=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
flow|docs::2.0::devel::core::mod_perl_specific>.

=head1 Debug notes

META: needs more organization

META: there is a new directive CoreDumpDirectory in 2.0.45, need to
check whether we should mention it.

META: there is a new compile-time option in perl-5.9.0+:
-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:
http://httpd.apache.org/dev/debugging.html
(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).
http://httpd.apache.org/~trawick/exception_hook.html




=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:

 % httpd -DONE_PROCESS -DNO_DETACH





=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
code:

  % 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
    "/home/stas/apache.org/modperl-perlmodule/src/modules/perl/libmodperl.so",
    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
  finish
  b modperl_hook_init
  continue

and then start the debugger with:

  % gdb /home/stas/httpd-2.0/bin/httpd -command \
  `pwd`/t/.gdb-jump-to-init

=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:

  .gdb-commands
  ------------
  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 \
  -DONE_PROCESS -DAPACHE2 -DPERL_USEITHREADS
  sharedlibrary mod_perl
  b modperl_hook_init
  # start: modperl_hook_init
  continue
  # restart: ap_run_pre_config
  continue
  # restart: modperl_hook_init
  continue
  b apr_poll
  continue
  
  # 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>
code.

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

=over

=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_Apache__Filter_print()> function implemented in
I<xs/Apache/Filter/Apache__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_Apache__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.

=back


=head1 Analyzing Dumped Core Files

META: need to review (unfinished)

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

=head2 Getting Ready to Debug

In order to debug the I<core> file we may need to recompile Perl and
mod_perl with debugging symbols inside. Usually you have to recompile
only mod_perl, but if the I<core> dump happens in the I<libmodperl.so>
library and you want to see the whole backtrace, you probably want to
recompile Perl as well.

Recompile Perl with I<-DDEBUGGING> during the ./Configure stage (or
even better with I<-Doptimize="-g"> which in addition to adding the
C<-DDEBUGGING> option, adds the I<-g> options which allows you to
debug the Perl interpreter itself).

After recompiling Perl, recompile mod_perl with C<MP_DEBUG=1> during
the I<Makefile.PL> stage.

Building mod_perl with C<PERL_DEBUG=1> will:

=over

=item 1 

add `-g' to EXTRA_CFLAGS

=item 1 

turn on MP_TRACE (tracing)

=item 1 

Set PERL_DESTRUCT_LEVEL=2

=item 1 

Link against C<libperld> if -e $Config{archlibexp}/CORE/libperld$Config{lib_ext}

=back

If you build a static mod_perl, remember that during I<make install>
Apache strips all the debugging symbols.  To prevent this you should
use the Apache I<--without-execstrip> C<./configure> option. So if you
configure Apache via mod_perl, you should do:

  panic% perl Makefile.PL USE_APACI=1 \
    APACI_ARGS='--without-execstrip' [other options]

Alternatively you can copy the unstripped binary manually. For example
we did this to give us an Apache binary called C<httpd_perl> which
contains debugging symbols:

  panic# cp httpd-2.x/httpd /home/httpd/httpd_perl/bin/httpd_perl

Now the software is ready for a proper debug.

=head2 Creating a Faulty Package

META: no longer need to create the package, use C<Debug::DumpCore>
from CPAN. Need to adjust the rest of the document to use it.

Next stage is to create a package that aborts abnormally with the
I<Segmentation fault> error. We will write faulty code on purpose,
so you will be able to reproduce the problem and exercise the
debugging technique explained here. Of course in a real case you
will have some real bug to debug, so in that case you may want to
skip this stage of writing a program with a deliberate bug.

We will use the C<Inline.pm> module to embed some code written in C
into our Perl script. The faulty function that we will add is this:

  void segv() {
      int *p;
      p = NULL;
      printf("%d", *p); /* cause a segfault */
  }

For those of you not familiar with C programming, I<p> is a pointer to
a segment of memory.  Setting it to C<NULL> ensures that we try to
read from a segment of memory to which the operating system does not
allow us access, so of course dereferencing C<NULL> pointer causes a
segmentation fault.  And that's what we want.

So let's create the C<Bad::Segv> package. The name I<Segv> comes from
the C<SIGSEGV> (segmentation violation signal) that is generated when
the I<Segmentation fault> occurs.

First we create the installation sources:

  panic% h2xs -n Bad::Segv -A -O -X
  Writing Bad/Segv/Segv.pm
  Writing Bad/Segv/Makefile.PL
  Writing Bad/Segv/test.pl
  Writing Bad/Segv/Changes
  Writing Bad/Segv/MANIFEST

Now we modify the I<Segv.pm> file to include the C code. Afterwards
it looks like this:

  package Bad::Segv;
  
  use strict;
  BEGIN {
      $Bad::Segv::VERSION = '0.01';
  }
  
  use Inline C => <<'END_OF_C_CODE';
    void segv() {
        int *p;
        p = NULL;
        printf("%d", *p); /* cause a segfault */
    }
  
  END_OF_C_CODE
  
  1;

Finally we modify I<test.pl>:

  use Inline SITE_INSTALL;
  
  BEGIN { $| = 1; print "1..1\n"; }
  END {print "not ok 1\n" unless $loaded;}
  use Bad::Segv;
  
  $loaded = 1;
  print "ok 1\n";

Note that we don't test Bad::Segv::segv() in I<test.pl>, since this
will abort the I<make test> stage abnormally, and we don't want this.

