libuEv | Simple event loop for Linux

Table of Contents

• Introduction
• API
• Example
• Build & Install
• Origin & References

Introduction

“Why an event loop, why not use threads?”

With the advent of light-weight processes (threads) programmers these days have a golden hammer they often swing without consideration. Event loops and non-blocking I/O is often a far easier approach, as well as less error prone.

The purpose of many applications is, with a little logic sprinkled on top, to act upon: network packets entering an interface, timeouts expiring, mouse clicks, or other types of events. Such applications are often very well suited to use an event loop.

Applications that need to churn massively parallel algorithms are more suitable for running multiple (independent) threads on several CPU cores. However, threaded applications must deal with the side effects of concurrency, like: race conditions, deadlocks, live locks, etc. Writing error free threaded applications is hard, debugging them can be even harder.

Sometimes the combination of multiple threads and an event loop per thread can be the best approach, but each application of course needs to be broken down individually to find the most optimal approach. Do keep in mind, however, that not all systems your application will run on have multiple CPU cores -- some small embedded systems still use a single CPU core, even though they run Linux, with multiple threads a program may actually run slower! Always profile your program, and if possible, test it on different architectures.

libuEv is a simple event loop in the style of the more established libevent, libev and the venerable Xt(3) event loop. The u (micro) in the name refers to both the small feature set and the small size overhead impact of the library. The primary target of libuEv is a modern Linux system.

Experienced developers may appreciate that libuEv is built on top of modern Linux APIs: epoll, timerfd and signalfd. Note, a certain amount of care is needed when dealing with APIs that employ signalfd. For details, see this article at lwn.net.

“Event driven software improves concurrency” -- Dave Zarzycki, Apple

API

Here is the interface to libuEv. It handles three different types of events: I/O (files, sockets, message queues, etc.), timers, and signals.

    /* Callback example, arg is passed from watcher's *_init()
     * w->fd holds the file descriptor, events is set by libuEv
     * to indicate if any of UEV_READ and/or UEV_WRITE is ready.
     */
    void callback       (uev_ctx_t *ctx, uev_t *w, void *arg, int events);

    /* Event loop functions, notice use of flags! */
    int uev_init        (uev_ctx_t *ctx);
    int uev_exit        (uev_ctx_t *ctx);
    int uev_run         (uev_ctx_t *ctx, int flags);         /* UEV_NONE, UEV_ONCE, and/or UEV_NONBLOCK */
    
    /* I/O watcher:     fd is non-blocking, events is UEV_READ and/or UEV_WRITE */
    int uev_io_init     (uev_ctx_t *ctx, uev_t *w, uev_cb_t *cb, void *arg, int fd, int events);
    int uev_io_set      (uev_t *w, int fd, int events);
    int uev_io_start    (uev_t *w);
    int uev_io_stop     (uev_t *w);
    
    /* Timer watcher:   timeout and period in milliseconds */
    int uev_timer_init  (uev_ctx_t *ctx, uev_t *w, uev_cb_t *cb, void *arg, int timeout, int period);
    int uev_timer_set   (uev_t *w, int timeout, int period); /* Change timeout or period */
    int uev_timer_start (uev_t *w);                          /* Restart a stopped timer */
    int uev_timer_stop  (uev_t *w);                          /* Stop a timer */
    
    /* Signal watcher:  signo is the signal to wait for, e.g., SIGTERM */
    int uev_signal_init (uev_ctx_t *ctx, uev_t *w, uev_cb_t *cb, void *arg, int signo);
    int uev_signal_set  (uev_t *w, int signo);               /* Change signal to wait for */
    int uev_signal_start(uev_t *w);                          /* Restart a stopped signal watcher */
    int uev_signal_stop (uev_t *w);                          /* Stop signal watcher */

To be able to set up callbacks to events the developer first need to create an event context, achieved by calling uev_init() with a pointer to a local uev_ctx_t variable.

Events are monitored by watchers in libuEv. A watcher is nothing more than a mechnism that polls a file descriptor. Register a watcher with a callback to the event context by passing the uev_ctx_t variable, along with an uev_t variable to each event's _init() function.

