What does malloc(0) return?

Others have answered how malloc(0) works. I will answer one of the questions that you asked that hasn't been answered yet (I think). The question is about realloc(malloc(0), 0):

What does malloc(0) return? Would the answer be same for realloc(malloc(0),0)?

The standard says this about realloc(ptr, size):

• if ptr is NULL, it behaves like malloc(size),
• otherwise (ptr is not NULL), it deallocates the old object pointer to by ptr and returns a pointer to a new allocated buffer. But if size is 0, C89 says that the effect is equivalent to free(ptr). Interestingly, I can't find that statement in C99 draft (n1256 or n1336). In C89, the only sensible value to return in that case would be NULL.

So, there are two cases:

• malloc(0) returns NULL on an implementation. Then your realloc() call is equivalent to realloc(NULL, 0). That is equivalent to malloc(0) from above (and that is NULL in this case).
• malloc(0) returns non-NULL. Then, the call is equivalent to free(malloc(0)). In this case, malloc(0) and realloc(malloc(0), 0) are not equivalent.

Note that there is an interesting case here: in the second case, when malloc(0) returns non-NULL on success, it may still return NULL to indicate failure. This will result in a call like: realloc(NULL, 0), which would be equivalent to malloc(0), which may or may not return NULL.

I am not sure if the omission in C99 is an oversight or if it means that in C99, realloc(ptr, 0) for non-NULL ptr is not equivalent to free(ptr). I just tried this with gcc -std=c99, and the above is equivalent to free(ptr).

Edit: I think I understand what your confusion is:

Let's look at a snippet from your example code:

ptr = malloc(0);
if (ptr == realloc(ptr, 1024))

The above is not the same as malloc(0) == realloc(malloc(0), 1024). In the second, the malloc() call is made twice, whereas in the first, you're passing a previously allocated pointer to realloc().

Let's analyze the first code first. Assuming malloc(0) doesn't return NULL on success, ptr has a valid value. When you do realloc(ptr, 1024), realloc() basically gives you a new buffer that has the size 1024, and the ptr becomes invalid. A conforming implementation may return the same address as the one already in ptr. So, your if condition may return true. (Note, however, looking at the value of ptr after realloc(ptr, 1024) may be undefined behavior.)

Now the question you ask: malloc(0) == realloc(malloc(0), 1024). In this case, let's assume that both the malloc(0) on the LHS and RHS returns non-NULL. Then, they are guaranteed to be different. Also, the return value from malloc() on the LHS hasn't been free()d yet, so any other malloc(), calloc(), or realloc() may not return that value. This means that if you wrote your condition as:

if (malloc(0) == realloc(malloc(0), 1024)
    puts("possible");

you won't see possible on the output (unless both malloc() and realloc() fail and return NULL).

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    void *p1;
    void *p2;

    p1 = malloc(0);
    p2 = realloc(p1, 1024);
    if (p1 == p2)
        puts("possible, OK");

    /* Ignore the memory leaks */
    if (malloc(0) == realloc(malloc(0), 1024))
        puts("shouldn't happen, something is wrong");
    return 0;
}

On OS X, my code didn't output anything when I ran it. On Linux, it prints possible, OK.

malloc(0) is Implementation Defined as far as C99 is concerned.

From C99 [Section 7.20.3]

The order and contiguity of storage allocated by successive calls to the calloc, malloc, and realloc functions is unspecified. The pointer returned if the allocation succeeds is suitably aligned so that it may be assigned to a pointer to any type of object and then used to access such an object or an array of such objects in the space allocated (until the space is explicitly deallocated). The lifetime of an allocated object extends from the allocation until the deallocation. Each such allocation shall yield a pointer to an object disjoint from any other object. The pointer returned points to the start (lowest byte address) of the allocated space. 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.

In C89, malloc(0) is implementation dependent - I don't know if C99 has fixed this or not. In C++, using:

char * p = new char[0];

is well defined - you get a valid, non-null pointer. Of course, you can't use the pointer to access what it points to without invoking undefined behaviour.

As to why this exists, it is convenient for some algorithms, and means you don't need to litter your code with tests for zero values.