Libevent：0异步IO简介

一：异步IO简介

大多数的初级编程者都是从阻塞IO调用开始网络编程的。阻塞（同步）IO调用指的是：调用会一直阻塞，不会返回，直到发生下面两种情况之一。要么操作完成，要么经历相当长的时间，网络协议栈自己放弃。

比如，当在TCP连接上调用connect时，操作系统会发送SYN包到TCP的远端主机。connect会一直阻塞而不返回，直到它接收到了远端主机发来的SYN+ACK包，或者经历太长的时间而自己放弃。

下面是一个简单的使用阻塞网络调用的客户端例子。它链接google，发送简单的HTTP请求，然后将响应输出到stdout。

Example:A simple blocking HTTP client

/* For  sockaddr_in */

#include  <netinet/in.h>

/* For socket functions */

#include <sys/socket.h>

/* For gethostbyname */

#include <netdb.h>

#include <unistd.h>

#include <string.h>

#include <stdio.h>

int main(int c,char
**v)

{

const char query[] =

"GET / HTTP/1.0\r\n"

"Host: www.google.com\r\n"

"\r\n";

const char hostname[] = "www.google.com";

struct sockaddr_in sin;

struct hostent *h;

const char *cp;

int fd;

ssize_t n_written, remaining;

char buf[1024];

    /* Look up the IP address for the hostname.   Watch out; this isn't

       threadsafe on most platforms. */

h = gethostbyname(hostname);

if (!h) {

fprintf(stderr, "Couldn't lookup%s: %s", hostname, hstrerror(h_errno));

return 1;

}

if (h->h_addrtype != AF_INET) {

fprintf(stderr, "No ipv6 support,sorry.");

return 1;

}

    /* Allocate a new socket */

fd = socket(AF_INET, SOCK_STREAM, 0);

if (fd < 0) {

perror("socket");

return 1;

}

    /* Connect to the remote host. */

sin.sin_family = AF_INET;

sin.sin_port = htons(80);

sin.sin_addr = *(struct in_addr*)h->h_addr;

if (connect(fd, (struct
sockaddr*) &sin, sizeof(sin))) {

perror("connect");

close(fd);

return 1;

}

    /* Write the query. */

    /* XXX Can send succeed partially? */

cp = query;

remaining = strlen(query);

while (remaining) {

n_written = send(fd, cp, remaining, 0);

if (n_written <= 0) {

perror("send");

return 1;

}

remaining -= n_written;

cp += n_written;

}

    /* Get an answer back. */

while (1) {

ssize_t result = recv(fd, buf,sizeof(buf), 0);

if (result == 0) {

break;

}elseif (result < 0) {

perror("recv");

close(fd);

return 1;

}

fwrite(buf, 1, result, stdout);

}

close(fd);

return 0;

}

上面例子中，所有的网络调用都是阻塞的：gethostbyname直到成功或失败的解析了www.google.com才会返回；connect直到TCP建链成功了才会返回；recv直到收到数据时才会返回；send直到将输出flushed到内核的写缓冲区之后才会返回。

当然，阻塞IO并不总是无用的。如果应用程序在同一时刻不需要做其他事，那么阻塞IO同样会很好的工作。

但是，如果你要编写一个需要同时处理多个连接的程序，比如需要从两个连接中读取数据，而且不知道那个连接会先收到数据，那么下面就是一个不好的例子：

BadExample

/* This  won't work. */

charbuf[1024];

int i, n;

while(i_still_want_to_read()) {

for (i=0; i<n_sockets; ++i) {

n = recv(fd[i], buf, sizeof(buf), 0);

if (n==0)

handle_close(fd[i]);

else if (n<0)

handle_error(fd[i], errno);

else

handle_input(fd[i], buf, n);

}

}

如果fd[2]上首先有数据到来，但是上面的代码只有在fd[0]和fd[1]上接收到数据之后，才能去处理fd[2]上的数据。

有时，可以通过多线程（进程）来处理这样的问题。一个最简单的方式就是每个链接用一个线程（进程）进行处理。这样每个链接都会有自己的线程（进程）处理，一个链接上的阻塞IO调用就不会影响到其他链接上的处理。

下面就是一个例子：在TCP的40713端口上进行监听的ROT13服务器，每次从输入中接收一行数据，经过简单的处理后进行输出。它使用fork产生新的进程来处理每个链接。

Example:Forking ROT13 server

/* For sockaddr_in */

#include  <netinet/in.h>

/* For socket functions */

#include <sys/socket.h>

 

