malloc(0) zero-size allocation returns NULL or non-null by implementation

Correct portable pattern for malloc(0)

The C standard (C99-C17) specifies that malloc(0) yields either NULL or a unique non-NULL pointer; the result is implementation-defined. The only portable guarantee is that the pointer may be passed to free and that free(NULL) is a no-op.

Steps

1. Allocate with void *p = malloc(0);
2. Immediately call free(p); - this is safe whether p is NULL or non-NULL.
3. Do not read from or write to p; any dereference is undefined behavior regardless of the value.
4. If the allocation is meant for later use, first verify that the requested size is greater than zero; otherwise treat the pointer as unusable.

Verification

• Write a minimal program that invokes malloc(0) on several libc versions and prints the returned pointer.
• Run the program with an allocator that logs the pointer passed to free to confirm that both NULL and non-NULL are accepted.
• Attempt a controlled read/write through the pointer; the program should crash or exhibit undefined behavior, proving dereferencing is unsafe.
• Check that free(NULL) does not segfault, verifying the no-op behavior.

#include 
#include 

int main(void) {
    void *p = malloc(0);
    printf("malloc(0) returned %p\n", (void *)p);
    free(p);               // safe for NULL and non-NULL
    return 0;
}