segfault only when NOT using debugger

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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.

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