C: Server/client socket program - Client error connecting

Question

I'm new to networking and trying to create a simple client, server socket program in C, where arguments determine whether the program should run as a client or server. I did this by using simple if statements (if a flag is given, run as server, else run as client), but I'm not sure how to test this. I run my code with the argument to be a server in one terminal (on localhost and port number 3000 for example), and open another terminal and run the code with the argument to be a client (also on localhost and the same port).

The expected result is to see the client prompt the user for a message (if connected successfully), and send that message to the server, which prints out the message, however, I don't get the prompt on the client terminal to enter a message.

(I got the code for server and client behavior from one of many websites online, but they separate the client.c and server.c, whereas I want to combine both into one .c program)

Here's my code below, the error is triggered by

if (connect(sockfd,(struct sockaddr *)&serv_addr,sizeof(serv_addr)) < 0)
       error("ERROR connecting");

in the client section of the code.

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <string.h>

void error(char *msg)
{
    perror(msg);
    exit(1);
}

int main(int argc, char *argv[])
{
 int sockfd, newsockfd, portno, clilen;
 char buffer[256];
 struct sockaddr_in serv_addr, cli_addr;
 int n, i, server = 0;  // 1 = server, 0 = client

 // check if server or client
 for (i = 0; i<argc; i++) {
   if (strcmp(argv[i], "-l") == 0)
      server = 1;
 }

 // client
 if (server == 0) {
   struct hostent *server;

   if (argc < 3) {
      fprintf(stderr,"usage %s hostname port\n", argv[0]);
      exit(0);
   }
   portno = atoi(argv[2]);
   sockfd = socket(AF_INET, SOCK_STREAM, 0);
   if (sockfd < 0)
       error("ERROR opening socket");
   server = gethostbyname(argv[1]);
   if (server == NULL) {
       fprintf(stderr,"FOUR*** ERROR, no such host\n");
       exit(0);
   }
   bzero((char *) &serv_addr, sizeof(serv_addr));
   serv_addr.sin_family = AF_INET;
   bcopy((char *)server->h_addr,
        (char *)&serv_addr.sin_addr.s_addr,
        server->h_length);
   serv_addr.sin_port = htons(portno);
   if (connect(sockfd,(struct sockaddr *)&serv_addr,sizeof(serv_addr)) < 0)
       error("ERROR connecting");
   printf("Please enter the message: ");
   bzero(buffer,256);
   fgets(buffer,255,stdin);
   n = write(sockfd,buffer,strlen(buffer));
   if (n < 0)
        error("ERROR writing to socket");
   bzero(buffer,256);
   n = read(sockfd,buffer,255);
   if (n < 0)
        error("ERROR reading from socket");
   printf("%s\n",buffer);
   return 0;
 }

 // server
 if (server == 1) {

   sockfd = socket(AF_INET, SOCK_STREAM, 0);
   if (sockfd < 0)
      error("ERROR opening socket");
   bzero((char *) &serv_addr, sizeof(serv_addr));
   portno = atoi(argv[1]);
   serv_addr.sin_family = AF_INET;
   serv_addr.sin_addr.s_addr = INADDR_ANY;
   serv_addr.sin_port = htons(portno);
   if (bind(sockfd, (struct sockaddr *) &serv_addr,
            sizeof(serv_addr)) < 0)
            error("ERROR on binding");
   listen(sockfd,5);
   clilen = sizeof(cli_addr);
   newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);
   if (newsockfd < 0)
        error("ERROR on accept");
   bzero(buffer,256);
   n = read(newsockfd,buffer,255);
   if (n < 0) error("ERROR reading from socket");
   printf("Here is the message: %s\n",buffer);
   n = write(newsockfd,"I got your message",18);
   if (n < 0) error("ERROR writing to socket");
   return 0;
 }
}

Here's the exact error output:

In one terminal window, I run the program as a server first:

$ ./socketz -l localhost 2003

Then in another terminal window, I run the program as a client:

$ ./socketz localhost 2003
ERROR connecting: Connection refused

Answer 1

The reason you can't connect is because your server process is not listening on port 2003. In particular, on this line:

 serv_addr.sin_port = htons(portno);

The value of portno is zero, which causes the value of serv_addr.sin_port to also be zero, which accept() interprets as meaning that it should just pick an available TCP port to bind to.

The root of the problem is here:

 portno = atoi(argv[1]);

... that line assigns a value to portno based on the first argument you entered when running the program, but you entered this:

 ./a.out -l localhost 2003

So the first argument is "-l", which is a non-number so it will cause atoi() to return 0. I think what you intended was portno = atoi(argv[3]); instead.