Equivalent of modulus (math function) in C?

Question

I have a piece of code where I see a warning saying

I am doing a comparison between signed and unsigned number .
Something like int <= CONSTANT/sizeof(expression)

What is the best way to correct this? I believe to take the modulus of signed number and then do the comparison, right? I mean I get the unsigned number after division by sizeof operator on an expression. So the other way could be to make this rhs signed

If so is there a function in c that would let me do this? I did a quick search and they say % for modulo which obviously is not what I am looking for.

This is the actual warning

warning: comparison between signed and unsigned integer expressions

and this is the actual line of code

functionA( ......, int num, .....) {

assert( num <= MAX_SIZE/sizeof(int));//where MAX_SIZE is #define MAX_SIZE 1000

}

Answer 1

Just cast one side to the other signedness. You have to make sure that the signed number is not negative if you cast that to unsigned - otherwise a comparison of -1 < 100 will not have the desired outcome, since (unsigned)(-1) == UINT_MAX -, or that the unsigned number doesn't overflow if you cast that to signed. In those cases, add an additional condition to treat them.

For the above particular case, I would use

assert(num <= (int)(MAX_SIZE/sizeof(int)));
// num <= MAX_SIZE/(int)sizeof(int) if I'm in a mean mood

if num might be negative and

assert((unsigned)num <= MAX_SIZE/sizeof(int));

if num is guaranteed to be nonnegative.

Answer 2

If you know the right operand is <= INT_MAX, you can cast it to int.

int bla;

...

if (bla < (int) sizeof expr) {
    ...
}

But if you can change the type of object bla to size_t (which is the type of the value that sizeof yields) it would be even better than to cast.

Answer 3

As I see this, your int num is allowed to take any negative value and positive values up to MAX_SIZE/sizeof(int). Otherwise you would have declared num as an unsigned integer for sure...

Would extending your assertion statement by this additional condition help in your case?

assert( num < 0 || /* make the signed check */
  (unsigned int)num <= MAX_SIZE/sizeof(int) /* the unsigned remainder */
  );