C produce error if no argument is given in command line

Question

In C, how can I produce an error if no arguments are given on the command line? I'm not using int main(int argc , * char[] argv). my main has no input so I'm getting my variable using scanf("%d", input)

Answer 1

Your question is inconsistent: if you want to get arguments from the command line, you must define main with argc and argv.

Your prototype for main is incorrect, it should be:

int main(int argc, char *argv[])

If the program is run without any command line arguments, arc will have the value 1. You can test it this way:

int main(int argc, char *argv[]) {
    if (argc < 2) {
        printf("error: missing command line arguments\n");
        return 1;
    }
    ...
}

If you define main with int main(void) you have no portable access to command line arguments. Reading standard input has nothing to do with command line arguments.

Answer 2

Given the code:

#include <stdio.h>

int main() {
  int input;
  int rc = scanf("%d", &input);
}

We can verify that scanf() was able to successfully get some input from the user by checking its return value. Only when rc == 1 has the user properly given us valid input.

If you'd like to know more, I recommend reading scanf's documentation.