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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!
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Overview. A segmentation fault (aka segfault) is a common condition that causes programs to crash; they are often associated with a file named core . Segfaults are caused by a program trying to read or write an illegal memory location.
You can't catch segfaults. Segfaults lead to undefined behavior - period (err, actually segfaults are the result of operations also leading to undefined behavior.
No, memory leaks by themselves would not cause a segmentation fault. However, memory leaks usually indicate sloppy code, and in sloppy code other issues, which would cause a segmentation fault, are likely to be present.
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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