -E

This problem is only known to be caused by installing gnu ld under Solaris.

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"exit signal Segmentation fault (11)" with mysql

If you build mod_perl and php in the same binary, you might get Segmentation fault followed by this error:

  exit signal Segmentation fault (11)

Solution: re-compile PHP without the built-in MySQL support (you can still connect to MySQL).

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foo ... at /dev/null line 0

Under mod_perl you may receive a warning or an error in the error_log which specifies /dev/null as the source file, and line 0 as an line number where the printing of the message was triggered. This is absolutely normal if the code is executed from within a handler, because there is no actual file associated with the handler. Therefore $0 is set to /dev/null and that's what you see.

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Preventing mod_perl Processes From Going Wild

See the sections ``Non-Scheduled Emergency Log Rotation'' and ``All RAM Consumed''

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Segfaults when using XML::Parser

If you have some of the processes segfault when using XML::Parser you should use


during the Apache configuration step.

Starting from mod_perl version 1.23 this option is disabled by default.

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My CGI/Perl Code Gets Returned as Plain Text Instead of Being Executed by the Webserver

See My CGI/Perl Code Gets Returned as Plain Text Instead of Being Executed by the Webserver.

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Incorrect line number reporting in error/warn log messages

See Use of uninitialized value at (eval 80) line 12.

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rwrite returned -1

This message happens when the client breaks the connection while your script is trying to write to the client. With Apache 1.3.x, you should only see the rwrite messages if LogLevel is set to debug.

There was a bug that reported this debug message regardless of the value of the LogLevel directive. It was fixed in mod_perl 1.19_01 (CVS version).

Generally LogLevel is either debug or info. debug logs everything, info is the next level, which doesn't include debug messages. You shouldn't use ``debug'' mode on your production server. At the moment there is no way to prevent users from aborting connections.

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Can't upgrade that kind of scalar ...

Fixed in mod_perl 1.23.

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caught SIGPIPE in process

  [modperl] caught SIGPIPE in process 1234
  [modperl] process 1234 going to Apache::exit with status...

That's the $SIG{PIPE} handler installed by mod_perl/Apache::SIG, which is called if a connection times out or if the client presses the 'Stop' button. It gives you an opportunity to do cleanups if the script was aborted in the middle of its execution. See Handling the 'User pressed Stop button' case for more info.

If your mod_perl version is earlier than 1.17 you might also get the message in the following section...

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Client hit STOP or Netscape bit it!

  Client hit STOP or Netscape bit it!
  Process 2493 going to Apache::exit with status=-2

You may see this message in mod_perl versions less than 1.17. See also caught SIGPIPE in process.

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Global symbol "$foo" requires explicit package name

The script below will print a warning like that above, moreover it will print the whole script as a part of the warning message:

  #!/usr/bin/perl -w
  use strict;
  print "Content-type: text/html\n\n";
  print "Hello $undefined";

The warning:

  Global symbol "$undefined" requires 
  explicit package name at /usr/apps/foo/cgi/tmp.pl line 4.
          eval 'package Apache::ROOT::perl::tmp_2epl;
  use Apache qw(exit);sub handler {
  #line 1 /usr/apps/foo/cgi/tmp.pl
  BEGIN {$^W = 1;}#!/usr/bin/perl -w
  use strict;
  print "Content-type: text/html\\n\\n";
  print "Hello $undefined";
  ;' called at 
  line 168
        Apache::ROOT::perl::tmp_2epl;use Apache qw(exit);sub han...') 
        called at 
        line 121
        called at /usr/apps/foo/cgi/tmp.pl line 4
          eval {...} called at /usr/apps/foo/cgi/tmp.pl line 4
  [Sun Nov 15 15:15:30 1998] [error] Undefined subroutine 
  &Apache::ROOT::perl::tmp_2epl::handler called at 
  line 135.
  [Sun Nov 15 15:15:30 1998] [error] Goto undefined subroutine 
  &Apache::Constants::SERVER_ERROR at 
  line 23.

The error is simple to fix. When you use the use strict; pragma (and you should...), Perl will insist that all variables are defined before being used, so the error will not arise.

The bad thing is that sometimes the whole script (possibly, thousands of lines) is printed to the error_log file as code that the server has tried to eval()uate.

May be you have a $SIG{__DIE__} handler installed (Carp::confess()?). If so that's what's expected.

You might wish to try something more terse such as "local $SIG{__WARN__} = \&Carp::cluck;" The confess method is very verbose and will tell you more than you might wish to know including full source.

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Use of uninitialized value at (eval 80) line 12.

Your code includes some undefined variable that you have used as if it was already defined and initialized. For example:

  $param = $q->param('test');
  print $param;


  $param = $q->param('test') || '';
  print $param;

In the second case, $param will always be defined, either with $q->param('test')'s return value or the default value ('' empty string in our example).

Also read about Finding the Line Which Triggered the Error or Warning.

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Undefined subroutine &Apache::ROOT::perl::test_2epl::some_function called at

See Names collisions with Modules and libs.

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Callback called exit

Callback called exit is just a generic message when some unrecoverable error occurs inside Perl during perl_call_sv() (which mod_perl uses to invoke all handler subroutines. Such problems seem to occur far less with 5.005_03 than 5.004.

Sometimes you discover that your server is not responding and its error_log has filled up the remaining space on the file system. When you get to see the contents of the error_log -- it includes millions of lines, like:

  Callback called exit at -e line 33, <HTML> chunk 1.

Why the looping?

Perl can get very confused inside an infinite loop in your code. It doesn't necessarily mean that your code did call exit(). Perl's malloc went haywire and called croak(), but no memory is left to properly report the error, so Perl is stuck in a loop writing that same message to stderr.

