| Gavin Kreuiter 2004-01-23, 5:02 pm |
| I am looking for some advice on how to debug a program when the
debugger "print" command actually clears the corruption. This is not
the usual non-initialised memory problem, because the program aborts
with a SIGBUS inside the debugger as well. But when I use the print
command inside the debugger, the program completes normally.
I am using gdb on a linux system. The offending C code is:
memcpy(new_entry, &newloc, IRECPTRLEN);
I display these values just before the memcpy:
printf("Calling memcpy(%p, %p, %d)\n", new_entry, &newloc,
IRECPTRLEN);
.... which works. When run straight from gdb (snipped a bit):
$ gdb xwif
(gdb) b src/c_library.c:598
Breakpoint 1 at 0x804bca3: file src/c_library.c, line 598.
(gdb) run
Starting program: /home/dev/bin/xwif -p
Calling memcpy(0x4001f000, 0xbffff04c, 4)
Breakpoint 1, c$keyed_write (p=0x80520a0, record=0x80658a0 "\002") at
src/c_library.c:598
598 memcpy(new_entry, &newloc, IRECPTRLEN);
(gdb) s
Program received signal SIGBUS, Bus error.
0x4207c46c in memcpy () from /lib/i686/libc.so.6
But when I use "print" before "step":
$ gdb xwif
(gdb) b src/c_library.c:598
Breakpoint 1 at 0x804bca3: file src/c_library.c, line 598.
(gdb) r
Starting program: /home/dev/bin/xwif -p
Calling memcpy(0x4001f000, 0xbffff04c, 4)
Breakpoint 1, c$keyed_write (p=0x80520a0, record=0x80658a0 "\002") at
src/c_library.c:598
598 memcpy(new_entry, &newloc, IRECPTRLEN);
(gdb) p new_entry
$1 = 0x4001f000 ""
(gdb) s
599 new_entry += IRECPTRLEN;
(gdb)
.... and it completes successfully.
I *know* that I am corrupting memory somewhere (I am calling mmap). I
wrote a small program to test the way I am using mmap(), and it works.
But when I try to include it in a much larger application, it aborts.
I am not asking you to debug my program, nor for help on mmap()
(although, if you really want to spend hours stepping through my code,
I won't object :-) But I am requesting help with techniques to debug
programs exhibiting symptoms like the above.
(I orignally posted this to comp.lang.c, but suspect that I might have
chosen the wrong newsgroup. Perhaps someone can also advise me how I
determine which group to post a query to; is there a FAQ on choosing
newsgroups?)
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