Now we build and install the package:

  panic% perl Makefile.PL
  panic% make && make test
  panic% su
  panic# make install

Running I<make test> is essential for C<Inline.pm> to prepare the binary
object for the installation during I<make install>.


META: stopped here!

Now we can test the package:

  panic% ulimit -c unlimited
  panic% perl -MBad::Segv -e 'Bad::Segv::segv()'
  Segmentation fault (core dumped)
  panic% ls -l core
  -rw-------  1  stas stas 1359872  Feb 6  14:08 core

Indeed, we can see that the I<core> file was dumped, which will be used
to present the debug techniques.

=head2 Getting the core File Dumped

Now let's get the I<core> file dumped from within the mod_perl
server. Sometimes the program aborts abnormally via the SIGSEGV signal
(I<Segmentation Fault>), but no I<core> file is dumped. And without
the I<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 I<core> file, the application has to:

=over

=item *

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 *

be running from a directory which at the moment of the
I<Segmentation fault> is writable by the process. Notice that the
program might change its current directory during its run, so it's
possible that the I<core> file will need to be dumped in a different
directory from the one the program was started from. For example when
mod_perl runs an C<Apache::Registry> script it changes its directory
to the one in which the script source is located.

=item *

be started from a shell process with sufficient resource allocations
for the I<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 I<core> file size to 8MB
with:

  panic% ulimit -c 8388608

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

=item *

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

=back

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

So let's write a simple script that uses C<Bad::Segv>:

  core_dump.pl
  ------------
  use strict;
  use Bad::Segv ();
  use Cwd()
  
  my $r = shift;
  $r->content_type('text/plain');
  
  my $dir = getcwd;
  $r->print("The core should be found at $dir/core\n");
  Bad::Segv::segv();

In this script we load the C<Bad::Segv> and C<Cwd> modules. After
that we acquire the request object and send the HTTP header. Now we
come to the real part--we get the current working directory, print out
the location of the I<core> file that we are about to dump and finally
we call Bad::Segv::segv() which dumps the I<core> file.

Before we run the script we make sure that the shell sets the I<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/httpd_perl/bin
  panic% limit coredumpsize unlimited
  panic% ./httpd_perl -X
      # issue a request here
  Segmentation fault (core dumped)

Our browser prints out:

  The core should be found at /home/httpd/perl/core

And indeed the core file appears where we were told it will (remember
that C<Apache::Registry> scripts change their directory to the
location of the script source):

  panic% ls -l /home/httpd/perl/core
  -rw------- 1 stas httpd 3227648 Feb 7 18:53 /home/httpd/perl/core

As you can see it's a 3MB I<core> file. Notice that mod_perl was
started as user I<stas>, which had write permission for directory
I</home/httpd/perl>.


=head2 Analyzing the core File

First we start C<gdb>:

  panic% gdb /home/httpd/httpd_perl/bin/httpd_perl /home/httpd/perl/core

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

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

  (gdb) where
  #0  0x4025ea08 in XS_Apache__Segv_segv ()
     from /usr/lib/perl5/site_perl/5.6.0/i386-linux/auto/Bad/Segv_C_0_01_e6b5959d800f515de36a7e7eeab28b39/Segv_C_0_01_e6b5959d800f515de36a7e7eeab28b39.so
  #1  0x40136528 in PL_curcopdb ()
     from /usr/lib/perl5/5.6.0/i386-linux/CORE/libperl.so

Well, you can see the last commands, but our perl and mod_perl are
probably without the debug symbols. So we recompile Perl and mod_perl
with debug symbols as explained earlier 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 I<core> file.

  panic% gdb /home/httpd/httpd_perl/bin/httpd_perl /home/httpd/perl/core

Now we can see the whole backtrace:

