Using client-side sockets in connected mode

In C language:

After creating the socket, you will have to use the connect function by passing it 3 parameters:

• fd: The first parameter is the integer returned from the socket creation system call.
• addr: The second is the memory address of a structure of type sockaddr. It is a generic structure that is used to encompass several other structures whose content varies according to the value of the first field (named sa_family in the structure sockaddr).
• len: The third is the size of the structure passed as an argument.

We will therefore have to declare a variant of the sockaddr structure corresponding to AF_INET. This is the struct sockaddr_in type. We will start by zeroing the content of this structure via the bzero function of string.h, then we will assign, to the first field, named here sin_family, the AF_INET value.

struct sockaddr_in	addr;

bzero(&addr, sizeof(addr));
addr.sin_family = AF_INET;
	

You must then fill in the structure, the field sin_addr with the IPv4 address of the server to contact and the field sin_port with the number of its TCP port.

Notes:

• The IPv4 address must be coded on 4 bytes, we will use a function of the arpa/inet.h library to convert its writing in the form of a character string (4 numerical values between 0 and 255 separated by periods) in the expected format. This is the inet_aton function which takes the string as the first parameter and the address of the field in which you want to place the result as the second parameter. The returned value is 0 unless the supplied string is not in IPv4 format.
• The port is an (unsigned) integer coded on 2 bytes and stored in the direction of most significant byte first and least significant byte then according to the convention chosen by the TCP/IP protocols. However, depending on the type of CPU, the convention used is not necessarily the same. With some types of CPU, it will be necessary to cross the two bytes and not with other types of CPU. To facilitate the process without asking questions, the arpa/inet.h library provides the htons() function which converts the unsigned short integer passed as a parameter from the host to network byte order and return the result.

inet_aton("127.0.0.1", &addr.sin_addr);
addr.sin_port = htons(80);
	

Having thus specified the address 127.0.0.1 and the port 80, all that remains is to call the connect function so that the client tries to establish the connection with the local web server.

connect(fd, (struct sockaddr *) &addr, sizeof(addr));
	

• Without the "cast" to the type corresponding to the generic structure struct sockaddr *, there is a warning from the C compiler.

Example:

#include <stdio.h>
#include <stdlib.h>

#include <sys/types.h>
#include <netdb.h>
#include <sys/socket.h>

#include <string.h>		// for the function bzero
#include <arpa/inet.h>	// for the function inet_addr

void customer_dialog(int fd) {

}

int main()
{
	int 	fd;
	struct sockaddr_in	addr;
	int		err;

	fd = socket (AF_INET, SOCK_STREAM, IPPROTO_TCP);
	if (fd<0) {
		printf("Socket creation error!\n");
		exit(1);
	}

	bzero(&addr, sizeof(addr));
	addr.sin_family = AF_INET;
	err = inet_aton("127.0.0.1", &addr.sin_addr);
	if (err == 0) {
		printf("Invalid IPv4 address!\n");
		exit(1);
	}
	addr.sin_port = htons(8080);
	err = connect(fd, (struct sockaddr *) &addr, sizeof(addr));
	if (err != 0) {
		printf("Server connection error!\n");
		exit(1);
	}

	printf("Connection established with the server!\n");
	customer_dialog(fd);
	return 0;
}
	

PHP and Python:

In languages like PHP or Python, there are libraries interfacing with system functions.

PHP

The example code written in C becomes in PHP:

function customer_dialog($socket) {

}

$socket = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);
if ($socket == false) die("Socket creation error!\n");
$err=socket_connect($socket, '127.0.0.1', 8080);
if ($err == false) die("Server connection error!\n");
echo "Connection established with the server!\n";
customer_dialog($socket);
	

Python

The example code written in C becomes in Python:

def customer_dialog(sock) :
	...

try :
	sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
	sock.connect(('127.0.0.1',8080))
	customer_dialog(sock)
except socket.error :
	print("Client launch error!")
	exit()
print("Connection established with the server!")