c - segfault only when NOT using debugger

Question

I have a multithreaded C program, which consistently generates a segmentation fault at a specific point in the program. When I run it with gdb, no fault is shown. Can you think of any reason why the fault might occur only when not using the debugger? It's pretty annoying not being able to use it to find the problem!

Answer 1

Classic Heisenbug. From Wikipedia:

Time can also be a factor in heisenbugs. Executing a program under control of a debugger can change the execution timing of the program as compared to normal execution. Time-sensitive bugs such as race conditions may not reproduce when the program is slowed down by single-stepping source lines in the debugger. This is particularly true when the behavior involves interaction with an entity not under the control of a debugger, such as when debugging network packet processing between two machines and only one is under debugger control.

The debugger may be changing timing, and hiding a race condition.

On Linux, GDB also disables address space randomization, and your crash may be specific to address space layout. Try (gdb) set disable-randomization off.

Finally, ulimit -c unlimited and post-mortem debugging (already suggested by Robie) may work.