#include <unistd.h>

#include <string.h>

#include <stdio.h>

#include <stdlib.h>

 

#define MAX_LINE 16384

 

char rot13_char(char c)

{

    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;

}

 

void child(int fd)

{

    char outbuf[MAX_LINE+1];

    size_t outbuf_used = 0;

    ssize_t result;

 

    while (1) 

    {

        char ch;

        result = recv(fd, &ch, 1, 0);

        if (result == 0) {

            break;

        } else if (result == -1) {

            perror("read");

            break;

        }

 

        /* We do this test to keep the userfrom overflowing the buffer. */

        if (outbuf_used < sizeof(outbuf)) {

            outbuf[outbuf_used++] = rot13_char(ch);

        }

 

        if (ch == '\n') {

            send(fd, outbuf, outbuf_used, 0);

            outbuf_used = 0;

            continue;

        }

    }

}

 

void run(void)

{

    int listener;

    struct sockaddr_in sin;

 

    sin.sin_family = AF_INET;

    sin.sin_addr.s_addr = 0;

    sin.sin_port = htons(40713);

 

    listener = socket(AF_INET, SOCK_STREAM, 0);

 

#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;

    }

 

 

 

    while (1) {

        struct sockaddr_storage ss;

        socklen_t slen = sizeof(ss);

        int fd = accept(listener, (struct sockaddr*)&ss, &slen);

        if (fd < 0) {

            perror("accept");

        } else {

            if (fork() == 0) {

                child(fd);

                exit(0);

            }

        }

    }

}

 

int main(int c, char **v)

{

    run();

    return 0;

}

这样，是否已经完美解决了同一时刻多连接的问题了呢？事实并非如此，第一，某些平台上，创建新进程（甚至是线程）是十分昂贵的。当然在实际环境中，可以使用线程池，而不是每次都创建新线程。第二，更重要的是，线程无法如你所愿的规模化使用。如果你的程序需要同时处理成千上万个链接的时候，处理成千上万个线程就不是那么高效的了。

    如果线程不是处理多连接的答案，那什么才是呢？

      在unix系统上，将socket设置为非阻塞:fcntl(fd, F_SETFL, O_NONBLOCK)。一旦将fd置为非阻塞，那么从此刻起，无论何时进行网络调用，该调用会立即返回，要么完成操作，返回成功，要么就是返回一个特定的错误码指出“当前无法完成任务，再试一次”。所以，上面2个链接的例子可以如下写：

BadExample: busy-polling all sockets

/* This will work, but the performance will beunforgivably bad. */

int i, n;

char buf[1024];

for (i=0;i < n_sockets; ++i)

fcntl(fd[i], F_SETFL, O_NONBLOCK);

while(i_still_want_to_read()) {

for (i=0; i < n_sockets; ++i) {

n = recv(fd[i], buf, sizeof(buf), 0);

if (n == 0) {

handle_close(fd[i]);

}else if (n < 0) {

if (errno == EAGAIN)

; /* The kernel didn't haveany data for us to read. */

else

handle_error(fd[i], errno);

} else {

handle_input(fd[i], buf, n);

}

}

}

上面就是使用非阻塞sockets的例子，它虽然可以工作，但是效率却很差，两个原因：第一，当每个链接都没有数据可读的时候，就会无限的轮训下去，用尽所有的CPU周期。第二，如果需要处理多个链接，那么不管是否有数据可读，每个链接都会进行一次内核调用。

所以，我们需要一种方法，可以告诉内核“一直等待，直到某个socket已经有准备好了，而且要告诉我那个socket准备好了”。

古老的解决方法是使用select，目前仍在使用。select使用三个socket fd集合（位数组）：可读、可写和异常。它会一直等待，直到集合中的某一个socket已经准备好了，而且，select返回时，会更改集合，使其只包含那些已经准备好了的socket fd。使用select的例子如下：

Example:Using select

/* If youonly have a couple dozen fds, this version won't be awful */

fd_setreadset;

int i, n;

charbuf[1024];

while (i_still_want_to_read()){

int maxfd = -1;

FD_ZERO(&readset);

/* Add all of the interesting fds toreadset */

for (i=0; i < n_sockets; ++i) {

if (fd[i]>maxfd) maxfd = fd[i];

FD_SET(fd[i], &readset);

}

/*Wait until one or more fds are ready to read */

select(maxfd+1, &readset, NULL, NULL,NULL);

