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Debugging mod_perl C Internals | |
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Debugging mod_perl C Internals | |
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The mod_perl Developer's Cookbook
By Geoffrey Young, Paul Lindner, Randy Kobes |
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mod_perl Pocket Reference
By Andrew Ford |
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Writing Apache Modules with Perl and C
By Lincoln Stein, Doug MacEachern |
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Embedding Perl in HTML with Mason
By Dave Rolsky, Ken Williams |
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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: Apache 2.0 Integration and mod_perl-specific functionality flow.
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: 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
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
If mod_perl is built as a DSO module, you cannot set the breakpoint in the mod_perl source files when the httpd program gets loaded into the debugger. The reason is simple: At this moment 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 apr_dso_load, let it load 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 modperl_hook_init.
To automate things you can put those in the .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
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 auto-solib-add flag to 0:
set auto-solib-add 0
as early as possible in ~/.gdbinit file.
With this setting in effect, you can load only the needed dynamic libraries with 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 modperl_hook_init(), so you can step through
the mod_perl startup. We had to use the ap_run_pre_config so we can
load the libmodperl.so library as explained earlier. Since httpd
restarts on the start, we have to continue until we hit
modperl_hook_init second time, where we can set the breakpoint at
apr_poll, the very point where httpd polls for new request and run
again continue so it'll stop at apr_poll. This particular script
passes over modperl_hook_init(), since we run the continue command
a few times to reach the apr_poll breakpoint. See the Precooked gdb Startup Scripts section for
standalone script examples.
When gdb stops at the function 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 APR::Pool we may run:
% t/TEST -run apr/pool
which will trigger the run of a handler in
t/response/TestAPR/pool.pm which in turn tests the 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 PerlIOAPR_open which resides in
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 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 sharedlibrary mod_perl instead of
saying 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 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 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 libpthread.so make sure to run:
sharedlibrary libpthread
somewhere after the program has started. See the 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 -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 Apache::Test framework's -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.
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 ddd substitute gdb with ddd:
% ddd -command=.debug-script-filename
The code/.debug-modperl-init startup script breaks at the
modperl_hook_init() function, which is useful for debugging code at
the modperl's initialization phase.
Similar to the previous startup script, the
code/.debug-modperl-register startup script breaks at the
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 httpd.conf file.
The code/.debug-modperl-xs startup script breaks at the
mpxs_Apache2__Filter_print() function implemented in
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 filter/api test is running it calls mpxs_Apache2__Filter_print() which is when the breakpoint is reached.
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 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.
When your application dies with the Segmentation fault error (which
is generated by the SIGSEGV signal) and optionally generates a
core file you can use gdb or a similar debugger to find out what
caused the Segmentation fault (or a segfault as we often call
it).
In order to debug the 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 core dump happens in the 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:
rebuild mod_perl with MP_DEBUG=1.
% perl Makefile.PL MP_DEBUG=1 ... % make && make test && make install
Building mod_perl with PERL_DEBUG=1 will:
add -g to EXTRA_CFLAGS
turn on MP_TRACE (tracing)
Set PERL_DESTRUCT_LEVEL=2
Link against libperld.so if $Config{archlibexp}/CORE/libperld$Config{lib_ext} exists.
If the segfault happens inside ap_ or apr_ calls, rebuild httpd
with --enable-maintainer-mode:
% CFLAGS="-g" ./configure --enable-maintainer-mode ... % make && make install
If the segfault happens inside Perl_ calls, rebuild perl with
-Doptimize='-g':
% ./Configure -Doptimize='-g' ... % make && make test && make install
Remember to recompile mod_perl if you've recompiled perl.
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.
Most likely you already have the segfault situation, but sometimes you want to create one. For example sometimes you need to make sure that your system is configured to dump core files.
For that purpose you can use Debug::DumpCore available from CPAN:
http://search.cpan.org/dist/Debug-FaultAutoBT/
% perl -MDebug::DumpCore -eDebug::DumpCore::segv Segmentation fault (core dumped)
Notice that you could use Perl's CORE::dump to achieve the same
goal:
% 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 your system is configured to dump core files then
Perl's CORE::dump will do just fine.
Now let's get the core file dumped from within the mod_perl
server. Sometimes the program aborts abnormally via the SIGSEGV signal
(Segmentation Fault), but no core file is dumped. And without
the core file it's hard to find the cause of the problem, unless
you run the program inside gdb or another debugger in first
place. In order to get the core file, the application has to:
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.
be running from a directory which at the moment of the 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 core file will need to be dumped in a different directory from the one the program was originally started from.
Under Apache 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 CoreDumpDirectory (available since
Apache 2.0.45) to tell Apache to dump the core file elsewhere.
be started from a shell process with sufficient resource allocations
for the 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 BSD::Resource to manipulate the
setting from within the code as well.
You can use ulimit for bash and limit for csh to check and
adjust the resource allocation. For example inside bash, you may
set the core file size to unlimited:
panic% ulimit -c unlimited
or for csh:
panic% limit coredumpsize unlimited
For example you can set an upper limit on the core file size to 8MB with:
panic% ulimit -c 8388608
So if the core file is bigger than 8MB it will be not created.
