Question

What happens if you try to free a memory which is not allocated using malloc/calloc?

Here is what I mean :

void main()
{
int temp = 0;
int *ptr = &temp;
free(ptr);
}

I thought free() would return some error code but free() does not have a return value.

Answer 1

If you call free() on the pointer which wasn’t allocated before, it will trigger undefined behavior.

From Linux man pages:

The free() function frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc() or realloc(). Otherwise, or if free(ptr) has already been called before, undefined behavior occurs. If ptr is NULL, no operation is performed.