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

int main(void)
{
    printf("Enter int in range %jd .. %jd:n > ", INTMAX_MIN, INTMAX_MAX);
    intmax_t i;
    if (scanf("%jd", &i) == 1)
        printf("Result: |%jd| = %jdn", i, imaxabs(i));
}

Enter int in range -9223372036854775808 .. 9223372036854775807:
 > -9223372036854775808
Result: |-9223372036854775808| = -9223372036854775808