malloc(-1) returns NULL but malloc(0) doesn't why? how can you allocate 0 memory space

Question

Possible Duplicate:
what's the point in malloc(0)?
what does malloc(0) return?

this code displays "unsuccesful" but if you replace -1 with 0 it wont be NULL. I don't get how you can allocate 0 memory space. I know there's no use but isn't NULL == 0L so it should be == 0 too..

#include <stdio.h>
#include <stdlib.h>
int main()
{
int *ptr;
if((ptr = malloc(-1)) == NULL)
    printf("unsuccessful: no memory space was allocated.\n");
else{
    printf("successful: memoryspace was allocated. \n");
    free(ptr);
}
getch();
return 0;

}

When `malloc` fails by returning `NULL` it should set `errno` and you should print `strerror(errno)` — Basile Starynkevitch, Sep 09 '12 at 07:24
"I don't get how you can allocate 0 memory space" - just don't call malloc() !! — Mitch Wheat, Sep 09 '12 at 07:25
Yeah, I was leaning towards multiple possibilities of allocating something (or maybe nothing) with one way of cleaning up. No need to check if `malloc` was called. It makes a bit more sense looking at it from a C++ class point of view, but I suppose it could happen in C. — chris, Sep 09 '12 at 07:26
*NULL == 0L* Not necessarily, no. In C (not C++) NULL is typed as a pointer with the value of zero `(void *)(0)`, so don't fall into the trap of equating NULL with zero in all cases. — cdarke, Sep 09 '12 at 07:35

score 12 · Accepted Answer · answered Sep 09 '12 at 07:24

It's implementation-defined:

7.22.3-1

If the space cannot be allocated, a null pointer is returned. If the size of the space requested is zero, the behavior is implementation-deﬁned: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object.

So it can return NULL but it doesn't have to. Also, to clarify why malloc(-1) fails, notice its prototype is:

void *malloc(size_t size);
             ^^^^^^

So you're converting -1 to an unsigned type, yielding a very large value.

malloc(-1) returns NULL but malloc(0) doesn't why? how can you allocate 0 memory space

1 Answers1