datatype promotion in c

Question

In the following code:

#include "stdio.h"
signed char a= 0x80;
unsigned char b= 0x01;


void main (void)
{
    if(b*a>1)
        printf("promoted\n");
    else if (b*a<1)
        printf("why doesnt promotion work?");

    while(1);
}

I expected "promoted' to be printed. But it doesnt. Why? If I can the datatypes to signed and unsigned int, and have a as a negative number, eg, 0x80000000 and b as a positive number, 0x01, "promoted" gets printed as expected.

PLZ HELP me understand what the problem is!

Answer 1

You've just been caught by the messy type-promotion rules of C.

In C, intermediates of integral type smaller than int are automatically promoted to int.

So you have:

0x80 * 0x01 = -128 * 1

0x80 gets signed extended to type int:

0xffffff80 * 0x00000001 = -128 * 1 = -128

So the result is -128 and thus is less than 1.

---

When you use type int and unsigned int, both operands get promoted to unsigned int. 0x80000000 * 0x01 = 0x80000000 as an unsigned integer is bigger than 1.

---

So here's the side-by-side comparison of the type promotion that's taking place:

(signed char) * (unsigned char) -> int
(signed int ) * (unsigned int ) -> unsigned int

(signed char)0x80       * (unsigned char)0x01 -> (int)         0xffffff80
(signed int )0x80000000 * (unsigned int )0x01 -> (unsigned int)0x80000000

(int)         0xffffff80 is negative   ->   prints "why doesnt promotion work?"
(unsigned int)0x80000000 is positive   ->   prints "promoted"

Here's a reference to the type-promotion rules of C.