libevent概述

libevent概述

libevent提供了一种机制，当特定的事件（文件读写、定时器触发）发生时，调用一个回调函数进行处理。事件类型也支持信号或周期定时器。它的目标是替换掉网络程序中的事件循环处理，这样开发程序时，只需要调用event_dispatch()，然后动态地增加、移除事件而不需要处理循环逻辑。

现在libevent支持/dev/poll, kqueue(2), event ports, POSIX select(2), Windows select(), poll(2), 和epoll(4). 内部事件机制完全与特定事件API独立。只需要很小的改动libevent就能支持新的事件支持，而不需要对框架进行重新设计。所以Libevent能够方便地进行跨平台的开发。Libevent也能够在多线程环境使用，这样需要独立的event_base结构。

Libevent 额外提供了复杂的框架来处理带缓存的网络操作，包含sockets、过滤器、限速、SSL、zero-copy文件传输、IOCP。Libevent包含了几个经常用到的协议实现，如DNS、HTTP和一个小型的rpc框架。

源代码下载：libevent-2.1.8-stable.tar.gz

非常多的开源项目使用了libevent，如Memcached、redis、Tor、NTP。

性能测试

示例

#include <netinet/in.h>
#include <sys/socket.h>
#include <fcntl.h>
#include <event2/event.h>
#include <assert.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>

#define MAX_LINE 16384

void do_read(evutil_socket_t fd, short events, void *arg);
void do_write(evutil_socket_t fd, short events, void *arg);

char
rot13_char(char c)
{
/* We don’t want to use isalpha here; setting the locale would change
* which characters are considered alphabetical. */
if ((c >= ‘a’ && c <= ‘m’) || (c >= ‘A’ && c <= ‘M’))
return c + 13;
else if ((c >= ‘n’ && c <= ‘z’) || (c >= ‘N’ && c <= ‘Z’))
return c - 13;
else
return c;
}

struct fd_state {
char buffer[MAX_LINE];
size_t buffer_used;
size_t n_written;
size_t write_upto;
struct event *read_event;
struct event *write_event;
};

struct fd_state *
alloc_fd_state(struct event_base *base, evutil_socket_t fd)
{
struct fd_state *state = malloc(sizeof(struct fd_state));
if (!state)
return NULL;
state->read_event = event_new(base, fd, EV_READ|EV_PERSIST, do_read, state);
if (!state->read_event) {
free(state);
return NULL;
}
state->write_event =
event_new(base, fd, EV_WRITE|EV_PERSIST, do_write, state);

if (!state->write_event) {
    event_free(state->read_event);
    free(state);
    return NULL;
}

state->buffer_used = state->n_written = state->write_upto = 0;

assert(state->write_event);
return state;

}

void
free_fd_state(struct fd_state *state)
{
event_free(state->read_event);
event_free(state->write_event);
free(state);
}

void
do_read(evutil_socket_t fd, short events, void *arg)
{
struct fd_state *state = arg;
char buf[1024];
int i;
ssize_t result;
while (1) {
assert(state->write_event);
result = recv(fd, buf, sizeof(buf), 0);
if (result <= 0)
break;

    for (i=0; i < result; ++i)  {
        if (state->buffer_used < sizeof(state->buffer))
            state->buffer[state->buffer_used++] = rot13_char(buf[i]);
        if (buf[i] == '\n') {
            assert(state->write_event);
            event_add(state->write_event, NULL);
            state->write_upto = state->buffer_used;
        }
    }
}

if (result == 0) {
    free_fd_state(state);
} else if (result < 0) {
    if (errno == EAGAIN) // XXXX use evutil macro
        return;
    perror("recv");
    free_fd_state(state);
}

}

void
do_write(evutil_socket_t fd, short events, void *arg)
{
struct fd_state *state = arg;

while (state->n_written < state->write_upto) {
    ssize_t result = send(fd, state->buffer + state->n_written,
                          state->write_upto - state->n_written, 0);
    if (result < 0) {
        if (errno == EAGAIN) // XXX use evutil macro
            return;
        free_fd_state(state);
        return;
    }
    assert(result != 0);

    state->n_written += result;
}

if (state->n_written == state->buffer_used)
    state->n_written = state->write_upto = state->buffer_used = 1;

event_del(state->write_event);

}

void
do_accept(evutil_socket_t listener, short event, void *arg)
{
struct event_base base = arg;
struct sockaddr_storage ss;
socklen_t slen = sizeof(ss);
int fd = accept(listener, (struct sockaddr)&ss, &slen);
if (fd < 0) { // XXXX eagain??
perror(“accept”);
} else if (fd > FD_SETSIZE) {
close(fd); // XXX replace all closes with EVUTIL_CLOSESOCKET */
} else {
struct fd_state *state;
evutil_make_socket_nonblocking(fd);
state = alloc_fd_state(base, fd);
assert(state); /XXX err/
assert(state->write_event);
event_add(state->read_event, NULL);
}
}

void
run(void)
{
evutil_socket_t listener;
struct sockaddr_in sin;
struct event_base *base;
struct event *listener_event;

base = event_base_new();
if (!base)
    return; /*XXXerr*/

sin.sin_family = AF_INET;
sin.sin_addr.s_addr = 0;
sin.sin_port = htons(40713);

listener = socket(AF_INET, SOCK_STREAM, 0);
evutil_make_socket_nonblocking(listener);

#ifndef WIN32
{
int one = 1;
setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one));
}
#endif

if (bind(listener, (struct sockaddr*)&sin, sizeof(sin)) < 0) {
    perror("bind");
    return;
}

if (listen(listener, 16)<0) {
    perror("listen");
    return;
}

listener_event = event_new(base, listener, EV_READ|EV_PERSIST, do_accept, (void*)base);
/*XXX check it */
event_add(listener_event, NULL);

event_base_dispatch(base);

}

int
main(int c, char **v)
{
setvbuf(stdout, NULL, _IONBF, 0);

run();
return 0;

}