C: Server/client socket program - Client error connecting - sockets

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.