Can someone explain me why freeing a twice in a row causes a crash, but freeing a first, then b, and then a again does not crash?
I know that a free will insert the heap chunk in a double linked free list. Freeing twice would insert the same chunk twice in the free list. But why is the crash happening?
int *a = malloc(8);
int *b = malloc(8);
free(a);
// free(a); // Would crash!
free(b);
free(a); // No crash.
Because in C lingo, undefined behavior is just that: undefined. Anything might happen.
Also see man 3 free:
[…] if
free(ptr)has already been called before, undefined behavior occurs.