  (gdb) bt
  #0  0x40323a30 in segv () at Segv_C_0_01_e6b5959d800f515de36a7e7eeab28b39.xs:9
  #1  0x40323af8 in XS_Apache__Segv_segv (cv=0x85f2b28)
      at Segv_C_0_01_e6b5959d800f515de36a7e7eeab28b39.xs:24
  #2  0x400fcbda in Perl_pp_entersub () at pp_hot.c:2615
  #3  0x400f2c56 in Perl_runops_debug () at run.c:53
  #4  0x4008b088 in S_call_body (myop=0xbffff788, is_eval=0) at perl.c:1796
  #5  0x4008ac4f in perl_call_sv (sv=0x82fc2e4, flags=4) at perl.c:1714
  #6  0x807350e in perl_call_handler ()
  #7  0x80729cd in perl_run_stacked_handlers ()
  #8  0x80701b4 in perl_handler ()
  #9  0x809f409 in ap_invoke_handler ()
  #10 0x80b3e8f in ap_some_auth_required ()
  #11 0x80b3efa in ap_process_request ()
  #12 0x80aae60 in ap_child_terminate ()
  #13 0x80ab021 in ap_child_terminate ()
  #14 0x80ab19c in ap_child_terminate ()
  #15 0x80ab80c in ap_child_terminate ()
  #16 0x80ac03c in main ()
  #17 0x401b8cbe in __libc_start_main () from /lib/libc.so.6

Reading the trace from bottom to top, we can see that it starts with Apache
calls, followed by Perl syscalls. At the top we can see the segv()
call which was the one that caused the Segmentation fault, we can also
see that the faulty code was at line 9 of I<Segv.xs> file (with MD5
signature of the code in the name of the file, because of the way
C<Inline.pm> works). It's a little bit tricky with C<Inline.pm> since
we have never created any I<.xs> files ourselves, (C<Inline.pm> does
it behind the scenes). The solution in this case is to tell
C<Inline.pm> not to cleanup the build directory, so we can see the
created I<.xs> file.

We go back to the directory with the source of C<Bad::Segv> and
force the recompilation, while telling C<Inline.pm> not to cleanup
after the build and to print a lot of other useful info:

  panic# cd Bad/Segv
  panic# perl -MInline=FORCE,NOCLEAN,INFO Segv.pm
  Information about the processing of your Inline C code:
  
  Your module is already compiled. It is located at:
  /home/httpd/perl/Bad/Segv/_Inline/lib/auto/Bad/Segv_C_0_01_e6b5959d800f515de36a7e7eeab28b39/Segv_C_0_01_e6b5959d800f515de36a7e7eeab28b39.so
  
  But the FORCE_BUILD option is set, so your code will be recompiled.
  I'll use this build directory:
  /home/httpd/perl/Bad/Segv/_Inline/build/Bad/Segv_C_0_01_e6b5959d800f515de36a7e7eeab28b39/
  
  and I'll install the executable as:
  /home/httpd/perl/Bad/Segv/_Inline/lib/auto/Bad/Segv_C_0_01_e6b5959d800f515de36a7e7eeab28b39/Segv_C_0_01_e6b5959d800f515de36a7e7eeab28b39.so
  
  The following Inline C function(s) have been successfully bound to Perl:
    void segv()

It tells us that the code was already compiled, but since we have
forced it to recompile we can look at the files after the build. So we
go into the build directory reported by C<Inline.pm> and find the
I<.xs> file there, where on line 9 we indeed find the faulty code:

  9: printf("%d",*p); // cause a segfault

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 I<core> file without any clue to which of handler or script has
triggered it. The special I<curinfo> C<gdb> macro comes to help:

  panic% gdb /home/httpd/httpd_perl/bin/httpd_perl /home/httpd/perl/core
  (gdb) source mod_perl-x.xx/.gdbinit
  (gdb) curinfo
  9:/home/httpd/perl/core_dump.pl

We start the C<gdb> debugger as before.  I<.gdbinit>, the file with
various useful C<gdb> macros is located in the source tree of
mod_perl. We use the C<gdb> source() function to load these macros,
and when we run the I<curinfo> macro we learn that the core was dumped
when I</home/httpd/perl/core_dump.pl> was executing the code at line
9.

These are the bits of information that are important in order to
reproduce and resolve a problem: the filename and line where the
faulty function was called (the faulty function is Bad::Segv::segv()
in our case) and the actual line where the Segementation fault occured
(the printf("%d",*p) call in XS code). The former is important for
problem reproducing, it's possible that if the same function was
called from a different script the problem won't show up (not the case
in our example, where the using of a value dereferenced from the NULL
pointer will always cause the Segmentation fault).


=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.

=over

=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 &
  done

The used truss(1) options are:

=over

=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.

=back

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
killed.

=item 1

Use gcore(1) to get a I<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 I<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.

=back

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:
http://sources.redhat.com/gdb/current/onlinedocs/gdb_5.html#SEC25

Stopping and starting multi-thread programs:
http://sources.redhat.com/gdb/current/onlinedocs/gdb_6.html#SEC40

=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:
http://sources.redhat.com/gdb/current/onlinedocs/gdb_21.html

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:
http://sources.redhat.com/gdb/current/onlinedocs/gdb_10.html#SEC69

=head1 Maintainers

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

Stas Bekman E<lt>stas (at) stason.orgE<gt>

=head1 Authors

=over

=item *

Stas Bekman E<lt>stas (at) stason.orgE<gt>

=back

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

=cut

