=head1 NAME

Debugging mod_perl C Internals

=head1 Description

This document explains how to debug C code under mod_perl, including
mod_perl core itself.

For certain debugging purposes you may find useful to read first the
following notes on mod_perl internals: L<Apache 2.0
Integration|docs::2.0::devel::core::apache_integration> and 
L<mod_perl-specific functionality

=head1 Debug notes

META: needs more organization (if you grok any of the following,
patches are welcome)

META: there is a new compile-time option in perl-5.8.8+:
-DDEBUG_LEAKING_SCALARS, which prints out the addresses of leaked SVs
and new_SV() can be used to discover where those SVs were allocated.
(see perlhack.pod for more info)

META: httpd has quite a lot of useful debug info:
(need to add this link to mp1 docs as well)

META: profiling: need a new entry of profiling.
+ running mod_perl under gprof: Defining GPROF when
compiling uses the moncontrol() function to disable gprof profiling in
the parent, and enable it only for request processing in children (or
in one_process mode).

META: Jeff Trawick wrote a few useful debug modules, for httpd-2.1:
mod_backtrace (similar to bt in gdb, but doesn't require the core
file) and mod_whatkilledus (gives the info about the request that
caused the segfault).

=head2 Entering Single Server Mode

Most of the time, when debugging Apache or mod_perl, one needs to
start Apache in a single server mode and not allow it to detach itself
from the initial process. This is accomplished with:


=head2 Setting gdb Breakpoints with mod_perl Built as DSO

If mod_perl is built as a DSO module, you cannot set the breakpoint in
the mod_perl source files when the I<httpd> program gets loaded into
the debugger. The reason is simple: At this moment I<httpd> has no
idea about mod_perl module yet. After the configuration file is
processed and the mod_perl DSO module is loaded then the breakpoints
in the source of mod_perl itself can be set.

The trick is to break at I<apr_dso_load>, let it load
I<libmodperl.so>, then you can set breakpoints anywhere in the modperl

  % gdb httpd
  (gdb) b apr_dso_load
  (gdb) run -DONE_PROCESS
  [New Thread 1024 (LWP 1600)]
  [Switching to Thread 1024 (LWP 1600)]

  Breakpoint 1, apr_dso_load (res_handle=0xbfffb48c, path=0x811adcc
    pool=0x80e1a3c) at dso.c:138
  141         void *os_handle = dlopen(path, RTLD_NOW | RTLD_GLOBAL);
  (gdb) finish
  Value returned is $1 = 0
  (gdb) b modperl_hook_init
  (gdb) continue

This example shows how to set a breakpoint at I<modperl_hook_init>.

To automate things you can put those in the I<.gdb-jump-to-init> file:

  b apr_dso_load
  run -DONE_PROCESS -d `pwd`/t -f `pwd`/t/conf/httpd.conf
  b modperl_hook_init

and then start the debugger with:

  % gdb /home/stas/httpd-2.0/bin/httpd -command \

=head2 Starting the Server Fast under gdb

When the server is started under gdb, it first loads the symbol tables
of the dynamic libraries that it sees going to be used. Some versions
of gdb may take ages to complete this task, which makes the debugging
very irritating if you have to restart the server all the time and it
doesn't happen immediately.

The trick is to set the C<auto-solib-add> flag to 0:

  set auto-solib-add 0

as early as possible in I<~/.gdbinit> file.

