Question

I have a main function,

int main (int argc, char **argv)
{}

I’m supposed to pass two different command-line arguments:

argv[1]=IP ADDRESS type
argv[2]=INTEGER type

For example, ./a.out IP-ADDRESS INTEGER. I am calling this binary with some other script.

The problem is while argv[1] is not available it behaves like INTEGER value as argv[1].

How can I put a check in that if argv is type INT don’t make it to the argv[1] or only IP-ADDRESS type is allow in argv[1]?

Answer 1

int main (int argc, char **argv)

is more commonly expressed as:

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

that is, argv is a pointer to an array of chars. What those chars represent is up to you, but generally these are going to be zero-delimited strings when the program is called on the command-line.

If the user does not supply positional parameters then maybe you should be using options. See getopt().

For example:

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

/* Options:
       -i : integer argument
       -p : ip address
*/
int main (int argc, char *argv[])
{
    int option;
    int integer = 0;
    char *ip_address = NULL;

    opterr = 0;
    while ( (option=getopt(argc, argv, "i:p:")) != EOF ) {
      switch ( option ) {
        case 'i': integer = atoi(optarg);
                  break;
        case 'p': ip_address = optarg;
                  break;
        case '?': fprintf(stderr,"Unknown option %c\n", optopt);
                  exit(1);
                  break;
      }
    }

    if (integer)    printf("INTEGER: %d\n", integer);
    if (ip_address) printf("IPADDRESS: %s\n", ip_address);

    return 0;
}

Sample runs:

./a.out -i 1234 -p 128.0.0.1
INTEGER: 1234
IPADDRESS: 128.0.0.1

./a.out -i 1234 
INTEGER: 1234

./a.out -p 128.0.0.1
IPADDRESS: 128.0.0.1

./a.out -p 128.0.0.1 -i 1234 
INTEGER: 1234
IPADDRESS: 128.0.0.1

./a.out -x stuff 
Unknown option x