Correct way to calculate the size for malloc() and realloc()?

Question

I've seen malloc() and realloc() used a bunch of different ways. After testing the various ways out, I was curious to know if I was using them correctly??

First I tried

int size = rowSize * colSize;
int newSize = size + rowSize;

int *randomNum;

randomNum = malloc(size * sizeof *randomNum);

randomNum = realloc(randomNum, newSize * sizeof *randomNum);

and that works!!

Then I tried,

int size = rowSize * colSize;
int newSize = size + rowSize;

int *randomNum;

randomNum = malloc(size * sizeof(int));

randomNum = realloc(randomNum, newSize * sizeof(int));

and that works also. So I guess I don't know why I'd use "sizeof *some_name" versus "sizeof(int)"?? Is there a reason to use one over the other?

Answer 1

They're the same. The reason to use your former example is to ease maintenance later if you decide to change types.

That said, don't use realloc like that - if it fails, you'll leak the original randomNum allocation.

Answer 2

After accept answer

@Carl Norum answer is good, but was mostly done with this and it adds a few points.

Use of sizeof(*pointer) or sizeof(type) is a style issue.
I find the 1st easier to maintain and less error prone. Applies for both malloc() , realloc() .

size_t count = rowSize * colSize;
int *randomNum = malloc(count * sizeof *randomNum);
int *randomNum = malloc(count * sizeof(int));

Example realloc()

int *randomNum = NULL;
...
size_t N = foo();

// If N is zero, (a strange case) one might want to increase it.
// Successfully allocating 0 bytes may return NULL or a non-NULL pointer.

int *randomNum2 = realloc(randomNum, N * sizeof(*randomNum));
if (randomNum2 == NULL && N > 0) {
  // randomNum still contains what it use to.
  Handle_allocation_failure();
  ...
  free(randomNum);
}
else {
  // The happy path
  randomNum = randomNum2;
  ...
  free(randomNum);
}