With this setting in effect, you can load only the needed dynamic
libraries with I<sharedlibrary> gdb command. Remember that in order to
set a breakpoint and step through the code inside a certain dynamic
library you have to load it first. For example consider this gdb
commands file:

  file ~/httpd/prefork/bin/httpd
  handle SIGPIPE pass
  handle SIGPIPE nostop
  set auto-solib-add 0
  b ap_run_pre_config
  run -d `pwd`/t -f `pwd`/t/conf/httpd.conf \
  sharedlibrary mod_perl
  b modperl_hook_init
  # start: modperl_hook_init
  # restart: ap_run_pre_config
  # restart: modperl_hook_init
  b apr_poll
  # load APR/PerlIO/PerlIO.so
  sharedlibrary PerlIO
  b PerlIOAPR_open

which can be used as:

  % gdb -command=.gdb-commands

This script stops in I<modperl_hook_init()>, so you can step through
the mod_perl startup. We had to use the I<ap_run_pre_config> so we can
load the I<libmodperl.so> library as explained earlier. Since httpd
restarts on the start, we have to I<continue> until we hit
I<modperl_hook_init> second time, where we can set the breakpoint at
I<apr_poll>, the very point where httpd polls for new request and run
again I<continue> so it'll stop at I<apr_poll>. This particular script
passes over modperl_hook_init(), since we run the C<continue> command
a few times to reach the I<apr_poll> breakpoint. See the L<Precooked
gdb Startup Scripts|/Precooked_gdb_Startup_Scripts> section for
standalone script examples.

When gdb stops at the function I<apr_poll> it's a time to start the
client, that will issue a request that will exercise the server
execution path we want to debug. For example to debug the
implementation of C<APR::Pool> we may run:

  % t/TEST -run apr/pool

which will trigger the run of a handler in
I<t/response/TestAPR/pool.pm> which in turn tests the C<APR::Pool>

But before that if we want to debug the server response we need to set
breakpoints in the libraries we want to debug. For example if we want
to debug the function C<PerlIOAPR_open> which resides in
I<APR/PerlIO/PerlIO.so> we first load it and then we can set a
breakpoint in it. Notice that gdb may not be able to load a library if
it wasn't referenced by any of the code. In this case we have to load
this library at the server startup. In our example we load:

  PerlModule APR::PerlIO

in I<httpd.conf>. To check which libraries' symbol tables can be
loaded in gdb, run (when the server has been started):

  gdb> info sharedlibrary

which also shows which libraries are loaded already.

Also notice that you don't have to type the full path of the library
when trying to load them, even a partial name will suffice. In our
commands file example we have used C<sharedlibrary mod_perl> instead of
saying C<sharedlibrary mod_perl.so>.

If you want to set breakpoints and step through the code in the Perl
and APR core libraries you should load their appropriate libraries:

  gdb> sharedlibrary libperl
  gdb> sharedlibrary libapr
  gdb> sharedlibrary libaprutil

Setting I<auto-solib-add> to 0 makes the debugging process unusual,
since originally gdb was loading the dynamic libraries automatically,
whereas now it doesn't. This is the price one has to pay to get the
debugger starting the program very fast. Hopefully the future versions
of gdb will improve.

Just remember that if you try to I<step-in> and debugger doesn't do
anything, that means that the library the function is located in
wasn't loaded. The solution is to create a commands file as explained
in the beginning of this section and craft the startup script the way
you need to avoid extra typing and mistakes when repeating the same
debugging process again and again.

Under threaded mpms (e.g. worker), it's possible that you won't be
able to debug unless you tell gdb to load the symbols from the threads
library. So for example if on your OS that library is called
I<libpthread.so> make sure to run:

  sharedlibrary libpthread

somewhere after the program has started. See the L<Precooked gdb
Startup Scripts|/Precooked_gdb_Startup_Scripts> section for examples.

Another important thing is that whenever you want to be able to see
the source code for the code you are stepping through, the library or
the executable you are in must have the debug symbols present. That
means that the code has to be compiled with I<-g> option for the gcc
compiler. For example if I want to set a breakpoint in /lib/libc.so, I
can do that by loading:

 gdb> sharedlibrary /lib/libc.so

But most likely that this library has the debug symbols stripped off,
so while gdb will be able to break at the breakpoint set inside this
library, you won't be able to step through the code. In order to do
so, recompile the library to add the debug symbols.

