malloc(sizeof(int)) vs malloc(sizeof(int *)) vs (int *)malloc(sizeof(int))

Question

I acknowledge that all three of these have a different meaning. But, I don't understand on what particular instances would each of these apply. Can anyone share an example for each of these? Thank you.

       malloc(sizeof(int))
       malloc(sizeof(int *))
(int *)malloc(sizeof(int))

Answer 1

malloc(sizeof(int)) means you are allocating space off the heap to store an int. You are reserving as many bytes as an int requires. This returns a value you should cast to int *. (A pointer to an int.) As some have noted, typical practice in C is to let implicit casting take care of this.

malloc(sizeof(int*)) means you are allocating space off the heap to store a pointer to an int. You are reserving as many bytes as a pointer requires. This returns a value you should cast to an int **. (A pointer to a pointer to an int.)

(int *)malloc(sizeof(int)) is exactly the same as the first call, but with the the result explicitly casted to a pointer to an int.

Note that on many architectures, an int is the same size as a pointer, so these will seem (incorrectly) to be all the same thing. In other words, you can accidentally do the wrong thing and have the resulting code still work.

Answer 2

The syntax pattern that is most foolproof is:

 int *p;
 p = malloc (cnt * sizeof *p);

This syntax will not force you to change the code if the type (and or size...) of *p changes, eg in

 struct mysstruct *q;
 q = malloc (cnt * sizeof *q);

Which will avoid problems like

struct mysstruct *z;
z = malloc (cnt * sizeof(struct hisstruct)); // Auch!

, plus: the sizeof expr form is also shorter.

---

UPDATE: to demonstrate the correctness of p = malloc(CNT * sizeof *p) this test program:

#include <stdio.h>
#include <stdlib.h>

struct mystruct {
        int i;
        char s[14];
        };
int main(void)
{
struct mystruct *p;
size_t siz;

siz = sizeof p;
printf("Sizeof p := %zu\n", siz);

siz = sizeof *p;
printf("Sizeof *p := %zu\n", siz);

printf("Allocating %zu (%u structs, each of size %zu) bytes to be assigned to p...\n"
        , 10u * sizeof *p
        , 10u, sizeof *p
        );
p = malloc(10 * sizeof *p);

return 0;
}

Which outputs here:

Sizeof p := 8
Sizeof *p := 20
Allocating 200 (10 structs, each of size 20) bytes to be assigned to p...