malloc double free behaviour

Question

The GNU manual page for malloc defines that when free() called twice with the same pointer (that was previously allocated by malloc())

Answer 1

you can not free twice the same pointer at the same time.

If you want to do a such behaviour without undefined behaviour.

you can use the following macro instead of the all free of your code

#define FREE(X) free(X); X=NULL


char *p;
p=malloc(50);
FREE(p);
FREE(p);

When you force the pointer to be NULL, this will avoid the undefined behaviour in the next free. Because free(NULL) do not cause undefined behaviour.