If debug code in response handler you usually start the client after
the server was started, when doing this a lot you may find it annoying
to need to wait before the client can be started. Therefore you can
use a few tricks to do it in one command. If the server starts fast
you can use sleep():

  % ddd -command=.debug-modperl-init & ; \
  sleep 2 ; t/TEST -verbose -run apr/pool

or the C<Apache::Test> framework's C<-ping=block> option:

  % ddd -command=.debug-modperl-init & ; \
  t/TEST -verbose -run -ping=block apr/pool

which will block till the server starts responding, and only then will
try to run the test.

=head2 Precooked gdb Startup Scripts

Here are a few startup scripts you can use with gdb to accomplish one
of the common debugging tasks. To execute the startup script, simply run:

 % gdb -command=.debug-script-filename

They can be run under gdb and any of the gdb front-ends. For example
to run the scripts under C<ddd> substitute C<gdb> with C<ddd>:

 % ddd -command=.debug-script-filename


=item * Debugging mod_perl Initialization

The F<code/.debug-modperl-init> startup script breaks at the
C<modperl_hook_init()> function, which is useful for debugging code at
the modperl's initialization phase.

=item * Debugging mod_perl's Hooks Registeration With httpd

Similar to the previous startup script, the
F<code/.debug-modperl-register> startup script breaks at the
C<modperl_register_hooks()>, which is the very first hook called in
the mod_perl land. Therefore use this one if you need to start
debugging at an even earlier entry point into mod_perl.

Refer to the notes inside the script to adjust it for a specific
I<httpd.conf> file.

=item * Debugging mod_perl XS Extensions

The F<code/.debug-modperl-xs> startup script breaks at the
C<mpxs_Apache2__Filter_print()> function implemented in
I<xs/Apache2/Filter/Apache2__Filter.h>. This is an example of debugging
code in XS Extensions. For this particular example the complete test
case is:

  % ddd -command=.debug-modperl-xs & \
  t/TEST -v -run -ping=block filter/api

When I<filter/api> test is running it calls
mpxs_Apache2__Filter_print() which is when the breakpoint is reached.

=item * Debugging code in shared objects created by C<Inline.pm>

This is not strictly related to mod_perl, but sometimes when trying to
reproduce a problem (e.g. for a p5p bug-report) outside mod_perl, the
code has to be written in C. And in certain cases, Inline can be just
the right tool to do it quickly. However if you want to interactively
debug the library that it creates, it might get tricky. So similar to
the previous sections, here is a gdb F<code/.debug-inline> startup
script that will save you a lot of time. All the details and a sample
perl script are inside the gdb script.


=head1 Analyzing Dumped Core Files

When your application dies with the I<Segmentation fault> error (which
is generated by the C<SIGSEGV> signal) and optionally generates a
F<core> file you can use C<gdb> or a similar debugger to find out what
caused the I<Segmentation fault> (or a I<segfault> as we often call

=head2 Getting Ready to Debug

In order to debug the F<core> file you may need to recompile Perl and
mod_perl with debugging symbols. Usually you have to recompile only
mod_perl, but if the F<core> dump happens in the I<libperl.so> library
and you want to see the whole backtrace, you need to recompile Perl as
well. It may also occur inside httpd or 3rd party module, in which
case you will need to recompile those. The following notes should help
to accomplish the right thing:


=item * mod_perl

rebuild mod_perl with C<MP_DEBUG=1>.

  % perl Makefile.PL MP_DEBUG=1 ...
  % make && make test && make install

Building mod_perl with C<PERL_DEBUG=1> will:


=item 1

add C<-g> to C<EXTRA_CFLAGS>

=item 1

turn on C<MP_TRACE> (tracing)

=item 1


=item 1

Link against F<libperld.so> if
F<$Config{archlibexp}/CORE/libperld$Config{lib_ext}> exists.


