Why do you specify the size when using malloc in C?

Question

Take the following code :

int *p = malloc(2 * sizeof *p);

p[0] = 10;  //Using the two spaces I
p[1] = 20;  //allocated with malloc before.

p[2] = 30;  //U         


        

Answer 1

Try this:

int main ( int argc, char *argv[] ) {
  int *p = malloc(2 * sizeof *p);
  int *q = malloc(sizeof *q);
  *q = 100;

  p[0] = 10;    p[1] = 20;    p[2] = 30;    p[3] = 40;
  p[4] = 50;    p[5] = 60;    p[6] = 70;


  printf("%d\n", *q);

  return 0;
}

On my machine, it prints:

50

This is because you overwrote the memory allocated for p, and stomped on q.

Note that malloc may not put p and q in contiguous memory because of alignment restrictions.