Tuesday, February 19, 2008

[Programming] Sockets programming on Linux/Unix

Last time we introduced the fundamental of sockets programming and gave an example (WinSock) on the Windows platform.
In deed, socket interface was first introduced in UNIX. In UNIX, socket interface is actually an extension of the concept of a pipe.
However, sockets are created and used differently from pipes because they make a clear distinction between client and server, and they can implement multiple clients attached to a single server.

Again, let's briefly go over the principles of sockets programming:
1) First, a server application creates a socket using the system call socket.
2) Next, ther server process gives the socket a name, using the system call bind.
3) Then the server process waits for a client to connect to the named socket using the system call listen, which creats a queue for incoming connections.
4) For incoming connections, the server can accept them using the system call accept.
5) When the server calls accept, a new socket is created that is distinct from the named socket. This new socket is used only for communication with this particular client. The named socket remains for further connections from other clients.
6) Once a connection is established, a two-way communication will be available between the server and the client. They can communicate with each other using sytem call read/write for AF_UNIX protocol or send/recv for AF_INET protocol.
7) After communication, a system call close or shutdown is called to release the named socket.

Client side:
1) The client creates an unamed socket by calling socket.
2) Then the client uses system call connect to establish with the server by using the server's named socket as an address.
3) If the connection is accepted by the server, then the client can communicate with server using read/write for AF_UNIX protocol or send/recv for AF_INET protocol.
4) Similarly, when communication is finished, a call of close/shutdown is called.

Here's a very simple example of socket server and client programs.



// Local server

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main()
{
int server_socketfd, client_socketfd;
int server_len, client_len;
struct sockaddr_un server_address;
struct sockaddr_un client_address;

unlink("server_socket");
server_socketfd = socket(AF_UNIX, SOCK_STREAM, 0);

server_address.sun_family = AF_UNIX;
strcpy(server_address.sun_path, "server_socket");
server_len = sizeof(server_address);
bind(server_socketfd, (struct sockaddr *)&server_address, server_len);

listen(server_socketfd, 10);
while(1)
{
char s_sendbuf[] = "Welcome to server.";
char s_recvbuf[255];
printf("Server waiting\n");

client_len = sizeof(client_address);
client_socketfd = accept(server_socketfd, (struct sockaddr *)&client_address, &client_len);


read(client_socketfd, s_recvbuf, 255);
printf("Message received from client: %s\n", s_recvbuf);
write(client_socketfd, s_sendbuf, strlen(s_sendbuf)+1);
close(client_socketfd);
}
}


// Local client


#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>

int main()
{
int socketfd;
int length, result;
struct sockaddr_un address;
char sendbuf[] = "Hi, this is client 1.";
char recvbuf[255];

socketfd = socket(AF_UNIX, SOCK_STREAM, 0);

address.sun_family = AF_UNIX;
strcpy(address.sun_path, "server_socket");
length = sizeof(address);

result = connect(socketfd, (struct sockaddr*)&address, length);

if(result==-1)
{
perror("Error:client 1");
exit(1);
}

write(socketfd, sendbuf, strlen(sendbuf)+1);
read(socketfd, recvbuf,255);
printf("Greeting from server: %s\n", recvbuf);
close(socketfd);
exit(0);
}

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