Sockets example in C

Hi!

This is my first post and it is about a very simple program using sockets on linux. The aim is not to have a clean program that handles errors, does safe communication etc, but a very simplistic introduction of sockets and network communications.

For understanding this article, you will need basic knowledge in C programming.

The application is made of 2 programs:

•  A server program that opens a socket and the constantly listen on the specified port. When it receives a message, it displays the received message and send the same message back.

• A client program that opens a socket and connects to the server on the specified address and sends a text message to the server. It then receives the message back and displays it before exiting the program.

So here is the code for the sockets_server.c

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

#define MSGSIZE     8192

main(int argc, char **argv)
{

char msg[MSGSIZE], msg_back[MSGSIZE];
int sd, sd_current, cc, fromlen, tolen;
int      addrlen, port;
struct   sockaddr_in sin;
struct   sockaddr_in pin;

//Wrong number of arguments
if (argc!=2)
{
    printf ("Incorrect number of argument\n");
    printf ("Usage : %s port\n", argv[0]);
    return 1;
}
   
//parsing the parameters
sscanf (argv[1],"%d",&port);

/* get an internet domain socket */
sd = socket(AF_INET, SOCK_STREAM, 0);

/* complete the socket structure */
memset(&sin, 0, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = INADDR_ANY;
sin.sin_port = htons(port);

/* bind the socket to the port number */
bind(sd, (struct sockaddr *) &sin, sizeof(sin));

/* show that we are willing to listen */
listen(sd, 5);

/* loop accepting connections */
while(1){    

    /* wait for a client to talk to us */
    addrlen = sizeof(pin);
    sd_current = accept(sd, (struct sockaddr *)  &pin, &addrlen);
    printf("Connection established with %s\n", inet_ntoa(pin.sin_addr));

    /* recieve a message from the client */
    recv(sd_current, msg, sizeof(msg), 0);
    printf("Message recieved: %s from %s\n",msg, inet_ntoa(pin.sin_addr));
   
    /* send the answer message to the client */
    send(sd_current, msg, strlen(msg), 0);
    printf("Message sent: %s to %s\n",msg, inet_ntoa(pin.sin_addr));
   
    /* close up the socket with the client */
    close(sd_current);
}
}

And here is the code for the sockets_client.c

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

#define HOSTSIZE 256
#define MSGSIZE     8192

main(int argc, char **argv)
{
char host[HOSTSIZE];
int sd, port;
struct sockaddr_in sin;
struct sockaddr_in pin;
struct hostent *hp;
char msg[MSGSIZE];

//Wrong number of arguments
if (argc!=4)
{
    printf ("Incorrect number of argument\n");
    printf ("Usage : %s host port msg\n", argv[0]);
    return 1;
}

//parsing the parameters
sscanf (argv[1],"%s",host);
sscanf (argv[2],"%d",&port);
strcpy (msg,argv[3]);
    
/* go find out about the desired host machine */
hp = gethostbyname(host);

/* fill in the socket structure with host information */
memset(&pin, 0, sizeof(pin));
pin.sin_family = AF_INET;
pin.sin_addr.s_addr = ((struct in_addr *)(hp->h_addr))->s_addr;
pin.sin_port = htons(port);

/* grab an Internet domain socket */
sd = socket(AF_INET, SOCK_STREAM, 0);

/* connect to PORT on HOST */
connect(sd,(struct sockaddr *)  &pin, sizeof(pin));

/* send a message to the server PORT on machine HOST */
send(sd, msg, strlen(msg), 0);
printf ("Sent message : %s \n", msg);

/* receive a message back */
recv(sd, msg, sizeof(msg), 0);
printf ("Received message back: %s \n", msg);

close(sd);
}

To compile that code, you will need to run the following commands:

gcc sockets_client.c -o client
gcc sockets_server.c -o server

Then to start the server  listening on port 1507, you need to run:

./server 1507

 And finally, to start the client and to send the message "Hello World" on the same machine on port 1507:

./client 127.0.0.1 1507 "Hello World"

Have fun!