Calculate how much bits is in int without sizeof()

Question

i am trying to create simple game. Task is to create program that calculate size of int in bites. Their code is

#include <stdio.h>
int intSize(void)
{
    unsigned int x,
        i = 0;
        x = ?;
    while ((?) != 0)
        ?;
    return ?;
}
int main()
{
    printf("Size is %d bits\n",?);
    return 0;
}

? is place where i should put my code.

This is what one of my tries

#include <stdio.h>
int intSize(void)
{
    unsigned int x,
        i = 0;
        x = 0;
    while ((x>>1) != 0)
        i++;
    return i;
}
int main()
{
    printf("Size ise %d bits\n", intSize());
    return 0;
}

I know that usigned int is 4bytes so i use bitwise operation to move bits it should be 32 but i am getting 1. I will be thankfull for any help.

Answer 1

Having

x = 0;
while ((x>>1) != 0)
  ...

the while stops immediately, so i is unchanged and the return value is 0.

The right way was to initialize x with~0u to have all bits valuing 1 (suppose 2-complements)

Out of that the test is not the right one and you missed to modify the value of x.

In

printf("Size ise %d bits\n", intSize);

you missed the () to call the function, currently you try to write its address, I say try because an address must be printf with the format %p.

Finally:

#include <stdio.h>

int intSize(void)
{
  unsigned int x,
     i = 0;

  x = ~0u;
  while ((x & 1) != 0)
    (i+= 1, x /= 2); /* not x >= 1 in case it is a rotate rather than a shift */
  return (int) i;
}

int main()
{
    printf("Size is %d bits\n", intSize());
    return 0;
}

Compilation and execution:

pi@raspberrypi:/tmp $ gcc -Wall s.c
pi@raspberrypi:/tmp $ ./a.out
Size is 32 bits
pi@raspberrypi:/tmp $ 

So in order the '?' are :

• ~0u
• x & 1
• (i+= 1, x /= 2)
• (int) i