CODEWITHSUNDEEP
cnullcommand-line-arguments
Vincents
Vincents

Reputation: 43

Command-line argument not getting passed in

I'm designing a pair of c programs that uses basic stream sockets to communicate between two programs, similar to a server-client interaction. Most of the programs seem to work fine, but the client end of the system keeps reporting an error that I can't quite work out.

The client program, once compiled, is executed using a command line argument, which as I understand is supposed to be the name of the server program. However, every time I run the following line

./client server

I get an error that only gets reported in the event that argv[1] is null. Here's the code from string_client.c:

if ((he=gethostbyname(argv[1])) == NULL) {  // get the host info 
  perror("gethostbyname");
  printf("%s", argv[1]);
  exit(1);
}

server is just the executable file from string_server.c, which is supposed to wait for an incoming request from the client and receive input from the client program(and works as far as that, as far as I can tell). I'm not sure about the gethostbyname method, but as that code was provided, I haven't questioned it yet.

Also, whenever I try printing out argv[1], it comes out as "la"... not sure what's happening there. What's going on with the command line? am I just using the wrong argument?

Upvotes: 0

Views: 291

Answers (1)

Grapsus
Grapsus

Reputation: 2762

Here you go:

#include <stdio.h>
#include <netdb.h>
extern int h_errno;

int main(int argc, char **argv)
{
    struct hostent *he;

    if(argc < 2)
    {
        printf("usage: %s hostname\n", argv[0]);
        return 255;
    }
    if ((he=gethostbyname(argv[1])) == NULL) {  // get the host info 
      perror("gethostbyname");
      printf("%s", argv[1]);
      return 1;
    }
    printf("%s resolved to %u.%u.%u.%u\n", argv[1], 
        (unsigned char)(he->h_addr_list[0][0]),
        (unsigned char)(he->h_addr_list[0][1]),
        (unsigned char)(he->h_addr_list[0][2]),
        (unsigned char)(he->h_addr_list[0][3]));

    return 0;
}

It works now:

$ gcc -Wall -o gethost gethost.c
$ ./gethost 
usage: ./gethost hostname
$ ./gethost google.com
google.com resolved to 173.194.40.196
$ ./gethost localhost
localhost resolved to 127.0.0.1

Upvotes: 3

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