comp.lang.c FAQ list · Question 16.8

Q: What do ``Segmentation violation'', ``Bus error'', and ``General protection fault'' mean? What's a ``core dump''?

A: These symptoms (and any similar messages having to do with memory access violations or protection faults) generally mean that your program tried to access memory it shouldn't have, invariably as a result of stack corruption or improper pointer use. Likely causes are:

overflow of local (``automatic,'' stack-allocated) arrays
inadvertent use of null pointers (see also questions 5.2 and 5.20)
uninitialized, misaligned, or otherwise improperly allocated pointers (see questions 7.1, 7.2, and 16.7)
stale aliases to memory that has been relocated (see question 7.29)
corruption of the malloc arena (see question 7.19)
attempts to modify read-only values (those declared const, and string literals--see question 1.32)
mismatched function arguments, especially involving pointers; two possibilities are scanf (see question 12.12) and fprintf (make sure it receives its first FILE * argument)

Under Unix, any of these problems almost invariably leads to a ``core dump'': a file named core, [footnote] created in the current directory, containing a memory image of the crashed process, for debugging.

The distinction between ``Bus error'' and ``Segmentation Violation'' may or may not be significant; different versions of Unix generate these signals under different sets of circumstances. Roughly speaking, a segmentation violation indicates an attempt to access memory which doesn't even exist, and a bus error indicates an attempt to access memory in an illegal way (perhaps due to an unaligned pointer; see question 16.7).