Critique of realloc() wrapper

Question

Seeking code correctness and best practices.

I would like to re-allocate various amounts of memory (0 or more) such that on systems that may return NULL for an allocation of 0 does not falsely imply a memory allocation error.

Given:

• void * (with a valid value or NULL already)
  
• size_t (new desired size)

Receive:

• void * (new pointer)
• int (failure status)

#include <assert.h>
#include <stdlib.h>

int ReallocAndTest(char **Buf, size_t NewSize) {
  assert(Buf);
  char *NewBuf = realloc(*Buf, NewSize);
  if ((NewBuf == NULL) && (NewSize > 0)) {
    return 1;  // return failure
  }
  *Buf = NewBuf;
  return 0;
}

I came across the below references, which imply a realloc() return of NULL is not always a failure. The lone special case occurs with size is 0. The return value may or may not be NULL.

C11dr 7.22.3.5 The realloc function 4 The realloc function returns a pointer to the new object (which may have the same value as a pointer to the old object), or a null pointer if the new object could not be allocated.

C11dr §J.1 Unspecified behavior. — The amount of storage allocated by a successful call to the calloc, malloc, or realloc function when 0 bytes was requested (7.22.3).

C11dr §J.3.12 Implementation-defined behavior. Whether the calloc, malloc, and realloc functions return a null pointer or a pointer to an allocated object when the size requested is zero (7.22.3).

• Discusses how a memory allocation of 0 is useful.
  Answer: when a routine has a malloc(size), testing for size != 0 is not needed.
  what's the point in malloc(0)?

• Asks if realloc(ptr), 0) is the same as free(ptr).
  Answer: Not necessarily.
  What happens if I re-alloc and the new size is 0. Is this equivalent with a free?

• Allocation wrapper set.
  Answer: Does not address realloc(ptr, 0).
  Are these memory-allocation wrapper functions kosher with all C compilers?

---

[Answer]

1. Use void* rather than char *.

2. With a 0 size, avoid returning the implementation defined
   ..a) NULL or
   ..b) ptr that cannot be referenced.
   and return a pointer to insure only 1 of the 2 is ever returned.

On point #2, I only slightly favor returning NULL.

Answer 1

1. Standard naming convention in C is usually lower_case_with_underscore.

2. You should be using void * rather than char *

3. I would consider making the return value deterministic when the new size is 0 by always returning NULL rather than the implementation defined behaviour (which could technically be a pointer which is not NULL but still not allowed to be dereferenced). free guarantees by spec that free(NULL) is legal.

Answer 2

With the variety of behaviors exhibited by different implementations, I think the only safe thing is to test for zero size before the realloc call and set a minimum allocation (eg 1 byte).

Note that realloc will probably set errno to ENOMEM when it fails, but I don't think this is guaranteed.