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A TCP server with kqueue

frevib profile image Frevib ・3 min read

Kernel queue (kqueue)

If you are programming with asynchronous (non-blocking) frameworks, then most likely this framework is using kernel polling APIs under the hood. Using an infinite loop, kernel polling APIs check for events and notify the caller when an event has occurred. For example when an HTTP request is fired, in each iteration of the loop a check is done to see if data already has returned from the server. The major OSes have implemented kernel polling in a different way:

  • Linux uses epoll
  • BSD and OSX use kqueue
  • Windows uses IOCP

epoll and kqueue are functionally almost the same, but there are some API differences. Let's write a bare minimum TCP server to understand how kqueue works.

Steps for creating a kqueue TCP server

  • Create a listening socket. This includes creating the socket with socket(2) and binding the socket to and IP and port with bind(2).

  • Create kevents we want to monitor with the helper macro EV_SET. Then create the kqueue itself with kqueue(2), and add the kevents to the kqueue using kevent(2).

  • Create an "event loop". This is just an infinite loop that checks if new clients have connected to our listening socket, and creates a new socket for the actual connection to the client using accept(8).

Code including Makefile on github

#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/event.h>
#include <string.h>
#include <unistd.h>

int main()
    // All needed variables.
    int socket_listen_fd,
        portno = 1815,
    struct kevent change_event[4],
    struct sockaddr_in serv_addr,

    // Create socket.
    if (((socket_listen_fd = socket(AF_INET, SOCK_STREAM, 0)) < 0))
        perror("ERROR opening socket");

    // Create socket structure and bind to ip address.
    bzero((char *)&serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = INADDR_ANY;
    serv_addr.sin_port = htons(portno);

    if (bind(socket_listen_fd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
        perror("Error binding socket");

    // Start listening.
    listen(socket_listen_fd, 3);
    client_len = sizeof(client_addr);

    // Prepare the kqueue.
    kq = kqueue();

    // Create event 'filter', these are the events we want to monitor.
    // Here we want to monitor: socket_listen_fd, for the events: EVFILT_READ 
    // (when there is data to be read on the socket), and perform the following
    // actions on this kevent: EV_ADD and EV_ENABLE (add the event to the kqueue 
    // and enable it).
    EV_SET(change_event, socket_listen_fd, EVFILT_READ, EV_ADD | EV_ENABLE, 0, 0, 0);

    // Register kevent with the kqueue.
    if (kevent(kq, change_event, 1, NULL, 0, NULL) == -1)

    // Actual event loop.
    for (;;)
        // Check for new events, but do not register new events with
        // the kqueue. Hence the 2nd and 3rd arguments are NULL, 0.
        // Only handle 1 new event per iteration in the loop; 5th
        // argument is 1.
        new_events = kevent(kq, NULL, 0, event, 1, NULL);
        if (new_events == -1)

        for (int i = 0; new_events > i; i++)
            int event_fd = event[i].ident;

            // When the client disconnects an EOF is sent. By closing the file
            // descriptor the event is automatically removed from the kqueue.
            if (event[i].flags & EV_EOF)
                printf("Client has disconnected");
            // If the new event's file descriptor is the same as the listening
            // socket's file descriptor, we are sure that a new client wants 
            // to connect to our socket.
            else if (event_fd == socket_listen_fd)
                // Incoming socket connection on the listening socket.
                // Create a new socket for the actual connection to client.
                socket_connection_fd = accept(event_fd, (struct sockaddr *)&client_addr, (socklen_t *)&client_len);
                if (socket_connection_fd == -1)
                    perror("Accept socket error");

                // Put this new socket connection also as a 'filter' event
                // to watch in kqueue, so we can now watch for events on this
                // new socket.
                EV_SET(change_event, socket_connection_fd, EVFILT_READ, EV_ADD, 0, 0, NULL);
                if (kevent(kq, change_event, 1, NULL, 0, NULL) < 0)
                    perror("kevent error");

            else if (event[i].filter & EVFILT_READ)
                // Read bytes from socket
                char buf[1024];
                size_t bytes_read = recv(event_fd, buf, sizeof(buf), 0);
                printf("read %zu bytes\n", bytes_read);

    return 0;

Discussion (1)

frosnerd profile image
Frank Rosner

Thank you very much for sharing!

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