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;

}

Answer 1

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.