Help with debugging LD_PRELOADed shared C libraries

[Thomas Ackermann <tja@anykey.de>]

Hello,
at first i asked at gcc-help, but it seems to be gdb-related:

I have a problem to debug my shared C libary which i use to fix an
existing binary.
I am loading this library via LD_PRELOAD variable, but sometimes the
programm crashes. Now i wan to find and fix the problem with gdb.

Regardless how i prelaod the library for the gdb session, gdb always
only gives "Cannot access memory at address ..." messages.

I tried "export LD_PRELOAD=/.../mylib.so" and then started "gdb mybin
core" or tried to set it within gdb: "set environment
LD_PRELOAD /.../mylib.so" and then continue with "file mybin" and "core
core", but:

Program terminated with signal 11, Segmentation fault.
#0 0x400cc9b8 in ?? ()
#0 0x400cc9b8 in ?? ()
End of crash report

That should be outputs from "bt", "info locals", "info
sharedlibrary" and "info frame".

Here in detail:

gdb mybin core
<...>
Program terminated with signal 11, Segmentation fault.
Cannot access memory at address 0x4001738c
#0 0x400cc9b8 in ?? ()
(gdb) bt
#0 0x400cc9b8 in ?? ()
Cannot access memory at address 0xbfffeddc
(gdb) info locals
No symbol table info available.

Both binary mybin and library mylib DO contain symbolic information as
confirmed via "nm".

The library was created with: "gcc -g3 -Wall -Wuninitialized
-D_GNU_SOURCE -fPIC -shared -ldl -o mylib.so mylib.c"

Can anybody give me a hint?