=item * httpd

If the segfault happens inside I<ap_> or I<apr_> calls, rebuild httpd
with C<--enable-maintainer-mode>:

  % CFLAGS="-g" ./configure --enable-maintainer-mode ...
  % make && make install

=item * perl

If the segfault happens inside I<Perl_> calls, rebuild perl with

  % ./Configure -Doptimize='-g' ...
  % make && make test && make install

Remember to recompile mod_perl if you've recompiled perl.

=item * 3rd party perl modules

if the trace happens in one of the 3rd party perl modules, make sure
to rebuild them, now that you've perl re-built with debugging
flags. They will automatically pick the right compile flags from perl.


Now the software is ready for a proper debug.

=head2 Causing a SegFault

Most likely you already have the segfault situation, but sometimes you
want to create one. For example sometimes you need to make sure that
L<your system is configured to dump core

For that purpose you can use C<Debug::DumpCore> available from CPAN:

  % perl -MDebug::DumpCore -eDebug::DumpCore::segv
  Segmentation fault (core dumped)

Notice that you could use Perl's C<CORE::dump> to achieve the same

  % perl -le 'dump'
  Abort (core dumped)

but the generated in that case backtrace is not very useful for
learning purposes. If all you want to test is whether L<your system is
configured to dump core files|/Getting_the_core_File_Dumped> then
Perl's C<CORE::dump> will do just fine.

=head2 Getting the core File Dumped

Now let's get the F<core> file dumped from within the mod_perl
server. Sometimes the program aborts abnormally via the SIGSEGV signal
(I<Segmentation Fault>), but no F<core> file is dumped. And without
the F<core> file it's hard to find the cause of the problem, unless
you run the program inside C<gdb> or another debugger in first
place. In order to get the F<core> file, the application has to:


=item 1

have the effective UID the same as real UID (the same goes for
GID). Which is the case of mod_perl unless you modify these settings
in the program.

=item 1

be running from a directory which at the moment of the I<Segmentation
fault> is writable by the process that received this signal. Notice
that the program might change its current directory during its run, so
it's possible that the F<core> file will need to be dumped in a
different directory from the one the program was originally started

Under Apache C<ServerRoot> is used as the default directory. Since
that directory is sually not writable by the user running Apache, it's
possible to use the directive C<CoreDumpDirectory> (available since
Apache 2.0.45) to tell Apache to dump the core file elsewhere.

=item 1

be started from a shell process with sufficient resource allocations
for the F<core> file to be dumped. You can override the default
setting from within a shell script if the process is not started
manually. In addition you can use C<BSD::Resource> to manipulate the
setting from within the code as well.

You can use C<ulimit> for C<bash> and C<limit> for C<csh> to check and
adjust the resource allocation. For example inside C<bash>, you may
set the core file size to unlimited:

  panic% ulimit -c unlimited

or for C<csh>:

  panic% limit coredumpsize unlimited

For example you can set an upper limit on the F<core> file size to 8MB

  panic% ulimit -c 8388608

So if the core file is bigger than 8MB it will be not created.

=item 1

Of course you have to make sure that you have enough disk space to
create a big core file (mod_perl F<core> files tend to be of a few MB
in size).


Note that when you are running the program under a debugger like
C<gdb>, which traps the C<SIGSEGV> signal, the F<core> file will not
be dumped. Instead it allows you to examine the program stack and
other things without having the F<core> file.

So let's write a simple script that uses C<Debug::DumpCore>:

  use strict;
  use warnings FATAL => 'all';
  use Apache2::RequestRec ();
  use Apache2::RequestIO ();
  use Debug::DumpCore ();
  use Cwd;
  my $r = shift;
  my $dir = getcwd();
  $r->print("The core should be found at $dir/core.$$\n");

In this script we load the C<Apache2::RequestRec>,
C<Apache2::RequestIO>, C<Debug::DumpCore> and C<Cwd> modules, then we
acquire the Apache request object and set the HTTP response
header. Now we come to the real part -- we get the current working
directory, print out the location of the F<core> file that we are
about to dump and finally we call C<Debug::DumpCore::segv()> which
dumps the F<core> file.

