main.c

#include "int128_lib.h"
#include <stdio.h>
#include <stdlib.h> // for malloc
#include <errno.h>  // for errno

int main()
{
    // reading an __int128 number
    char *number = malloc(sizeof(char) * 129);
    printf("Please input your number: ");
    fgets(number, 129, stdin);
    __int128 intputInteger128;
    char *p;
    errno = 0;
    // the third argument is the base the number is interpreted as, now it's interpreted as a decimal number
    strtoi128(number, &p, 10, &intputInteger128);

    // error checks: the string does not represent an integer
    if (errno != 0) // you should add  p != '\0' but it's often buggy and doesn't work
    {
        fprintf(stderr, "You entered something invalid but not a 128 integer");
        return 1;
    }
    else
    {
        // No error
        // we print the number
        print_int128n(intputInteger128);
    }
    return 0;
}