UNIX & GNU/Linux - System calls - Using socket()

The socket() system call function will help us to create an end point.

This end point will allow for example to connect a client to a server.
Indeed, both of them (client and server) will have a socket() system call function on their implementation.

In this tutorial of socket() we are going to see how to create a socket for a client and for a server.

Let's see first of all with the server snippet.

Socket() with the server

For the server, I'm using three files: server.c, debug.c and the header h.h.

server.c

/* server.c */
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <unistd.h>

#include "h.h"

void my_socket(t_s *s)
{
    s->name    = "TCP";
    s->domain    = AF_INET;
    s->type    = SOCK_STREAM;
    s->pe        = getprotobyname(s->name);
    s->fd        = socket(s->domain, s->type, s->pe->p_proto);
    check_error(s->fd, -1);
}

int main()
{
    t_s s;

    my_socket(&s);
    debug(&s);
    check_error(close(s.fd), -1);
    return 0;
}

debug.c

/* debug.c */
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>

#include "h.h"

void debug(t_s *s)
{
    printf("domain = %d\n", s->domain);
    printf("type = %d\n", s->type);
    printf("fd = %d\n", s->fd);
    printf("name = %s\n", s->name);
    printf("p_proto = %d\n", s->pe->p_proto);
}

void check_error(int test, int error)
{
    if (test == error)
    {
        fprintf(stderr, "ERROR: %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }
}

h.h

#ifndef H_H_
#define H_H_

#include <netdb.h>

/**
 * Structure
 */
typedef struct mystruct
{
    int domain;
    int type;
    int fd;
    char *name;
    struct protoent *pe;
} t_s;

/**
 * Prototype
 */
void debug(t_s *);
void check_error(int, int);

#endif /* H_H_ */

Compiling

So let's compile and execute them:

gcc server.c debug.c -o server ; ./server

Result

domain = 2
type = 1
fd = 3
name = TCP
p_proto = 6

Do the same for the client side.