Before we run the script we make sure that the shell sets the F<core>
file size to be unlimited, start the server in single server mode as a
non-root user and generate a request to the script:

  panic% cd /home/httpd/bin
  panic% limit coredumpsize unlimited
  panic% ./httpd -DONE_PROCESS -DNO_DETACH
      # issue a request here
  Segmentation fault (core dumped)

Our browser prints out:

  The core should be found at /home/httpd/bin/core.12345

And indeed the core file appears where we were told it will be:

  panic% ls -l /home/httpd/bin/core.12345
  -rw-------  1 stas stas 13758464 Nov 23 18:33 /home/httpd/bin/core.12345

As you can see it's about 14MB F<core> file. Notice that mod_perl was
started as user I<stas>, which had write permission for directory

Notice that on certain platforms you get no PID digits appended to the
core file name, so sometimes, it'll be just F<core>.

=head2 Analyzing the core File

First we start C<gdb>:

  panic% gdb /home/httpd/bin/httpd /home/httpd/bin/core.12345

with the location of the mod_perl executable and the core file as the

To see the backtrace you run the I<where> or the I<bt> command:

  (gdb) bt
  #0  0x407ab26c in crash_now_for_real (
      suicide_message=0x407ad300 "Cannot stand this life anymore")
      at DumpCore.xs:10
  #1  0x407ab293 in crash_now (
      suicide_message=0x407ad300 "Cannot stand this life anymore",
      attempt_num=42) at DumpCore.xs:17
  #2  0x407ab39b in XS_Debug__DumpCore_segv (my_perl=0x86a9298, cv=0x8d36750)
      at DumpCore.xs:26
  #3  0x40540649 in Perl_pp_entersub () from .../libperl.so
  #7  0x404530cc in modperl_callback () from .../mod_perl.so

Well, you can see the last commands, but our perl and mod_perl are
probably without the debug symbols. This is not the kind of trace you
should send as a part of your bug report, because a lot of important
information that should aid resolve the reported problem is missing.

Therefore the next step is to recompile Perl and mod_perl (and may be
Apache) with debug symbols as L<explained
earlier|/Getting_Ready_to_Debug> in this chapter.

Now when we repeat the process of starting the server, issuing a
request and getting the core file, after which we run C<gdb> again
against the executable and the dumped F<core.6789> file.

  panic% gdb /home/httpd/bin/httpd /home/httpd/bin/core.6789

Now we can see the whole backtrace:

