Sockets and Networking (UNIX)

Network programming under UNIX is relatively simple in C.

This guide assumes you already have a good general idea about C, UNIX and networks.

A simple client

To start with, we'll look at one of the simplest things you can do: initialize a stream connection and receive a message from a remote server.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>
 
#define MAXRCVLEN 500
#define PORTNUM 2300
 
int main(int argc, char *argv[])
{
   char buffer[MAXRCVLEN + 1]; /* +1 so we can add null terminator */
   int len, mysocket;
   struct sockaddr_in dest; 
 
   mysocket = socket(AF_INET, SOCK_STREAM, 0);
  
   memset(&dest, 0, sizeof(dest));                /* zero the struct */
   dest.sin_family = AF_INET;
   dest.sin_addr.s_addr = htonl(INADDR_LOOPBACK); /* set destination IP number - localhost, 127.0.0.1*/ 
   dest.sin_port = htons(PORTNUM);                /* set destination port number */
 
   connect(mysocket, (struct sockaddr *)&dest, sizeof(struct sockaddr_in));
  
   len = recv(mysocket, buffer, MAXRCVLEN, 0);
 
   /* We have to null terminate the received data ourselves */
   buffer[len] = '\0';
 
   printf("Received %s (%d bytes).\n", buffer, len);
 
   close(mysocket);
   return EXIT_SUCCESS;
}

This is the very bare bones of a client; in practice, we would check every function that we call for failure, however, error checking has been left out for clarity.

As you can see, the code mainly revolves around dest which is a struct of type sockaddr_in. This struct stores information about the machine we want to connect to.

mysocket = socket(AF_INET, SOCK_STREAM, 0);

The socket() function tells our OS that we want a file descriptor for a socket which we can use for a network stream connection; what the parameters mean is mostly irrelevant for now.

memset(&dest, 0, sizeof(dest));                /* zero the struct */
dest.sin_family = AF_INET;
dest.sin_addr.s_addr = inet_addr("127.0.0.1"); /* set destination IP number */ 
dest.sin_port = htons(PORTNUM);                /* set destination port number */

Now we get on to the interesting part:

The first line uses memset() to zero the struct.

The second line sets the address family. This should be the same value that was passed as the first parameter to socket(); for most purposes AF_INET will serve.

The third line is where we set the IP of the machine we need to connect to. The variable dest.sin_addr.s_addr is just an integer stored in Big Endian format, but we don't have to know that as the inet_addr() function will do the conversion from string into Big Endian integer for us.

The fourth line sets the destination port number. The htons() function converts the port number into a Big Endian short integer. If your program is going to be run solely on machines which use Big Endian numbers as default then dest.sin_port = 21 would work just as well. However, for portability reasons htons() should always be used.

Now that all of the preliminary work is done, we can actually make the connection and use it:

connect(mysocket, (struct sockaddr *)&dest, sizeof(struct sockaddr_in));

This tells our OS to use the socket mysocket to create a connection to the machine specified in dest.

len = recv(mysocket, buffer, MAXRCVLEN, 0);

Now this receives up to MAXRCVLEN bytes of data from the connection and stores them in the buffer string. The number of characters received is returned by recv(). It is important to note that the data received will not automatically be null terminated when stored in the buffer, so we need to do it ourselves with buffer[len] = '\0'.

And that's about it!

The next step after learning how to receive data is learning how to send it. If you've understood the previous section then this is quite easy. All you have to do is use the send() function, which uses the same parameters as recv(). If in our previous example buffer had the text we wanted to send and its length was stored in len we would write send(mysocket, buffer, len, 0). send() returns the number of bytes that were sent. It is important to remember that send(), for various reasons, may not be able to send all of the bytes, so it is important to check that its return value is equal to the number of bytes you tried to send. In most cases this can be resolved by resending the unsent data.

A simple server

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>
 
#define PORTNUM 2300
 
int main(int argc, char *argv[])
{
    char* msg = "Hello World !\n";
  
    struct sockaddr_in dest; /* socket info about the machine connecting to us */
    struct sockaddr_in serv; /* socket info about our server */
    int mysocket;            /* socket used to listen for incoming connections */
    socklen_t socksize = sizeof(struct sockaddr_in);

    memset(&serv, 0, sizeof(serv));           /* zero the struct before filling the fields */
    serv.sin_family = AF_INET;                /* set the type of connection to TCP/IP */
    serv.sin_addr.s_addr = htonl(INADDR_ANY); /* set our address to any interface */
    serv.sin_port = htons(PORTNUM);           /* set the server port number */    

    mysocket = socket(AF_INET, SOCK_STREAM, 0);
  
    /* bind serv information to mysocket */
    bind(mysocket, (struct sockaddr *)&serv, sizeof(struct sockaddr));

    /* start listening, allowing a queue of up to 1 pending connection */
    listen(mysocket, 1);
    int consocket = accept(mysocket, (struct sockaddr *)&dest, &socksize);
  
    while(consocket)
    {
        printf("Incoming connection from %s - sending welcome\n", inet_ntoa(dest.sin_addr));
        send(consocket, msg, strlen(msg), 0); 
        close(consocket);
        consocket = accept(mysocket, (struct sockaddr *)&dest, &socksize);
    }

    close(mysocket);
    return EXIT_SUCCESS;
}

Superficially, this is very similar to the client. The first important difference is that rather than creating a sockaddr_in with information about the machine we're connecting to, we create it with information about the server, and then we bind() it to the socket. This allows the machine to know the data received on the port specified in the sockaddr_in should be handled by our specified socket.

The listen() function then tells our program to start listening using the given socket. The second parameter of listen() allows us to specify the maximum number of connections that can be queued. Each time a connection is made to the server it is added to the queue. We take connections from the queue using the accept() function. If there is no connection waiting on the queue the program waits until a connection is received. The accept() function returns another socket. This socket is essentially a "session" socket, and can be used solely for communicating with connection we took off the queue. The original socket (mysocket) continues to listen on the specified port for further connections.

Once we have "session" socket we can handle it in the same way as with the client, using send() and recv() to handle data transfers.

Note that this server can only accept one connection at a time; if you want to simultaneously handle multiple clients then you'll need to fork() off separate processes, or use threads, to handle the connections.

Useful network functions

int gethostname(char *hostname, size_t size);

The parameters are a pointer to an array of chars and the size of that array. If possible, it finds the hostname and stores it in the array. On failure it returns -1.

struct hostent *gethostbyname(const char *name);

This function obtains information about a domain name and stores it in a hostent struct. The most useful part of a hostent structure is the (char**) h_addr_list field, which is a null terminated array of the IP addresses associated with that domain. The field h_addr is a pointer to the first IP address in the h_addr_list array. Returns NULL on failure.

FAQs

What about stateless connections?

If you don't want to exploit the properties of TCP in your program and would rather just use a UDP connection, then you can just replace SOCK_STREAM with SOCK_DGRAM in your call to socket() and use the result in the same way. It is important to remember that UDP does not guarantee delivery of packets and order of delivery, so checking is important.

If you want to exploit the properties of UDP, then you can use sendto() and recvfrom(), which operate like send() and recv() except you need to provide extra parameters specifying who you are communicating with.

How do I check for errors?

The functions socket(), recv() and connect() all return -1 on failure and use errno for further details.