Echo server & client in C.

build.cmd

@echo off
tcc echo_server.c
tcc echo_client.c

echo_client.c

#include <stdio.h>
#include <string.h>
#include <winsock2.h>

#pragma comment(lib, "ws2_32")

void main() {
	WSADATA wsa;
	if (WSAStartup(MAKEWORD(2, 2), &wsa) != 0) {
		printf("WSAStartup() failed\n");
		return;
	}

	SOCKET client;
	if ((client = socket(AF_INET, SOCK_STREAM, 0)) == INVALID_SOCKET) {
		printf("socket() failed\n");
		return;
	}

	struct sockaddr_in addr = {};
	addr.sin_family = AF_INET;
	addr.sin_addr.s_addr = inet_addr("127.0.0.1");
	addr.sin_port = 8080;

	if (connect(client, &addr, sizeof(addr)) == SOCKET_ERROR) {
		printf("connect() failed\n");
		return;
	}

	char* msg = "Hello world!";
	int len = strlen(msg);

	int total = 0;
	do {
		int sent;
		if ((sent = send(client, msg, len, 0)) == SOCKET_ERROR) {
			printf("send() failed\n");
			return;
		}
		total += sent;
	} while (total < len);

	printf("sent: %s\n", msg);

	char msg2[1024] = {};
	int len2;
	if ((len2 = recv(client, msg2, sizeof(msg2) - 1, 0)) == SOCKET_ERROR) {
		printf("recv() failed\n");
		return;
	}

	printf("received: %s\n", msg2);

	if (closesocket(client) == SOCKET_ERROR) {
		printf("closesocket(client) failed\n");
		return;
	}

	if (WSACleanup() != 0) {
		printf("WSACleanup() failed\n");
		return;
	}
}

echo_server.c

#include <stdio.h>
#include <winsock2.h>

#pragma comment(lib, "ws2_32")

void main() {
	WSADATA wsa;
	if (WSAStartup(MAKEWORD(2, 2), &wsa) != 0) {
		printf("WSAStartup() failed\n");
		return;
	}

	SOCKET server;
	if ((server = socket(AF_INET, SOCK_STREAM, 0)) == INVALID_SOCKET) {
		printf("socket() failed\n");
		return;
	}

	struct sockaddr_in addr = {};
	addr.sin_family = AF_INET;
	addr.sin_addr.s_addr = INADDR_ANY;
	addr.sin_port = 8080;

	if (bind(server, &addr, sizeof(addr)) == SOCKET_ERROR) {
		printf("bind() failed\n");
		return;
	}

	if (listen(server, 1) == SOCKET_ERROR) {
		printf("listen() failed\n");
		return;
	}

	SOCKET client;
	if ((client = accept(server, 0, 0)) == INVALID_SOCKET) {
		printf("accept() failed\n");
		return;
	}

	char msg[1024] = {};
	int len;
	if ((len = recv(client, msg, sizeof(msg), 0)) == SOCKET_ERROR) {
		printf("recv() failed\n");
		return;
	}

	printf("received: %s\n", msg);
	msg[0] = 'X';

	int total = 0;
	do {
		int sent;
		if ((sent = send(client, msg, len, 0)) == SOCKET_ERROR) {
			printf("send() failed\n");
			return;
		}
		total += sent;
	} while (total < len);

	printf("sent: %s\n", msg);

	if (closesocket(client) == SOCKET_ERROR) {
		printf("closesocket(client) failed\n");
		return;
	}

	if (closesocket(server) == SOCKET_ERROR) {
		printf("closesocket(server) failed\n");
		return;
	}

	if (WSACleanup() != 0) {
		printf("WSACleanup() failed\n");
		return;
	}
}