When all watchers are registered call the event loop with uev_run() and the argument to the event context. The flags parameter is slightly mysterious, but can be used to integrate libuEv into another event loop. With flags set to UEV_ONCE the event loop returns after having served the first event. If flags is set to UEV_ONCE | UEV_NONBLOCK the event loop returns immediately if no event is available.

Summary:

1. Prepare an event context with uev_init()
2. Register event callbacks with uev_io_init(), uev_signal_init() or uev_timer_init()
3. Enter the event loop with uev_run()
4. Leave the event loop with uev_exit(), possibly from a callback

Note 1: Make sure to use non-blocking stream I/O! Most hard to find bugs in event driven applications are due to sockets and files being opened in blocking mode. Be careful out there!

Note 2: As mentioned above, a certain amount of care is needed when dealing with signalfd. This means that if your application, for instance, uses system() you must redesign that to use fork(), and then in the child, unblock all signals blocked by your parent process, before you run exec(). This because Linux does not unblock signals for your children, and neither does most (all?) C-libraries. An example of this, from finit, implementing run() as a better replacement to system(), which sucks anyawy :)

Example

Here follows a very brief example to illustrate how one can use libuEv to act upon joystick input.

#include <err.h>
#include <errno.h>
#include <stdio.h>
#include <stdint.h>
#include <fcntl.h>
#include <unistd.h>

#include "uev.h"

struct js_event {
	uint32_t time;		/* event timestamp in milliseconds */
	int16_t  value;		/* value */
	uint8_t  type;		/* event type */
	uint8_t  number;	/* axis/button number */
} e;

static void joystick_cb(uev_ctx_t *ctx, uev_t *w, void *arg, int events)
{
	read (w->fd, &e, sizeof(e));

	switch (e.type) {
	case 1:
		if (e.value) printf("Button %d pressed\n", e.number);
		else 	     printf("Button %d released\n", e.number);
		break;

	case 2:
		printf("Joystick axis %d moved, value %d!\n", e.number, e.value);
		break;
	}
}

int main(void)
{
	int       fd = open("/dev/input/js1", O_RDONLY, O_NONBLOCK);
	uev_t     js1_watcher;
	uev_ctx_t ctx;

	if (fd < 0)
		errx(errno, "Cannot find a joystick attached.");

	uev_init(&ctx);
	uev_io_init(&ctx, &js1_watcher, joystick_cb, NULL, fd, UEV_READ);

	puts("Starting, press Ctrl-C to exit.");

	return uev_run(&ctx, 0);
}

To compile the program, save the code as joystick.c and call GCC with gcc -o joystick joystick.c io.c timer.c signal.c main.c from this directory, skips using a Makefile altogether. Alternatively, call the Makefile with make joystick from this directory.

More complete and relevant example uses of libuEv is the TFTP/FTP server uftpd, and the Linux /sbin/init replacement finit. Both use libuEv as a GIT submodule.

Also, see the bench.c program (make bench from within the library) for reference benchmarks against libevent and libev.

Build & Install

The library is built for and developed on GNU/Linux systems, so it may use GCC and GNU Make specific extensions here and there. This is not on purpose and patches to correct this are most welcome. Particularly patches to support *BSD and its kqueue interface.

• make all: The library
• make test: Test and showcase
• make install: Honors $prefix and $DESTDIR environment variables

Size of libuEv:

$ make strip
CC      main.o
CC      io.o
CC      timer.o
CC      signal.o
ARCHIVE libuev.a
LINK    libuev.so.1
STRIP   libuev.a
STRIP   libuev.so.1
text	   data	    bss	    dec	    hex	filename
1177	      0	      0	   1177	    499	main.o     (ex libuev.a)
 308	      0	      0	    308	    134	io.o       (ex libuev.a)
 682	      0	      0	    682	    2aa	timer.o    (ex libuev.a)
 563	      0	      0	    563	    233	signal.o   (ex libuev.a)
6306	    768	      8	   7082	   1baa	libuev.so.1

Origin & References

libuEv was originally based on LibUEvent by Flemming Madsen but has been completely rewritten with a much clearer API. Now more similar to the famous libev by Mark Lehmann. Another small event library used for inspiration is the very small Picoev by Oku Kazuho.

libuEv is developed and maintained by Joachim Nilsson