  (gdb) bt
  #0  0x407ab26c in crash_now_for_real (
      suicide_message=0x407ad300 "Cannot stand this life anymore")
      at DumpCore.xs:10
  #1  0x407ab293 in crash_now (
      suicide_message=0x407ad300 "Cannot stand this life anymore",
      attempt_num=42) at DumpCore.xs:17
  #2  0x407ab39b in XS_Debug__DumpCore_segv (my_perl=0x86a9298, cv=0x8d36750)
      at DumpCore.xs:26
  #3  0x40540649 in Perl_pp_entersub (my_perl=0x86a9298) at pp_hot.c:2890
  #4  0x4051ca4d in Perl_runops_debug (my_perl=0x86a9298) at dump.c:1449
  #5  0x404c1ea3 in S_call_body (my_perl=0x86a9298, myop=0xbfffed90, is_eval=0)
      at perl.c:2298
  #6  0x404c19cf in Perl_call_sv (my_perl=0x86a9298, sv=0x8cd0914, flags=4)
      at perl.c:2216
  #7  0x404530cc in modperl_callback (my_perl=0x86a9298, handler=0x81ba6d8,
      p=0x8d16828, r=0x8d16860, s=0x813d238, args=0x8d018d8)
      at modperl_callback.c:102
  #8  0x404539ce in modperl_callback_run_handlers (idx=6, type=4, r=0x8d16860,
      c=0x0, s=0x813d238, pconf=0x0, plog=0x0, ptemp=0x0,
      run_mode=MP_HOOK_RUN_FIRST) at modperl_callback.c:263
  #9  0x40453c2d in modperl_callback_per_dir (idx=6, r=0x8d16860,
      run_mode=MP_HOOK_RUN_FIRST) at modperl_callback.c:351
  #10 0x4044c728 in modperl_response_handler_run (r=0x8d16860, finish=0)
      at mod_perl.c:911
  #11 0x4044cadb in modperl_response_handler_cgi (r=0x8d16860) at mod_perl.c:1006
  #12 0x080db2bc in ap_run_handler (r=0x8d16860) at config.c:151
  #13 0x080dba19 in ap_invoke_handler (r=0x8d16860) at config.c:363
  #14 0x080a9953 in ap_process_request (r=0x8d16860) at http_request.c:246
  #15 0x080a3ef8 in ap_process_http_connection (c=0x8d10920) at http_core.c:250
  #16 0x080e7efc in ap_run_process_connection (c=0x8d10920) at connection.c:42
  #17 0x080e82f8 in ap_process_connection (c=0x8d10920, csd=0x8d10848)
      at connection.c:175
  #18 0x080d9b6d in child_main (child_num_arg=0) at prefork.c:609
  #19 0x080d9c44 in make_child (s=0x813d238, slot=0) at prefork.c:649
  #20 0x080d9d6a in startup_children (number_to_start=2) at prefork.c:721
  #21 0x080da177 in ap_mpm_run (_pconf=0x81360a8, plog=0x817e1c8, s=0x813d238)
      at prefork.c:940
  #22 0x080e0de8 in main (argc=11, argv=0xbffff284) at main.c:619

That's the perfect back trace to send as a part of the bug report.

Reading the trace from bottom to top, we can see that it starts with
Apache calls, followed by mod_perl calls which end up in
C<modperl_callback()> which calls the Perl program via

Notice that in our example we knew what script has caused the
Segmentation fault. In a real world the chances are that you will find
the F<core> file without any clue to which of handler or script has
triggered it. The special I<curinfo> C<gdb> macro comes to help:

For perl enabled with threads that's:

  define curinfo
     printf "%d:%s\n", my_perl->Tcurcop->cop_line, \

For a non-threaded version that's:

  define curinfo
     printf "%d:%s\n", PL_curcop->cop_line, \

Simply past the correct version at the gdb prompt (in this example the
perl is threaded):

  (gdb) define curinfo
  Type commands for definition of "curinfo".
  End with a line saying just "end".
  >   printf "%d:%s\n", my_perl->Tcurcop->cop_line, \

and now we can call it:

  (gdb) curinfo
  No symbol "my_perl" in current context.

Oops, the function where the segfault has happened doesn't have the
perl context, so we need to look at the backtrace and find the first
function which accepts the C<my_perl> argument (this is because we use
a threaded perl). In this example this is the second frame:

  #2  0x407ab39b in XS_Debug__DumpCore_segv (my_perl=0x86a9298, cv=0x8d36750)
      at DumpCore.xs:26

therefore we need to go two frames up:

  (gdb) up 2
  #2  0x407ab39b in XS_Debug__DumpCore_segv (my_perl=0x86a9298, cv=0x8d36750)
      at DumpCore.xs:26
  26      in DumpCore.xs

and now we call C<curinfo> again:

  gdb) curinfo

Et voilĂ , we can see that the segfault was triggered on line 14 of
F<core_dump.pl>, which has the line:


And we are done.

These are the bits of information that are important to extract and
include in your bug report in order for us to be able to reproduce and
resolve a problem. In this example it was the full backtrace, the
filename and line where the faulty function was called (the faulty
function is C<Debug::DumpCore::segv()>) and the actual line where the
Segmentation fault occured (C<crash_now_for_real> at

=head2 Analyzing the core File Automatically

If the core file(s) are found in the mod_perl source directory, when
running F<t/REPORT> the core file backtraces will be automatically
extracted and added to the report if the perl module C<Devel::GDB> is

See the function C<dump_core_file()> in
F<Apache-Test/lib/Apache/TestReport.pm> if you want to see how it is
invoked or refer to the C<Devel::GDB> manpage.

