Simple Kqueue Echo Server in C

Shaowen Zhu — Sat, 23 Jul 2022 13:28:00 +0000

There are three important functions when use kqueue.

int kqueue(void);

int kevent(int  kq, 
    const struct kevent* changelist, int nchanges, 
    struct kevent *eventlist, int nevents, 
    const struct timespec *timeout);

EV_SET(kev, ident, filter, flags, fflags, data, udata);

The kqueue() system call creates a new kernel event queue and returns a descriptor.

The kevent() system call is used to register events with the queue, and return any pending events to the user.

change kqueue

When you want to change the queue, like delete a socket file descriptor from kqueue for example, or add listen socket to kqueue, you can call kevent() like below:

int kq = kqueue();
struct kevent event;
EV_SET(&event, listen_fd, EVFILT_READ, EV_ADD, 0, 0, NULL);
kevent(kq, &event, 1, NULL, 0, NULL);

As the codes above, when we want to change(delete/add) a event to a kqueue, there are two steps, first thing we should do is to initialize kevent with a file descriptor we want to monitor, secondly is to add this kevent to kqueue using kevent().

monitor kqueue

When we want to monitor kqueue, we don't have to provide changelist and nchanges parameter of kevent(). We just need to provide these two parameters: eventlist and nevents, like below:

struct kevent event_list[MAX_EVENTS];
int num_events = kevent(kq, NULL, 0, event_list, MAX_EVENTS, NULL);

remove kevent

When a client disconnect, we don't care(monitor) the kevent associated with this client anymore, so we need to remove this kevent from kqueue. But we don't have to call EV_SET() and kevent(), we just need to close this file descriptor. Cause the man page says below:

Calling close() on a file descriptor will remove any kevents that reference the descriptor.

source code

reference

kqueue

Streaming Server Using Kqueue

DEV Community: Shaowen Zhu

Simple Kqueue Echo Server in C