Libevent Analysis Notes (1) - Installation and Usage

Introduction to Libevent

The libevent API provides a mechanism to execute a callback function when a specific event occurs on a file descriptor or after a timeout has been reached. Furthermore, libevent also support callbacks due to signals or regular timeouts. The libevent API provides such a mechanism:
Execute a specified callback function when a specific event occurs in a specified file descriptor, or when a timeout is reached.
Additionally, libevent callback functionality also supports triggering by signals or regular timeouts.
Note that its license is a BSD-style license, which can be used in commercial products without having to open source.

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1. Compilation and Installation

​ In Ubuntu, you can install directly using apt:

~$  sudo apt-get install libevent

​ Or use:

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~$  wget http://monkey.org/~provos/libevent-1.4.13-stable.tar.gz
~$  tar –xzvf  libevent-1.4.13-stable.tar.gz
~$  cd libevent-1.4.13-stable
~$  ./configure
~$  make
~$  sudo make install

2. Library Usage

You can understand the basic usage of libevent from these simple examples. When compiling, you need to add the “-levent” parameter.

2.1. I/O Events (corresponding to EV_READ, EV_WRITE)

Below is a simplified server program snippet that uses libevent to return system time.

1void get_time(int fd,short event,struct event *arg) ///Get system time and write it back/
2{
3……
4 localtime_r(&now,&t);
5 asctime_r(&t,buf);
6 write( fd,buf,strlen(buf) );
7……
8}
9void con_accept(int fd,short event,void *arg) ///Callback function provided to the event, accepts a connection/
10{
11 struct sockaddr_in s_in;
12 socklen_t len = sizeof(s_in);
13 int ns = accept(fd,(struct sockaddr *) &s_in,&len);
14 struct event *ev = malloc(sizeof(struct event));
15 event_set(ev,ns,EV_WRITE,(void )get_time,ev);
16 event_add(ev,NULL);
17}
18
19int main()
20{
21 int sockfd = socket(PF_INET,SOCK_STREAM,0);
22 struct sockaddr_in s_in;
23……
24 bind(sockfd,(struct sockaddr) &s_in,sizeof(s_in));
25 listen(sockfd,5);
26 event_init(); //libevent initialization
27
28 struct event ev;
29 event_set(&ev,sockfd,EV_READ|EV_PERSIST,con_accept,&ev);
30//Set event properties to readable, persistent, callback function is con_accept()
31 event_add(&ev,NULL);//Add event, no timeout set
32 event_dispatch();//Enter libevent main loop
33 return 0;
34}
35

2.2. Signal Handling Events (corresponding to EV_SIGNAL)

1 static void signal_cb(int fd, short event, void *arg)
2{
3 struct event *signal = arg;
4 printf("%s: got signal %d\n", func, EVENT_SIGNAL(signal));
5 if (called >= 2)
6 event_del(signal); //If called more than twice, delete this signal
7 called++;
8}
9
10int main (int argc, char **argv)
11{
12 struct event signal_int;
13 event_init();//libevent initialization
14 event_set(&signal_int, SIGINT, EV_SIGNAL|EV_PERSIST, signal_cb, &signal_int);
15//Set event properties to signal-triggered, persistent, callback function is signal_cb()
16 event_add(&signal_int, NULL); //Add event
17 event_dispatch();//Enter libevent main loop
18 return 0;
19}
20

2.3. Regular Timeout Handling (corresponding to EV_TIMEOUT)

1static void timeout_cb(int fd, short event, void *arg)
2{
3 struct timeval tv;
4 struct event *timeout = arg;
5 int newtime = time(NULL);
6 printf("%s: called at %d: %d\n", func, newtime,
7 newtime - lasttime);
8 lasttime = newtime;
9 evutil_timerclear(&tv);
10 tv.tv_sec = 2;
11 event_add(timeout, &tv);
12}
13
14int main (int argc, char **argv)
15{
16 struct event timeout;
17 struct timeval tv;
18 event_init();//libevent initialization
19 evtimer_set(&timeout, timeout_cb, &timeout);
20//Actually this function corresponds to event_set(timeout, -1, 0, timeout_cb,&timeout)
21evutil_timerclear(&tv); //If there’s a time clearing function, it directly corresponds to timerclear(), otherwise set tv_sec and tv_usec to 0
22
23 tv.tv_sec = 2;
24 event_add(&timeout, &tv);
25 lasttime = time(NULL);
26 event_dispatch();
27 return 0;
28}
29

Original link: http://www.cppblog.com/xguru/archive/2010/06/24/118597.html