Is it required to free a pointer variable before using realloc?

Question

Is it necessary to free memory before using realloc again for a pointer variable. Which of the following is correct?

for(i = 0; i < n; i++){
   myArray = (int *)realloc(myArray, i*sizeof(int));
}

for(i = 0; i < n; i++){
   myArray = (int *)realloc(myArray, i*sizeof(int));
   free(myArray);
   myArray = NULL;
}

Answer 1

The specific usefulness of realloc is that you don't need to free before using it: it exists to grow memory that has already been allocated.

So it is not required and would be uncommon. When passed a NULL pointer, realloc behaves as malloc. If you're using free before calling it, you might as well be using malloc.

Neither example is correct since you've omitted error handling. All the allocators can return NULL and the usage of realloc is a little tricky in this respect. Read the docs and examples carefully. Specifically, ptr = realloc(ptr, ... is always a bad idea because if realloc fails and returns NULL, then you've just lost your reference and leaked memory. Instead use a tmp variable, e.g.:

tmp = realloc(ptr, newSize);
if (tmp != NULL)
    ptr = tmp;
else handle_error();