Dialog via created sockets

In C language:

Once the connection has been established between the client and the server, there is a two-way communication channel identifiable by the IP addresses and TCP port of each of its ends. On each side, we can write or read in its socket via the write and read functions of the unistd.h library (as we would for a single file).

Writing

The write function takes 3 parameters:

• fd: The first parameter is the integer identifying the socket.
• buf: The second parameter is the memory address of a buffer containing the message to be sent in the communication channel.
• count: The third parameter is an integer indicating the number of characters to send from the buffer to the communication channel.

The function returns an integer:

• if it is strictly positive, it corresponds to the number of characters written in the channel. In some cases, it may be lower than the value provided by the third parameter.
• If it is 0, it indicates that there is nothing more to read because the other end of the communication channel has been closed.
• If it is strictly negative, it corresponds to an error code whose certain constants are as follows:
• EBADF: indicates an error on the first parameter which is not a valid file descriptor.
• EFAULT: indicates an error on the second parameter because the memory address that was provided is outside the process space.
• EINTR: The arrival of a signal interrupted the write before any data was written.

Reading

The read function is a blocking function (i.e. it waits until there is something to read) which takes 3 parameters:

• fd: The first parameter is the integer identifying the socket.
• buf: The second parameter is the memory address of a buffer that will receive the message read from the socket.
• count: The third parameter is an integer (strictly positive) indicating the maximum number of characters that can be placed in the buffer passed as the second parameter.

The function returns an integer:

• if it is strictly positive, it corresponds to the number of characters read and placed in the buffer.
• If it is 0, it indicates that there is nothing more to read because the other end of the communication channel has been closed.
• If it is strictly negative, it corresponds to an error code whose certain constants are as follows:
• EBADF: indicates an error on the first parameter which is not a valid file descriptor.
• EFAULT: indicates an error on the second parameter because the memory address that was provided is outside the process space.
• EINTR: The arrival of a signal interrupted reading before any data was read.

Closing

The shutdown function takes 2 parameters:

• fd: The first parameter is the integer identifying the socket.

The close function takes 1 parameter:

• fd: This parameter is the integer identifying the socket.

Example:

void customer_dialog(int fd) {
	char buf[256];
	while ((len=read(fd,&buf,sizeof(buf)-1)) != 0) {
		if (len > 0) {
			buf[len]=0x00;
			printf("%s\n",buf);
		} else if (len != EINTR) {
			printf("Socket read error!\n");
			exit(1);
		}
	}
	close(fd);
}
	

void server_dialog(int fd) {
	char msg[] = "Hello customer.\nI'm the server!\n";
	int i, j, l, m=5;

	l=strlen(msg);
	for (i = 0; i < l ; i+=j ){
		j=l-i;
		j=write(fd, &msg[i], (j<m)?j:m);
		if (j < 0 && j != EINTR) {
			printf("Socket write error!\n");
			exit(1);
		}
	}
}
	

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) {
	$m=5;
	while (true) {
		$buf = socket_read($socket,$m);
		if ($buf == '') break;
		if ($buf == false) {
			echo "Erreur lecture socket !\n";
			exit(1);
		}
		echo $buf;
	}
}
	

function customer_dialog($socket) {
	$msg="Hello customer.\nI'm the server!\n";
	socket_write($socket,$msg);
}
	

Python

The example code written in C becomes in Python:

def communication_client(sock) :
	try :
		m=5
		while (True) :
			buf=sock.recv(m)
			if (buf =='') :
				break
			print(buf,end='')
	except socket.error :
		print("Error accepting incoming connection!")
		exit()
	

def server_dialog(sock) :
	msg="Hello customer.\nI'm the server!\n"
	sock.send(msg)