Re: gerbv crashes (was Re: gEDA-user: gerbv 1.0 on RH8)

[Mario Klebsch <mario@xxxxxxxxxx>]

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Am 04.02.2005 um 22:12 schrieb Stefan Petersen:

> Hello again!
>
> According to your gdb dump it segfaults in a function called 
> 'update_statusbar' when calling 'gtk_label_set_text' in gtk.
>
> This can not happen, it is impossible. At least not where gdb dump 
> shows. Broken gtk/glib/libc?

This can easily happen. The program segaults in calloc(), which is a 
malloc() like function. Due to thw way malloc() works, segfaults can 
occur (without the lib being broken) if the user program does things 
like writing past the end of malloc()ed buffers, writing to free()ed 
buffers or even free()in stuff, that isn'tn malloc()ed anymore (double 
calls to free()).

The Bugs often are miles away from the location, that segfaults. To 
find bugs like this one, you have to use a specual 
malloc()/free()-debugger like efence (Electric Fence). This is not a 
program (like gdb) but a library itself, which supplies malloc(), 
free() &co. efence is build in a special way, that common errors in 
using malloc()ed memory do not crash malloc()/fre()/... but let your 
program crash where the bug is located.

You have to link efence to your program, there even is a shared lib, 
which can be loaded using

	LD_PRELOAD=/path/to/libefence.so /some/program

efence checks for double free()s, delevers memory in a way, that 
accesses past the buffer end do segfault, and after free()in some 
memory, the address never gets used again, so accesses to free()ed 
memory segfault, too. OTOH, memory allocation with efence is much 
slower and long running programs can easily run out of memory. So only 
use it for debugging, not for production software.

73, Mario
- -- 
Mario Klebsch                                           mario@xxxxxxxxxx
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