=head2 Obtaining core Files under Solaris

There are two ways to get core files under Solaris. The first is by
configuring the system to allow core dumps, the second is by stopping
the process when it receives the SIGSEGV signal and "manually"
obtaining the core file.

=head3 Configuring Solaris to Allow core Dumps

By default, Solaris 8 won't allow a setuid process to write a core
file to the file system. Since apache starts as root and spawns
children as 'nobody', core dumps won't produce core files unless you
modify the system settings.

To see the current settings, run the coreadm command with no
parameters and you'll see:

  % coreadm
      global core file pattern:
        init core file pattern: core
             global core dumps: disabled
        per-process core dumps: enabled
       global setid core dumps: disabled
  per-process setid core dumps: disabled
      global core dump logging: disabled

These settings are stored in the I</etc/coreadm.conf> file, but you
should set them with the coreadm utility. As super-user, you can run
coreadm with -g to set the pattern and path for core files (you can
use a few variables here) and -e to enable some of the disabled
items. After setting a new pattern, enabling global, global-setid, and
log, and rebooting the system (reboot is required), the new settings
look like:

  % coreadm
      global core file pattern: /usr/local/apache/cores/core.%f.%p
        init core file pattern: core
             global core dumps: enabled
        per-process core dumps: enabled
       global setid core dumps: enabled
  per-process setid core dumps: disabled
      global core dump logging: enabled

Now you'll start to see core files in the designated cores directory
and they will look like I<core.httpd.2222> where httpd is the name of
the executable and the 2222 is the process id. The new core files will
be read/write for root only to maintain some security, and you should
probably do this on development systems only.

=head3 Manually Obtaining core Dumps

On Solaris the following method can be used to generate a core file.


=item 1

Use truss(1) as I<root> to stop a process on a segfault:

  panic% truss -f -l -t \!all -s \!SIGALRM -S SIGSEGV -p <pid>

or, to monitor all httpd processes (from bash):

  panic% for pid in `ps -eaf -o pid,comm | fgrep httpd | cut -d'/' -f1`;
  do truss -f -l -t \!all -s \!SIGALRM -S SIGSEGV -p $pid 2>&1 &

The used truss(1) options are:


=item *

C<-f> - follow forks.

=item *

C<-l> - (that's an el) includes the thread-id and the pid (the pid is
what we want).

=item *

C<-t> - specifies the syscalls to trace,

=item *

!all - turns off the tracing of syscalls specified by C<-t>

=item *

C<-s> - specifies signals to trace and the C<!SIGALRM> turns off the
numerous alarms Apache creates.

=item *

C<-S> - specifies signals that stop the process.

=item *

C<-p> - is used to specify the pid.


Instead of attaching to the process, you can start it under truss(1):

  panic% truss -f -l -t \!all -s \!SIGALRM -S SIGSEGV \
         /usr/local/bin/httpd -f httpd.conf 2>&1 &

=item 1

Watch the I<error_log> file for reaped processes, as when they get
SISSEGV signals. When the process is reaped it's stopped but not

=item 1

Use gcore(1) to get a F<core> of stopped process or attach to it with
gdb(1).  For example if the process id is 662:

  panic% gcore 662
  gcore: core.662 dumped

Now you can load this F<core> file in gdb(1).

=item 1

C<kill -9> the stopped process. Kill the truss(1) processes as well,
if you don't need to trap other segfaults.


Obviously, this isn't great to be doing on a production system since
truss(1) stops the process after it dumps core and prevents Apache
from reaping it.  So, you could hit the clients/threads limit if you
segfault a lot.

