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;
}
		

This version was mainly generated by a LLM, but nevertheless it's working. Calls to read() and write() are blocking, but this is acceptable for this simple example... production server probably will want to make the sockets file descriptors non blocking to improve performance, but this will increase complexity as error handling is different for async write/read, and if you defer processing data you'll need to dynamically allocate a buffer for each client in order to avoid overwriting data that isn't processed yet.

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';