How do I convert a number from the command line to an inter using strtol in C?

Question

I started a course in C today and I am not really sure how you would use the strtol function to convert a string of an integer from command line arguments instead of using the atoi function (for some reason, I am not allowed to use the atoi function.

How would I change the following code to use the strtol function instead? If possible, please explain what the variables you create (if any) do with relevance to the strtol function.

int main( int argc, char * argv[] ) {
    int num1 = atoi(argv[1]);
    int num2 = atoi(argv[2]);
    
    printf("num1 = %d, num2 = %d", num1, num2);
    return 0;
}

Answer 1

convert a number from the command line to an integer using strtol

[Found many strtol() posts, yet not a direct good C answer to OP's question on SO.]

Good strtol() usage involves base, errno and the end pointer and testing for various outcomes.

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

int main(int argc, char *argv[]) {
  for (int a = 1; a < argc; a++) { // check command line arguments after argv[0].
    int base = 10; // convert strings as encoded with decimal digits.
    char *endptr; // location to store the end of conversion.
    errno = 0;
    long val = strtol(argv[a], &endptr, base);
    printf("<%s> --> %ld:", argv[a], val);
    if (argv[a] == endptr) {
      printf(" No conversion.
");
    } else if (errno == ERANGE) { // ***
      printf(" Out of `long` range.
");
    } else if (errno) {
      printf(" Implementation specific error %d detected.
", errno);
    } else if (*endptr) {
      printf(" Trailing junk <%s> after the numeric part.
", endptr);
    } else {
      printf(" Success.
");
    }
  }
}

---

*** Better code uses the value of errno promptly after strtol() to insure it reflects an error due to strtol() and not some later function.