=head1 Debugging Threaded MPMs

=head2 Useful Information from gdb Manual

Debugging programs with multiple threads:

Stopping and starting multi-thread programs:

=head2 libpthread

when using:

  set auto-solib-add 0

make sure to:

  sharedlibrary libpthread

(or whatever the shared library is used on your OS) without which you
may have problems to debug the threaded mpm mod_perl.

=head1 Defining and Using Custom gdb Macros

GDB provides two ways to store sequences of commands for execution as
a unit: user-defined commands and command files. See:

Apache 2.0 source comes with a nice pack of macros and can be found in
I<httpd-2.0/.gdbinit>. To use it issue:

  gdb> source /wherever/httpd-2.0/.gdbinit

Now if for example you want to dump the contents of the bucket
brigade, you can do:

  gdb> dump_brigade my_brigade

where C<my_brigade> is the pointer to the bucket brigade that you want
to debug.

mod_perl 1.0 has a similar file (I<modperl/.gdbinit>) mainly including
handy macros for dumping Perl datastructures, however it works only
with non-threaded Perls. But otherwise it's useful in debugging
mod_perl 2.0 as well.

=head1 Expanding C Macros

Perl, mod_perl and httpd C code makes an extensive use of C macros,
which sometimes use many other macros in their definitions, so it
becomes quite a task to figure out how to figure out what a certain
macro expands to, especially when the macro expands to different
values in differnt environments. Luckily there are ways to automate
the expansion process.

=head2 Expanding C Macros with C<make>

The mod_perl I<Makefile>'s include a rule for macro expansions which
you can find by looking for the C<c.i.> rule. To expand all macros in
a certain C file, you should run C<make filename.i>, which will create
I<filename.i> with all macros expanded in it. For example to create
I<apr_perlio.i> with all macros used in I<apr_perlio.c>:

  % cd modperl-2.0/xs/APR/PerlIO
  % make apr_perlio.i

the I<apr_perlio.i> file now lists all the macros:

  % less apr_perlio.i
  # 1 "apr_perlio.c"
  # 1 "<built-in>"
  #define __VERSION__ "3.1.1 (Mandrake Linux 8.3 3.1.1-0.4mdk)"

=head2 Expanding C Macros with C<gdb>

With gcc-3.1 or higher and gdb-5.2-dev or higher you can expand macros
in gdb, when you step through the code. e.g.:

  (gdb) macro expand pTHX_
  expands to:  PerlInterpreter *my_perl __attribute__((unused)),
  (gdb) macro expand PL_dirty
  expands to: (*Perl_Tdirty_ptr(my_perl))

For each library that you want to use this feature with you have to
compile it with:

  CFLAGS="-gdwarf-2 -g3"

or whatever is appropriate for your system, refer to the gcc manpage
for more info.

To compile perl with this debug feature, pass C<-Doptimize='-gdwarf-2
-g3'> to C<./Configure>. For Apache run:

  CFLAGS="-gdwarf-2 -g3" ./configure [...]

for mod_perl you don't have to do anything, as it'll pick the
C<$Config{optimize}> Perl flags automatically, if Perl is compiled
with C<-DDEBUGGING> (which is implied on most systems, if you use
C<-Doptimize='-g'> or similar.)

Notice that this will make your libraries B<huge>! e.g. on Linux 2.4
Perl 5.8.0's normal I<libperl.so> is about 0.8MB on linux, compiled
with C<-Doptimize='-g'> about 2.7MB and with C<-Doptimize='-gdwarf-2
-g3'> 12.5MB. C<httpd> is also becomes about 10 times bigger with this
feature enabled. I<mod_perl.so> instead of 0.2k becomes 11MB. You get
the idea. Of course since you may want this only during the
development/debugging, that shouldn't be a problem.

The complete details are at:

=head1 Maintainers

Maintainer is the person(s) you should contact with updates,
corrections and patches.

Stas Bekman [http://stason.org/]

=head1 Authors


=item *

Stas Bekman [http://stason.org/]


Only the major authors are listed above. For contributors see the
Changes file.