/* Process all of the fds that are stillset in readset */

for (i=0; i < n_sockets; ++i) {

if (FD_ISSET(fd[i], &readset)) {

n = recv(fd[i], buf, sizeof(buf),0);

if (n == 0) {

handle_close(fd[i]);

} else if (n < 0) {

if (errno == EAGAIN)

; /* The kernel didn'thave any data for us to read. */

else

handle_error(fd[i],errno);

} else {

handle_input(fd[i], buf, n);

}

}

}

}

一个完整的使用select的ROT13的服务器例子如下：

/* Forsockaddr_in */

#include<netinet/in.h>

/* For socketfunctions */

#include<sys/socket.h>

/* Forfcntl */

#include<fcntl.h>

/* forselect */

#include<sys/select.h>

#include<assert.h>

#include<unistd.h>

#include<string.h>

#include<stdlib.h>

#include<stdio.h>

#include<errno.h>

#defineMAX_LINE 16384

char rot13_char(charc)

{

/* We don't want to use isalpha here;setting the locale would change

* which characters are consideredalphabetical. */

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;

}

structfd_state {

char buffer[MAX_LINE];

size_t buffer_used;

int writing;

size_t n_written;

size_t write_upto;

};

structfd_state * alloc_fd_state(void)

{

struct fd_state *state =malloc(sizeof(struct fd_state));

if (!state)

return NULL;

state->buffer_used = state->n_written= state->writing =

state->write_upto = 0;

return state;

}

void free_fd_state(structfd_state *state)

{

free(state);

}

void make_nonblocking(intfd)

{

fcntl(fd, F_SETFL, O_NONBLOCK);

}

int do_read(intfd, struct fd_state *state)

{

char buf[1024];

int i;

ssize_t result;

while (1) {

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') {

state->writing = 1;

state->write_upto =state->buffer_used;

}

}

}

if (result == 0) {

return 1;

} else if (result < 0) {

if (errno == EAGAIN)

return 0;

return -1;

}

return 0;

}

int do_write(intfd, struct fd_state *state)

{

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)

return 0;

return -1;

}

assert(result != 0);

state->n_written += result;

}

if (state->n_written ==state->buffer_used)

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

state->writing = 0;

return 0;

}

void run(void)

{

int listener;

struct fd_state *state[FD_SETSIZE];

struct sockaddr_in sin;

int i, maxfd;

fd_set readset, writeset, exset;

sin.sin_family = AF_INET;

sin.sin_addr.s_addr = 0;

sin.sin_port = htons(40713);

for (i = 0; i < FD_SETSIZE; ++i)

state[i] = NULL;

listener = socket(AF_INET, SOCK_STREAM, 0);

make_nonblocking(listener);

#ifndefWIN32

{

int one = 1;

setsockopt(listener, SOL_SOCKET,SO_REUSEADDR, &one, sizeof(one));

}

#endif

if (bind(listener, (structsockaddr*)&sin, sizeof(sin)) < 0) {

perror("bind");

return;

}

if (listen(listener, 16)<0) {

perror("listen");

return;

}

FD_ZERO(&readset);

FD_ZERO(&writeset);

FD_ZERO(&exset);

while (1) {

maxfd = listener;

FD_ZERO(&readset);

FD_ZERO(&writeset);

FD_ZERO(&exset);

FD_SET(listener, &readset);

for (i=0; i < FD_SETSIZE; ++i) {

if (state[i]) {

if (i > maxfd)

maxfd = i;

FD_SET(i, &readset);

if (state[i]->writing) {

FD_SET(i, &writeset);

}

}

}

if (select(maxfd+1, &readset,&writeset, &exset, NULL) < 0) {

perror("select");

return;

}

if (FD_ISSET(listener, &readset)) {

struct sockaddr_storage ss;

socklen_t slen = sizeof(ss);

int fd = accept(listener, (structsockaddr*)&ss, &slen);

if (fd < 0) {

perror("accept");

} else if (fd > FD_SETSIZE) {

close(fd);

} else {

make_nonblocking(fd);

state[fd] = alloc_fd_state();

assert(state[fd]);/*XXX*/

}

}

for (i=0; i < maxfd+1; ++i) {

int r = 0;

if (i == listener)

continue;

if (FD_ISSET(i, &readset)) {

r = do_read(i, state[i]);

}

if (r == 0 && FD_ISSET(i,&writeset)) {

r = do_write(i, state[i]);

}

if (r) {

free_fd_state(state[i]);

state[i] = NULL;

close(i);

}

}

}

}

int main(intc, char **v)

