Assign values to a variable

Question

I have the following program:

#include <stdio.h>
int a,b;
int main(int argc,char *argv[]){
    int c;
    printf("%d",a+b);
    printf("%d",c);
    return 0;
}

How do I pass a value to the global or local variable via terminal? When I output the program, I want you to return the sum given a command via terminal, example:

I compile the program:

cc exemplo.c -o exemplo

Then to do the output:

./exemplo < a=1 b=2

Answer 1

In its main function, the argc represents the number of arguments being passed, while the argv is an array with each argument. In your example, by invoking ./exemplo 1 2, would be 3 arguments (argc = 3), being them:

argv[0] = "example"

argv[1] = "1"

argv[2] = "2"

This way, you can use them like this:

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

int a,b;
int main(int argc,char *argv[]){
    int c;

    // string to long(string, endptr, base)
    // Converte o argv de String para Inteiro
    a = strtol(argv[1], NULL, 10);
    b = strtol(argv[2], NULL, 10);

    printf("%d",a+b);
    printf("%d",c);
    return 0;
}

Test and give me feedback. I’m currently without a C compiler, but edit if something is out when I test.

Answer 2

You need to get the data from array of arguments that the main() takes. Takes each of the elements without taking 0 which is the name of the executable.

How the argument comes as string, need to make a conversion with strtol().

It has a series of checks that would be good to do, how to know if came the amount of arguments needed, if the number conversion worked and others that might be useful to the problem.

You don’t need variables to display the sum. But if you’re going to use variables for something else, don’t make them global, you almost never need.

You don’t need this static variable, I just did so by the limitation of ideone.

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

static char *argv[] = { "app", "123", "456" }; //gambiarra pra simular a passagem de argumento pela linha de comando

int main(/*int argc, char *argv[] */) {
    printf("Soma: %ld", strtol(argv[1], NULL, 10) + strtol(argv[2], NULL, 10));
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.