libev入门
引言
实现应用的捷径是充分利用开放源代码和开放标准等资源。为了实现视频服务器，研究了开源视频服务器DarwinStreamingServer，Reactor并发编程设计模式，同时还评估了Boost.Asio、ACE、libevent以及 libev等网络编程相关的库。得出的结论是基于DarwinStreamingServer的设计思想，采用Reactor设计模式实现一个更加高效并可扩展的视频服务器，网络库则选择libev。
本文的目的是在理解libev的基础上，学习如何编写基于libev的应用程序。
libev是什么
要了解libev，有必要先了解libevent。早在2000年libevent就发布了第一个版本。通过libevent的编程接口，可以实现在指定的事件发生后调用回调函数，指定的事件是文件描述符上的读写事件等。除了文件描述符的事件，libevent还支持定时器以及信号等。欲详细了解libevent可以参照其站点：http://monkey.org/~provos/libevent/。
以下是libev站点上的自述：
A full-featured and high-performance event loop that is loosely modelled after libevent, but without its limitations and bugs. It is used, among others, in the GNU Virtual Private Ethernet and rxvt-unicode packages, and in the Deliantra MORPG Server and Client.
理解libev
Libev是一个event loop：向libev注册感兴趣的events，比如Socket可读事件，libev会对所注册的事件的源进行管理，并在事件发生时触发相应的程序。
To do this, it must take more or less complete control over your process (or thread) by executing the event loop handler, and will then communicate events via a callback mechanism.
通过event watcher来注册事件，which are relatively small C structures you initialise with the details of the event, and then hand it over to libev by starting the watcher.
先来解释watcher，libev通过分配和注册watcher对不同类型的事件进行监听。不同事件类型的watcher又对应不同的数据类型，watcher的定义模式是
struct ev_TYPE或者ev_TYPE，其中TYPE为具体的类型。当前libev定义了如下类型的watcher：
- ev_io
- ev_timer
- ev_periodic
- ev_signal
- ev_child
- ev_stat
- ev_idle
- ev_prepare and ev_check
- ev_embed
- ev_fork
- ev_cleanup
- ev_async
下面是一个libev使用的例子，通过注册io类型的watcher来监视STDIN可读事件的发生：
static void my_cb (struct ev_loop *loop, ev_io *w, int revents)
{
ev_io_stop (w);
ev_break (loop, EVBREAK_ALL);
}
struct ev_loop *loop = ev_default_loop (0);
ev_io stdin_watcher;，
ev_init (&stdin_watcher, my_cb);
ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
ev_io_start (loop, &stdin_watcher);
ev_run (loop, 0);
上面的示例代码中用到的与watcher相关的函数有ev_init，ev_io_set，ev_io_start，ev_io_stop。ev_init对一个watcher的与具体类型无关的部分进行初始化。ev_io_set对watcher的与io类型相关的部分进行初始化，显然如果是TYPE类型那么相应的函数就是ev_TYPE_set。可以采用ev_TYPE_init函数来替代ev_init和ev_TYPE_set。ev_io_start激活相应的watcher，watcher只有被激活的时候才能接收事件。ev_io_stop停止已经激活的watcher。
{
ev_io_stop (w);
ev_break (loop, EVBREAK_ALL);
}
struct ev_loop *loop = ev_default_loop (0);
ev_io stdin_watcher;，
ev_init (&stdin_watcher, my_cb);
ev_io_set (&stdin_watcher, STDIN_FILENO, EV_READ);
ev_io_start (loop, &stdin_watcher);
ev_run (loop, 0);
接下来看看event loop的概念。示例程序中的ev_run、ev_break以及ev_loop_default都是event loop控制函数。event loop定义为struct ev_loop。有两种类型的event loop，分别是default类型和dynamically created类型，区别是前者支持子进程事件。ev_default_loop和ev_loop_new函数分别用于创建default类型或者dynamically created类型的event loop。
event_run函数告诉系统应用程序开始对事件进行处理，有事件发生时就调用watcher callbacks。除非调用了ev_break或者不再有active的watcher，否则会一直重复这个过程。
libev编程
先来看libev提供的例子。这段代码等待键盘事件的发生，或者超时，两个事件都会触发程序结束。操作系统环境是ubuntu server 10.10，libev是下载的源码，并没有采用ubuntu server自己提供的版本，源码的地址是 http://dist.schmorp.de/libev/libev-4.04.tar.gz。例子代码如下：
// 只需include一个头文件
#include <ev.h>
#include <stdio.h> // for puts
// every watcher type has its own typedef'd struct
// with the name ev_TYPE
ev_io stdin_watcher;
ev_timer timeout_watcher;
// all watcher callbacks have a similar signature
// this callback is called when data is readable on stdin
static void
stdin_cb (EV_P_ ev_io *w, int revents)
{
puts ("stdin ready");
// for one-shot events, one must manually stop the watcher
// with its corresponding stop function.
ev_io_stop (EV_A_ w);
// this causes all nested ev_run's to stop iterating
ev_break (EV_A_ EVBREAK_ALL);
}
// another callback, this time for a time-out
static void
timeout_cb (EV_P_ ev_timer *w, int revents)
{
puts ("timeout");
// this causes the innermost ev_run to stop iterating
ev_break (EV_A_ EVBREAK_ONE);
}
int
main (void)
{
// use the default event loop unless you have special needs
struct ev_loop *loop = EV_DEFAULT;
// initialise an io watcher, then start it
// this one will watch for stdin to become readable
ev_io_init (&stdin_watcher, stdin_cb, 0, EV_READ);
ev_io_start (loop, &stdin_watcher);
// initialise a timer watcher, then start it
// simple non-repeating 5.5 second timeout
ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
ev_timer_start (loop, &timeout_watcher);
// now wait for events to arrive
ev_run (loop, 0);
// break was called, so exit
return 0;
}
#include <ev.h>
#include <stdio.h> // for puts
// every watcher type has its own typedef'd struct
// with the name ev_TYPE
ev_io stdin_watcher;
ev_timer timeout_watcher;
// all watcher callbacks have a similar signature
// this callback is called when data is readable on stdin
static void
stdin_cb (EV_P_ ev_io *w, int revents)
{
puts ("stdin ready");
// for one-shot events, one must manually stop the watcher
// with its corresponding stop function.
ev_io_stop (EV_A_ w);
// this causes all nested ev_run's to stop iterating
ev_break (EV_A_ EVBREAK_ALL);
}
// another callback, this time for a time-out
static void
timeout_cb (EV_P_ ev_timer *w, int revents)
{
puts ("timeout");
// this causes the innermost ev_run to stop iterating
ev_break (EV_A_ EVBREAK_ONE);
}
int
main (void)
{
// use the default event loop unless you have special needs
struct ev_loop *loop = EV_DEFAULT;
// initialise an io watcher, then start it
// this one will watch for stdin to become readable
ev_io_init (&stdin_watcher, stdin_cb, 0, EV_READ);
ev_io_start (loop, &stdin_watcher);
// initialise a timer watcher, then start it
// simple non-repeating 5.5 second timeout
ev_timer_init (&timeout_watcher, timeout_cb, 5.5, 0.);
ev_timer_start (loop, &timeout_watcher);
// now wait for events to arrive
ev_run (loop, 0);
// break was called, so exit
return 0;
}
用如下命令编译：
gcc -lev -o sample sample.c
参考资料
- libev的文档
- http://software.schmorp.de/pkg/libev.html