Debugging .core files for when it all goes wrong¶
This is a short tutorial on debugging compiled c / c++ programs as a last resort for finding out what is going on.
It is a tough nut to crack but even with some simple tools and a bit of perseverance you can make decent headway into a problem that otherwise has no traction.
Debugging C/C++ programs¶
SOmetimes a program dies, and leaves a file of its last memory state. Using the gdb debugger we can see what happened
recently I installed firefox 3 from ports. Then it died when I tried printing.
$gdb core firefox-bin.core # gdb core firefox-bin.core GNU gdb 6.1.1 [FreeBSD] Copyright 2004 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "i386-marcel-freebsd"...core: No such file or directory. Core was generated by `firefox-bin'. Program terminated with signal 11, Segmentation fault. #0 0x299c5c0b in ?? ()
The problem here is the last call made before everything went blooey was in memory address 0x299c5c0b (Hex for 698113035). unfortunately if this binary had been compiled with debugging symbols I could see the name of that function not ??
Normally bt (backtrace) would help but it is useless too...
so I recompiled firefox with debugging symbols, and tried to break it again. (I altered the options file in /var/db/ports/firefox3/ - WITHOUT_DEBUG=false. Yes double megatives do not help my mental state right now)