apache with a segmentation fault
i deployed a python app on a live server and the only output i got was a blank page. there was no error in the virtual hosts log file and it stopped somewhere during the execution of the script. in the apache error log file i found the following line:
child pid 15136 exit signal Segmentation fault (11)
a very informative and helpful message. after a bit of googling i found out that the gdb would help. gdb is short for GNU Project Debugger. in the file /usr/share/doc/apache2.2-common/README.backtrace there is a short howto to get a stacktrace of a segmentation fault in apache… at least on debian based systems. here is a short overview of what there is to do. at first it’s necessary to install the following packages:
apt-get install apache2-dbg libapr1-dbg libaprutil1-dbg gdb
add the line
CoreDumpDirectory /var/cache/apache2
to your apache config, usually /etc/apache/apache2.conf.
after a restart of apache it should now create a memory dump named /var/cache/apache2/core which can be analysed with gdb. it might be necessary to set the maximum size of the coredump like following (inclusive restart of apache):
/etc/init.d/apache2 stop
ulimit -c unlimited
/etc/init.d/apache2 start
to analyze the core dump you need to execute the gdb like following:
gdb /usr/sbin/apache2 /var/cache/apache2/core
(gdb) bt full
...
(gdb) quit
if you use the threaded mpm (unlikely) then you need to use
gdb /usr/sbin/apache2 /var/cache/apache2/core
(gdb) thread apply all bt full
...
(gdb) quit
my dump produced following output:
#0 0xb7dd86a5 in free () from /lib/libc.so.6
#1 0xb6675011 in RelinquishMagickMemory () from /usr/lib/libMagick.so.9
#2 0xb6625ba0 in DestroyDrawInfo () from /usr/lib/libMagick.so.9
#3 0xb57d9857 in Magick::Options::~Options () from /usr/lib/libMagick++.so.10
#4 0xb57d6725 in Magick::ImageRef::~ImageRef () from /usr/lib/libMagick++.so.10
#5 0xb57cbfe6 in Magick::Image::~Image () from /usr/lib/libMagick++.so.10
#6 0xb59ed7f3 in boost::python::objects::value_holder::~value_holder
() from /var/lib/python-support/python2.5/PythonMagick/_PythonMagick.so
#7 0xb581adea in ?? () from /usr/lib/libboost_python-gcc42-1_34_1-py25.so.1.34.1
#8 0xb6ce6f4f in ?? () from /usr/lib/libpython2.5.so.1.0
#9 0×0889a39c in ?? ()
#10 0xb6d8f7e0 in ?? () from /usr/lib/libpython2.5.so.1.0
#11 0xbf80d088 in ?? ()
#12 0xb6ce6c60 in ?? () from /usr/lib/libpython2.5.so.1.0
#13 0×00000000 in ?? ()
i was using pythonMagick which uses Magick++ wich uses ImageMagick. it was a bit irritating that Magick++ version 10 used ImageMagick version 9 instead of 10. after removing ImageMagick version 9 the problem was gone. no idea why it used the wrong version.