Question

#include <stdlib.h>

struct strt;
typedef struct {
    int i;
    struct strt *next;
} strt;

strt *s1 = malloc(sizeof(strt));
strt *s2=s1->next = malloc(sizeof(strt));

free(s1);

Now, does free(s1) deallocate the block pointed to by s1 only or those pointed to by s1 and s1->next/s2?

I understand this must be a question asked a thousand times, but I
couldn’t manage to describe this the the search engine, neither could
I find direct mention of the same issue in documentation of the free() function.

Answer 1

You should have a free per malloc. So you have to free s1 and s2 separately. Either:

free(s1);
free(s2);

Or:

free(s1->next);
free(s1);

In the second case, the statements’s order is important, because you can’t acceed to the objet referenced by s1 if s1 is freed.

C11 (n1570), § 7.22.3.3 The free function
The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation.