Example epoll event loop based TCP server

epoll_tcp_server.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <sys/epoll.h>

#define BUFFER_SIZE 256
#define PORT 4000
#define IP_ADDRESS "127.0.0.1"
#define MAX_EVENTS 10

int main() {
    int server_fd, client_fd, epoll_fd;
    struct sockaddr_in server_addr, client_addr;
    socklen_t client_len = sizeof(client_addr);
    char buffer[BUFFER_SIZE];

    // Create a socket
    server_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (server_fd < 0) {
        perror("socket creation failed");
        exit(EXIT_FAILURE);
    }

    // Set address and port number for the server
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(PORT);
    inet_pton(AF_INET, IP_ADDRESS, &server_addr.sin_addr);

    // Bind the socket to the address and port
    if (bind(server_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }

    // Listen for incoming connections
    if (listen(server_fd, 3) < 0) {
        perror("listen failed");
        exit(EXIT_FAILURE);
    }

    printf("Server listening on %s:%d...\n", IP_ADDRESS, PORT);

    // Create an epoll instance
    epoll_fd = epoll_create1(0);
    if (epoll_fd < 0) {
        perror("epoll creation failed");
        exit(EXIT_FAILURE);
    }

    // Add the server socket to the epoll instance
    struct epoll_event event;
    event.events = EPOLLIN;
    event.data.fd = server_fd;
    if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, server_fd, &event) < 0) {
        perror("epoll_ctl failed");
        exit(EXIT_FAILURE);
    }

    while (1) {
        // Wait for events on the epoll instance
        struct epoll_event events[MAX_EVENTS];
        int num_events = epoll_wait(epoll_fd, events, MAX_EVENTS, -1);
        if (num_events < 0) {
            perror("epoll_wait failed");
            exit(EXIT_FAILURE);
        }

        // Handle each event
        for (int i = 0; i < num_events; i++) {
            if (events[i].data.fd == server_fd) {
                // Accept a new connection
                client_fd = accept(server_fd, (struct sockaddr *)&client_addr, &client_len);
                if (client_fd < 0) {
                    perror("accept failed");
                    continue;
                }

                printf("Connection accepted from client IP address %s and port %d...\n",
                       inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));

                // Add the client socket to the epoll instance
                event.events = EPOLLIN;
                event.data.fd = client_fd;
                if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, client_fd, &event) < 0) {
                    perror("epoll_ctl failed");
                    continue;
                }
            } else {
                // Read from the client
                int bytes_read = read(events[i].data.fd, buffer, BUFFER_SIZE);
                if (bytes_read < 0) {
                    perror("read failed");
                    continue;
                } else if (bytes_read == 0) {
                    printf("Client disconnected...\n");
                    close(events[i].data.fd);
                    epoll_ctl(epoll_fd, EPOLL_CTL_DEL, events[i].data.fd, NULL);
                    continue;
                }

                printf("Received message from client: %s\n", buffer);
				
		// Check for newline character
		if (buffer[0] == '\n') {
			printf("Client sent newline, closing connection...\n");
			close(events[i].data.fd);
			epoll_ctl(epoll_fd, EPOLL_CTL_DEL, events[i].data.fd, NULL);
			continue;
		}

                // Write back to the client
                int bytes_written = write(events[i].data.fd, buffer, bytes_read);
                if (bytes_written < 0) {
                    perror("write failed");
                    continue;
                }
            }
        }
    }

    return 0;
}
		

Before line 105 it should probably terminate the string before printing it to avoid printing after the end of the message:

buffer[bytes_read] = '\0';