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

When you use * (p+3), you're addressing out of bounds even with using 2*sizeof(* p), hence you're accessing an invalid memory block, perfect for seg faults.

You specify the size b/c otherwise, the function doesn't know how big of a block out of the heap memory to allocate to your program for that pointer.