Allocating memory with an undefined pointer

Question

I was surprised when gcc -Wall compiled this without warning. Is this really legitimate C? What are the risks of writing code like this?

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

typedef struct {
  int a;
  int b;
} MyStruct;

int main(void) {
  MyStruct *s = malloc(sizeof(*s)); // as opposed to malloc(sizeof(MyStruct))
  s->a = 5;
  printf("%d\n", s->a);
}

Answer 1

Not only it's legitimate, it's even preferable to the alternative. This way you let the compiler deduce the actual type instead of doing it manually.