Proper usage of realloc()

Question

From man realloc:The realloc() function returns a pointer to the newly allocated memory, which is suitably aligned for any kind of variable and may be different from ptr, or NULL if the request fails.

So in this code snippet:

ptr = (int *) malloc(sizeof(int));
ptr1 = (int *) realloc(ptr, count * sizeof(int));
if(ptr1 == NULL){           //reallocated pointer ptr1
    printf("Exiting!!\n");
    free(ptr);
    exit(0);
}else{
    free(ptr);          //to deallocate the previous memory block pointed by ptr so as not to leave orphaned blocks of memory when ptr=ptr1 executes and ptr moves on to another block
    ptr = ptr1;         //deallocation using free has been done assuming that ptr and ptr1 do not point to the same address                     
}

Is it sufficient to just assume that the reallocated pointer points to a different block of memeory and not to the same block.Because if the assumption becomes false and realloc returns the address of the original memory block pointed to by ptr and then free(ptr) executes(for the reason given in the comments) then the memory block would be erased and the program would go nuts. Should I put in another condition which will compare the equality of ptr and ptr1 and exclude the execution of the free(ptr) statement?

Answer 1

Just don't call free() on your original ptr in the happy path. Essentially realloc() has done that for you.

ptr = malloc(sizeof(int));
ptr1 = realloc(ptr, count * sizeof(int));
if (ptr1 == NULL) // reallocated pointer ptr1
{       
    printf("\nExiting!!");
    free(ptr);
    exit(0);
}
else
{
    ptr = ptr1;           // the reallocation succeeded, we can overwrite our original pointer now
}

Answer 2

Applying fixes as edits, based on the good comments below.

Reading this comp.lang.c question, reveals 3 cases:

1. "When it is able to, it simply gives you back the same pointer you handed it."
2. "But if it must go to some other part of memory to find enough contiguous space, it will return a different pointer (and the previous pointer value will become unusable)."
3. "If realloc cannot find enough space at all, it returns a null pointer, and leaves the previous region allocated."

This can be translated directly to code:

int* ptr = (int*)malloc(sizeof(int));
int* tmp = (int*)realloc(ptr, count * sizeof(int));
if(tmp == NULL)
{
    // Case 3, clean up then terminate.
    free(ptr);
    exit(0);
}
else if(tmp == ptr)
{
    // Case 1: They point to the same place, so technically we can get away with
    // doing nothing.
    // Just to be safe, I'll assign NULL to tmp to avoid a dangling pointer.
    tmp = NULL;
}
else
{
    // Case 2: Now tmp is a different chunk of memory.
    ptr = tmp;
    tmp = NULL;
}

So, if you think about it, the code you posted is fine (almost). The above code simplifies to:

int* ptr = (int*)malloc(sizeof(int));
int* tmp = (int*)realloc(ptr, count * sizeof(int));
if(tmp == NULL)
{
    // Case 3.
    free(ptr);
    exit(0);
}
else if(ptr != tmp)
{
    ptr = tmp;
}
// Eliminate dangling pointer.
tmp = NULL;

Note the extra else if(ptr != tmp), which excludes Case 1, where you wouldn't want to call free(ptr) because ptr and tmp refer to the same location. Also, just for safety, I make sure to assign NULL to tmp to avoid any dangling pointer issues while tmp is in scope.

Answer 3

OP: ... may be different from ptr, or NULL if the request fails.
A: Not always. NULL may be legitimately returned (not a failure), if count is 0.

OP: Is it sufficient to just assume that the reallocated pointer points to a different block of memory and not to the same block.
A: No

OP: Should I put in another condition which will compare the equality of ptr and ptr1 and exclude the execution of the free(ptr) statement?
A: No.

If realloc() returns NULL (and count is not 0), the value of ptr is still valid, pointing to the un-resized data. free(ptr) or not depends on your goals.

If realloc() returns not NULL, do not free(ptr), it is all ready freed.

Example: https://codereview.stackexchange.com/questions/36662/critique-of-realloc-wrapper

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

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

Answer 4

realloc will return the same address to ptr if it have enough space to extend the actual chunk of memory pointed by ptr. Otherwise, it will move the data to the new chunk and free the old chunk. You can not rely on ptr1 being different to ptr. Your program behaves undefined.

If realloc returns another address, it first deallocates the old one so you don't have to do it yourself.

By the way, never cast the return of malloc/realloc :). Your code should be like this:

ptr=malloc(sizeof(int));
ptr=realloc(ptr,count*sizeof(int));
if(ptr==NULL)
{   
    // error!    
    printf("\nExiting!!");
    // no need to free, the process is exiting :)
    exit(0);
}

Answer 5

If realloc moves your data, it will free the old pointer for you behind the scenes. I don't have a copy of the C11 standard, but it is guaranteed in the C99 standard.

Answer 6

You should not free your original pointer if the realloc succeeds. Whether you free that pointer if the realloc fails depends on the needs of your particular application; if you absolutely cannot continue without that additional memory, then this would be a fatal error and you would deallocate any held storage and exit. If, OTOH, you can still continue (perhaps execute a different operation and hope that memory will come available later), the you'd probably want to hold on to that memory and a attempt a another realloc later.

Chapter and verse:

7.22.3.5 The realloc function

Synopsis

1

     #include <stdlib.h>
     void *realloc(void *ptr, size_t size);

Description

2 The realloc function deallocates the old object pointed to by ptr and returns a pointer to a new object that has the size specified by size. The contents of the new object shall be the same as that of the old object prior to deallocation, up to the lesser of the new and old sizes. Any bytes in the new object beyond the size of the old object have indeterminate values.

3 If ptr is a null pointer, the realloc function behaves like the malloc function for the specified size. Otherwise, if ptr does not match a pointer earlier returned by a memory management function, or if the space has been deallocated by a call to the free or realloc function, the behavior is undefined. If memory for the new object cannot be allocated, the old object is not deallocated and its value is unchanged.

Returns

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.

Emphasis added. Note clause 4; the returned pointer may be the same as your original pointer.