Perl 5.005+ plus is recommended for its improved malloc.c and other features that improve mod_perl and are turned on by default.

See also Out_of_memory!

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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''. Common causes 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:


  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 ways to control httpd processes.

Note that Perl 5.005 and later have PERL_EMERGENCY_SBRK turned on by default.

The other trick is to have a startup script initialize Carp::confess, like so:

  use Carp ();
  eval { Carp::confess("init") };

this way, when the real problem happens, Carp::confess doesn't eat memory in the emergency pool ($^M).

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server reached MaxClients setting, consider raising the MaxClients setting

See Choosing MaxClients.

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syntax error at /dev/null line 1, near "line arguments:"

  syntax error at /dev/null line 1, near "line arguments:"
  Execution of /dev/null aborted due to compilation errors.
  parse: Undefined error: 0

There is a chance that your /dev/null device is broken. Try:

  % echo > /dev/null

Alternatively you should try to remove this special file and recreate it:

  # rm /dev/null
  # mknod /dev/null c 1 3
  # chmod a+rw /dev/null

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Can't call method "register_cleanup" (CGI.pm)

  Can't call method "register_cleanup" on an
  undefined value at /usr/lib/perl5/5.00503/CGI.pm line 263.

caused by this code snippet in CGI.pm:

  if ($MOD_PERL) {
    undef $NPH;

One solution is to add to httpd.conf:

  PerlPostReadRequestHandler 'sub { Apache->request(shift) }'

But even better, switch to Apache::Cookie:

  use Apache;
  use Apache::Cookie;
  sub handler {
    my $r = shift;
    my $cookies = Apache::Cookie->new($r)->parse;
    my %bar = $cookies->{foo}->value;

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Shutdown and Restart

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Evil things might happen when using PerlFreshRestart

Unfortunately, not all perl modules are robust enough to survive reload. For them this is an unusual situation. PerlFreshRestart does not much more than:

  while (my($k,$v) = each %INC) {
    delete $INC{$k};
    require $k;

Besides that, it flushes the Apache::Registry cache, and empties any dynamic stacked handlers (e.g. PerlChildInitHandler).

Lots of Segfaults and other problems were reported by users who had turned PerlFreshRestart On. Most of them have gone away when it was turned off. It doesn't mean that you shouldn't use it, if it works for you. Just beware of the dragons...

Note that if you have mod_perl enabled Apache built as DSO and you restart it, the whole Perl interpreter is completely torn down (perl_destruct())and restarted. The value of PerlFreshRestart is irrelevant at this point.

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Constant subroutine XXX redefined

That's a mandatory warning inside Perl which happens only if you modify your script and Apache::Registry reloads it. Perl is warning you that the subroutine(s) were redefined. It is mostly harmless. If you don't like seeing these warnings, just kill -USR1 (graceful restart) Apache when you modify your scripts.

You aren't supposed to see these warnings if you don't modify the code with perl 5.004_05 or 5.005+.and higher. If you still experience a problem with code within a CGI script, moving all the code into a module (or a library) and require()ing it should solve the problem.

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Can't undef active subroutine

  Can't undef active subroutine at
  /usr/apps/lib/perl5/site_perl/5.005/aix/Apache/Registry.pm line 102.
  Called from package Apache::Registry, filename
  /usr/apps/lib/perl5/site_perl/5.005/aix/Apache/Registry.pm, line 102

This problem is caused when a client drops the connection while httpd is in the middle of a write. httpd times out, sending a SIGPIPE, and Perl (in that child) is stuck in the middle of its eval context. This is fixed by the Apache::SIG module which is called by default. This should not happen unless you have code that is messing with $SIG{PIPE}. It's also triggered only when you've changed your script on disk and mod_perl is trying to reload it.

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[warn] child process 30388 did not exit, sending another SIGHUP

From mod_perl.pod: With Apache versions 1.3.0 and higher, mod_perl will call the perl_destruct() Perl API function during the child exit phase. This will cause proper execution of END blocks found during server startup as well as invoking the DESTROY method on global objects which are still alive. It is possible that this operation may take a long time to finish, causing problems during a restart. If your code does not contain any END blocks or DESTROY methods which need to be run during child server shutdown, this destruction can be avoided by setting the PERL_DESTRUCT_LEVEL environment variable to -1. Be aware that `your code' includes any modules you use and they may well have DESTROY and END blocks...

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Processes Get Stuck on Graceful Restart

If you see a process stuck in ``G'' (Gracefully finishing) after a doing a graceful restart (sending kill -SIGUSR1) it means that the process is hanging in perl_destruct() while trying to cleanup. This cleanup normally isn't a requirement, you can disable it by setting the PERL_DESTRUCT_LEVEL environment variable to -1. See the section ``Speeding up the Apache Termination and Restart'' for more information.

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httpd keeps on growing after each restart

See the HUP Signal explanation at the section: Server Stopping and Restarting

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Windows OS specific notes

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Apache::DBI causes the server to exit when it starts up, with:

  [Mon Oct 25 15:06:11 1999] file .\main\http_main.c, line 5890,
  assertion "start_mutex" failed

Solution: build mod_perl with PERL_STARTUP_DONE_CHECK set (e.g. insert


at the top of mod_perl.h or add it to the defines in the MSVC++ and similar applications' Options dialog).

Apache loads all Apache modules twice, to make sure the server will successfully restart when asked to. This flag disables all PerlRequire and PerlModule statements on the first load, so they can succeed on the second load. Without that flag, the second load fails. [ TOC ]

Your corrections of the technical and grammatical errors are very welcome. You are encouraged to help me improve this guide. If you have something to contribute please send it directly to me.
The Writing Apache Modules with Perl and C book can be purchased online from O'Reilly and Amazon.com.

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Written by Stas Bekman.
Last Modified at 05/07/2001

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