What causes a SIGSEGV

Wikipedia has the answer, along with a number of other sources.

A segfault basically means you did something bad with pointers. This is probably a segfault:

char *c = NULL;
...
*c; // dereferencing a NULL pointer

Or this:

char *c = "Hello";
...
c[10] = 'z'; // out of bounds, or in this case, writing into read-only memory

Or maybe this:

char *c = new char[10];
...
delete [] c;
...
c[2] = 'z'; // accessing freed memory

Same basic principle in each case - you're doing something with memory that isn't yours.

There are various causes of segmentation faults, but fundamentally, you are accessing memory incorrectly. This could be caused by dereferencing a null pointer, or by trying to modify readonly memory, or by using a pointer to somewhere that is not mapped into the memory space of your process (that probably means you are trying to use a number as a pointer, or you incremented a pointer too far). On some machines, it is possible for a misaligned access via a pointer to cause the problem too - if you have an odd address and try to read an even number of bytes from it, for example (that can generate SIGBUS, instead).