Of course you have to make sure that you have enough disk space to create a big core file (mod_perl core files tend to be of a few MB in size).
Note that when you are running the program under a debugger like
gdb, which traps the SIGSEGV signal, the core file will not
be dumped. Instead it allows you to examine the program stack and
other things without having the core file.
So let's write a simple script that uses Debug::DumpCore:
core_dump.pl
------------
use strict;
use warnings FATAL => 'all';
use Apache2::RequestRec ();
use Apache2::RequestIO ();
use Debug::DumpCore ();
use Cwd;
my $r = shift;
$r->content_type('text/plain');
my $dir = getcwd();
$r->print("The core should be found at $dir/core.$$\n");
$r->rflush;
Debug::DumpCore::segv();
In this script we load the Apache2::RequestRec,
Apache2::RequestIO, Debug::DumpCore and 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 core file that we are
about to dump and finally we call Debug::DumpCore::segv() which
dumps the core file.
Before we run the script we make sure that the shell sets the 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 core file. Notice that mod_perl was started as user stas, which had write permission for directory /home/httpd/bin.
Notice that on certain platforms you get no PID digits appended to the core file name, so sometimes, it'll be just core.
First we start 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 arguments.
To see the backtrace you run the where or the 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 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 gdb again
against the executable and the dumped 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
modperl_callback() which calls the Perl program via
Perl_call_sv.
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 core file without any clue to which of handler or script has
triggered it. The special curinfo gdb macro comes to help:
For perl enabled with threads that's:
define curinfo
printf "%d:%s\n", my_perl->Tcurcop->cop_line, \
my_perl->Tcurcop->cop_file
end
For a non-threaded version that's:
define curinfo
printf "%d:%s\n", PL_curcop->cop_line, \
((XPV*)(*(XPVGV*)PL_curcop->cop_filegv->sv_any)\
->xgv_gp->gp_sv->sv_any)->xpv_pv
end
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, \
my_perl->Tcurcop->cop_file
>end
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 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 curinfo again:
gdb) curinfo 14:/home/httpd/cgi-bin/core_dump.pl
Et voilà, we can see that the segfault was triggered on line 14 of core_dump.pl, which has the line:
Debug::DumpCore::segv();
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 Debug::DumpCore::segv()) and the actual line where the
Segmentation fault occured (crash_now_for_real at
DumpCore.xs:10).
If the core file(s) are found in the mod_perl source directory, when
running t/REPORT the core file backtraces will be automatically
extracted and added to the report if the perl module Devel::GDB is
installed.
See the function dump_core_file() in
Apache-Test/lib/Apache/TestReport.pm if you want to see how it is
invoked or refer to the Devel::GDB manpage.
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.
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 /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 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.
On Solaris the following method can be used to generate a core file.
Use truss(1) as 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:
-f - follow forks.
-l - (that's an el) includes the thread-id and the pid (the pid is
what we want).
-t - specifies the syscalls to trace,
!all - turns off the tracing of syscalls specified by -t
-s - specifies signals to trace and the !SIGALRM turns off the
numerous alarms Apache creates.
-S - specifies signals that stop the process.
-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 &
Watch the error_log file for reaped processes, as when they get SISSEGV signals. When the process is reaped it's stopped but not killed.
Use gcore(1) to get a 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 core file in gdb(1).
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.
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
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.
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 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 my_brigade is the pointer to the bucket brigade that you want
to debug.
mod_perl 1.0 has a similar file (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.
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.
makeThe mod_perl Makefile's include a rule for macro expansions which
you can find by looking for the c.i. rule. To expand all macros in
a certain C file, you should run make filename.i, which will create
filename.i with all macros expanded in it. For example to create
apr_perlio.i with all macros used in apr_perlio.c:
% cd modperl-2.0/xs/APR/PerlIO % make apr_perlio.i
the 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)" ...
gdbWith 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 -Doptimize='-gdwarf-2
-g3' to ./Configure. For Apache run:
CFLAGS="-gdwarf-2 -g3" ./configure [...]
for mod_perl you don't have to do anything, as it'll pick the
$Config{optimize} Perl flags automatically, if Perl is compiled
with -DDEBUGGING (which is implied on most systems, if you use
-Doptimize='-g' or similar.)
Notice that this will make your libraries huge! e.g. on Linux 2.4
Perl 5.8.0's normal libperl.so is about 0.8MB on linux, compiled
with -Doptimize='-g' about 2.7MB and with -Doptimize='-gdwarf-2
-g3' 12.5MB. httpd is also becomes about 10 times bigger with this
feature enabled. 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
Maintainer is the person(s) you should contact with updates, corrections and patches.
Stas Bekman [http://stason.org/]
Stas Bekman [http://stason.org/]
Only the major authors are listed above. For contributors see the Changes file.
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Last modified Sat Feb 15 17:38:39 2014
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