/*
 * Copyright (c) 2020, Dive into Systems, LLC (https://diveintosystems.org/)
 * An example of command line arguments in C
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

/* argc: a count of the number of command line arguments, the executable
 *       name (command) is included so argc is at least 1 for every program
 * argv: an array of strings, there is one string for each command line arg
 */
int main(int argc, char *argv[]) {

    int age, i;
    float gpa;
    long long id;
    char *name;

    // let's just print out the argv strings to see what they are:
    for (i = 0; i < argc; i++) {
        printf("Command line arg %2d: %s\n", i, argv[i]);
    }
    printf("\n");

    // In a program that takes command line arguments, usually there
    // is code to check that the user passed in the correct number of
    // command line arguments, and if not, often an error message with
    // some information on how to call this execuatable is printed out
    // (what command line this program expects, and in which order), 
    // and then a call to exit stops exection.  Sometimes some command
    // line options are optional, in which case default values should
    // be set by the program, and a check for them existing should be
    // made before trying to access what would be their argv[i] value

    // (1) check that the user passed the correct number of command line args
    if (argc != 5) {
        printf("usage: ./prog name age id_num gpa\n");
        exit(1);
    }

    // (2) get command line argument's values, sometimes converting them
    //     from strings to other types
    // atoi, atol, atoll, atof convert a string of numeric characters to
    // the equivalent int, long, long long, or float value
    age = atoi(argv[2]);
    id = atoll(argv[3]);
    gpa = atof(argv[4]);

    // Let's copy the name argument to a dynamically allocated
    // string and modify it  (just to illustrate malloc, strlen, strcpy)
    name = malloc( sizeof(char) * (strlen(argv[1]) + 1) );
    if (!name) {
        printf("Error: malloc failed....\n");
        exit(1);
    }

    if (strcpy(name, argv[1]) != NULL) {
        printf("name: %s age: %d id: %lld gpa: %3.2f\n", name, age, id, gpa);
    } else {
        printf("Error: strcpy failed....\n");
    }

    free(name);
    name = NULL;

    return 0;
}