TCP echo client-server in C

client.c

#include <arpa/inet.h>
#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

int main() {
	const char* server_name = "localhost";
	const int server_port = 8877;

	struct sockaddr_in server_address;
	memset(&server_address, 0, sizeof(server_address));
	server_address.sin_family = AF_INET;

	// creates binary representation of server name
	// and stores it as sin_addr
	// http://beej.us/guide/bgnet/output/html/multipage/inet_ntopman.html
	inet_pton(AF_INET, server_name, &server_address.sin_addr);

	// htons: port in network order format
	server_address.sin_port = htons(server_port);

	// open a stream socket
	int sock;
	if ((sock = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
		printf("could not create socket\n");
		return 1;
	}

	// TCP is connection oriented, a reliable connection
	// **must** be established before any data is exchanged
	if (connect(sock, (struct sockaddr*)&server_address,
	            sizeof(server_address)) < 0) {
		printf("could not connect to server\n");
		return 1;
	}

	// send

	// data that will be sent to the server
	const char* data_to_send = "Gangadhar Hi Shaktimaan hai";
	send(sock, data_to_send, strlen(data_to_send), 0);

	// receive

	int n = 0;
	int len = 0, maxlen = 100;
	char buffer[maxlen];
	char* pbuffer = buffer;

	// will remain open until the server terminates the connection
	while ((n = recv(sock, pbuffer, maxlen, 0)) > 0) {
		pbuffer += n;
		maxlen -= n;
		len += n;

		buffer[len] = '\0';
		printf("received: '%s'\n", buffer);
	}

	// close the socket
	close(sock);
	return 0;
}

server.c

#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

/**
 * TCP Uses 2 types of sockets, the connection socket and the listen socket.
 * The Goal is to separate the connection phase from the data exchange phase.
 * */

int main(int argc, char *argv[]) {
	// port to start the server on
	int SERVER_PORT = 8877;

	// socket address used for the server
	struct sockaddr_in server_address;
	memset(&server_address, 0, sizeof(server_address));
	server_address.sin_family = AF_INET;

	// htons: host to network short: transforms a value in host byte
	// ordering format to a short value in network byte ordering format
	server_address.sin_port = htons(SERVER_PORT);

	// htonl: host to network long: same as htons but to long
	server_address.sin_addr.s_addr = htonl(INADDR_ANY);

	// create a TCP socket, creation returns -1 on failure
	int listen_sock;
	if ((listen_sock = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
		printf("could not create listen socket\n");
		return 1;
	}

	// bind it to listen to the incoming connections on the created server
	// address, will return -1 on error
	if ((bind(listen_sock, (struct sockaddr *)&server_address,
	          sizeof(server_address))) < 0) {
		printf("could not bind socket\n");
		return 1;
	}

	int wait_size = 16;  // maximum number of waiting clients, after which
	                     // dropping begins
	if (listen(listen_sock, wait_size) < 0) {
		printf("could not open socket for listening\n");
		return 1;
	}

	// socket address used to store client address
	struct sockaddr_in client_address;
	int client_address_len = 0;

	// run indefinitely
	while (true) {
		// open a new socket to transmit data per connection
		int sock;
		if ((sock =
		         accept(listen_sock, (struct sockaddr *)&client_address,
		                &client_address_len)) < 0) {
			printf("could not open a socket to accept data\n");
			return 1;
		}

		int n = 0;
		int len = 0, maxlen = 100;
		char buffer[maxlen];
		char *pbuffer = buffer;

		printf("client connected with ip address: %s\n",
		       inet_ntoa(client_address.sin_addr));

		// keep running as long as the client keeps the connection open
		while ((n = recv(sock, pbuffer, maxlen, 0)) > 0) {
			pbuffer += n;
			maxlen -= n;
			len += n;

			printf("received: '%s'\n", buffer);

			// echo received content back
			send(sock, buffer, len, 0);
		}

		close(sock);
	}

	close(listen_sock);
	return 0;
}