How to get a value from optarg

Question

Hi I am writing a simple client-server program. In this program I have to use getopt() to get the port number and ip address like this:

server -i 127.0.0.1 -p 10001

I do not know how can I get values from optarg, to use later in the program.

Answer 1

How about like this:

char buf[BUFSIZE+1];
snprintf(buf,BUFSIZE,"%s",optarg);

Or in a more complete example:

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

#define BUFSIZE 16

int main( int argc, char **argv )
{
    char c;
    char port[BUFSIZE+1];
    char addr[BUFSIZE+1];

    while(( c = getopt( argc, argv, "i:p:" )) != -1 )
        switch ( c )
        {
            case 'i':
                snprintf( addr, BUFSIZE, "%s", optarg );
                break;
            case 'p':
                snprintf( port, BUFSIZE, "%s", optarg );
                break;
            case '?':
                fprintf( stderr, "Unrecognized option!\n" );
                break;
        }

    return 0;
}

For more information see the documentation of Getopt.

Answer 2

You use a while loop to move through all the arguments and process them like so ...

#include <unistd.h>

int main(int argc, char *argv[])
{
    int option = -1;
    char *addr, *port;

    while ((option = getopt (argc, argv, "i:p:")) != -1)
    {
         switch (option)
         {
         case 'i':
             addr = strdup(optarg);
             break;
         case 'p':
             port = strdup(optarg);
             break;
         default:
              /* unrecognised option ... add your error condition */
              break;
         }
    }

    /* rest of program */

    return 0;
}

Answer 3

It is one of the numerous flaws of the getopt documentation: it does not state clearly that optarg must be copied for later use (using strdup(), for instance) because it may be overwritten by later options or just plain freed by getopt.

Answer 4

In the case of an ip and port you don't need to store the strings. Just parse them and store the values in a sockaddr.

#include <arpa/inet.h>                  // for inet_ntop, inet_pton                                                                                                                                                                                                                                                                                                          
#include <getopt.h>                     // for getopt, optarg                                                                                                                                                                                                                                                                                                                
#include <netinet/in.h>                 // for sockaddr_in, etc                                                                                                                                                                                                                                                                                                              
#include <stdio.h>                      // for fprintf, printf, stderr                                                                                                                                                                                                                                                                                                       
#include <stdlib.h>                     // for atoi, EXIT_SUCCESS                                                                                                                                                                                                                                                                                                            
#include <string.h>                     // for memset                                                                                                                                                                                                                                                                                                                        
#include <sys/socket.h>                 // for AF_INET                                                                                                                                                                                                                                                                                                                       

int main(int argc, char *argv[])                                                                                                                                                                                                                                                                                                                                             
{                                                                                                                                                                                                                                                                                                                                                                            
    struct sockaddr_in sa;                                                                                                                                                                                                                                                                                                                                                   
    char c;                                                                                                                                                                                                                                                                                                                                                                  

    memset(&sa, 0, sizeof(sa));                                                                                                                                                                                                                                                                                                                                              
    sa.sin_family = AF_INET;                                                                                                                                                                                                                                                                                                                                                 
    sa.sin_addr.s_addr = htonl(INADDR_ANY);                                                                                                                                                                                                                                                                                                                                         
    sa.sin_port = 0;                                                                                                                                                                                                                                                                                                                                                         

    while ((c = getopt(argc, argv, "i:p:")) != -1)                                                                                                                                                                                                                                                                                                                           
    {                                                                                                                                                                                                                                                                                                                                                                        
        switch (c)                                                                                                                                                                                                                                                                                                                                                           
        {                                                                                                                                                                                                                                                                                                                                                                    
            case 'p':                                                                                                                                                                                                                                                                                                                                                        
                sa.sin_port = htons(atoi(optarg));                                                                                                                                                                                                                                                                                                                           
                break;                                                                                                                                                                                                                                                                                                                                                       
            case 'i':                                                                                                                                                                                                                                                                                                                                                        
                inet_pton(AF_INET, optarg, &(sa.sin_addr));                                                                                                                                                                                                                                                                                                                  
                break;                                                                                                                                                                                                                                                                                                                                                       
            case '?':                                                                                                                                                                                                                                                                                                                                                        
                fprintf(stderr, "Unknown option\n");                                                                                                                                                                                                                                                                                                                         
                break;                                                                                                                                                                                                                                                                                                                                                       
        } /* -----  end switch  ----- */                                                                                                                                                                                                                                                                                                                                     
    }                                                                                                                                                                                                                                                                                                                                                                        

    char str[INET_ADDRSTRLEN];                                                                                                                                                                                                                                                                                                                                               
    inet_ntop(AF_INET, &(sa.sin_addr), str, INET_ADDRSTRLEN);                                                                                                                                                                                                                                                                                                                
    printf("%s:%d\n", str, ntohs(sa.sin_port));                                                                                                                                                                                                                                                                                                                              

    return EXIT_SUCCESS;                                                                                                                                                                                                                                                                                                                                                     
} /* ----------  end of function main  ---------- */