How to get a value from optarg - c

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

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 2

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 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 ---------- */