Question

Is there a function in C that returns the maximum value of a variable like this (I will name the function “maxvalue” in example below)?

int a;
printf("%d", maxvalue(a)); // 32767
unsigned int b;
printf("%d", maxvalue(b)); // 65535

So basically the function returns values like INT_MAX when the variable is signed INT, UINT_MAX when unsigned int, etc.

Answer 1

Such a function is not defined by the C standard library. You can try defining a macro that calculates it:

#define MAX_VALUE(a) (((unsigned long long)1 << (sizeof(a) * CHAR_BIT)) - 1)

When using it, be careful it is assigned to a type large enough. For example:

#include <stdlib.h>
#include <stdio.h>
#include <limits.h>

#define IS_TYPE_SIGNED(a) ((a-1) < 0)
#define MAX_VALUE_UNSIGNED(a) (((unsigned long long)1 << \
        (sizeof(a) * CHAR_BIT)) - 1)
#define MAX_VALUE_SIGNED(a) (MAX_VALUE_UNSIGNED(a) >> 1)
#define MAX_VALUE(a) (IS_TYPE_SIGNED(a) ? \
        MAX_VALUE_SIGNED(a) : MAX_VALUE_UNSIGNED(a))

int main(void)
{
    unsigned int i = 0;
    signed int j = 0;

    printf("%llu\n", MAX_VALUE(i));
    printf("%llu\n", MAX_VALUE(j));
    return EXIT_SUCCESS;
}

This prints out:

4294967295
2147483647