{

setvbuf(stdout, NULL, _IONBF, 0);

run();

return 0;

}

但是问题还没有解决。因为产生和读取select的位数组耗费的时间与最大的socket fd数成正比，所以当socket fd数变得很大时，select调用的性能就会下降很多。

不同的操作系统都提供了不同的select替代函数。包括poll, epoll, kqueue, evports和/dev/poll。所有这些接口都具有比select更好的性能，而且除了poll之外，他们在增加socket，删除socket，通知哪个socket准备好这些方面，都可以达到O(1)的性能。

不幸的是，所有这些不同的接口都没有形成标准。linux提供了epoll，BSDs提供了kqueue，Solaris提供了evports和/dev/poll，而且这些操作系统提供的接口相互独立。所以，当你需要编写一个可移植的、高性能异步应用时，你需要一个封装所有这些接口的抽象，而且提供那个最高效的接口。

这就是libeventAPI能提供的最底层的功能。它提供了一系列的select替代接口，并且使用当前操作系统所具有的，最高效的版本。

下面是另一个ROT13服务器的例子。该实例使用libevent2替代select。去除了fd_sets，而是使用event_base添加和删除事件，当然这是通过poll，epoll，kqueue等来实现的。

Example:A low-level ROT13 server with Libevent

/* Forsockaddr_in */

#include<netinet/in.h>

/* Forsocket functions */

#include<sys/socket.h>

/* Forfcntl */

#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_LINE16384

voiddo_read(evutil_socket_t fd, short events, void *arg);

voiddo_write(evutil_socket_t fd, short events, void *arg);

char

rot13_char(charc)

{

/* We don't want to use isalpha here;setting the locale would change

* which characters are consideredalphabetical. */

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;

}

structfd_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;

};

structfd_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(structfd_state *state)

{

event_free(state->read_event);

event_free(state->write_event);

free(state);

}

void do_read(evutil_socket_tfd, 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 evutilmacro

return;

perror("recv");

free_fd_state(state);

}

}

void do_write(evutil_socket_tfd, 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 useevutil 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_tlistener, short event, void *arg)

{

struct event_base *base = arg;

struct sockaddr_storage ss;

socklen_t slen = sizeof(ss);

int fd = accept(listener, (structsockaddr*)&ss, &slen);

if (fd < 0) { // XXXX eagain??

perror("accept");

} else if (fd > FD_SETSIZE) {

close(fd); // XXX replace all closeswith 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);

#ifndefWIN32

{

int one = 1;

setsockopt(listener, SOL_SOCKET,SO_REUSEADDR, &one, sizeof(one));

}

#endif

if (bind(listener, (structsockaddr*)&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(intc, char **v)

{

setvbuf(stdout, NULL, _IONBF, 0);

run();

return 0;

}

（上面的代码需要注意的是，使用evutil_socket_t，而不是int作为socket的类型；使用evutil_make_socket_nonblocking而不是fcntl(O_NONBLOCK)，将socket转为非阻塞。这些改变使得我们的代码可以兼容win32平台下的网络API。）

二：更方便并且兼容windows

    我们的代码虽然更加高效了，但是也变得更加复杂了。回到我们使用fork的例子，我们没有为每个链接都管理一个缓存，我们只是在每个进程上使用了独立的栈缓存。实际上，我们无需明确的跟踪哪个socket在读或写，在代码中它是隐含的。我们也无需一个跟踪多少操作已经完成的结构体，我们可以仅仅使用循环和栈变量即可。

    另外，如果你对windows上的网络编程很熟悉，则可以看出，使用libevent的上面的例子没有达到最佳的性能。在Windows上，高效的异步IO与并不是类似于select那样的机制，而是使用IOCP（IO Completion Ports）API。与其他高效网络API不同的是，IOCP并不通知你的程序哪个socket已经准备好操作了，相反的，程序告诉windows网络栈开始一个网络操作，而IOCP告诉程序操作已经完成了。

    幸运的是，libevent2的“bufferevents”接口可以解决上面的问题：它使得程序编写更加简单，而且可以在windows上、unix上都提供最高效的接口。下面是最后一个ROT13服务器的例子，它使用了bufferevents API：

Example:A simpler ROT13 server with Libevent

/* Forsockaddr_in */

#include<netinet/in.h>

/* Forsocket functions */

#include<sys/socket.h>

/* Forfcntl */

#include<fcntl.h>

#include<event2/event.h>

#include<event2/buffer.h>

#include<event2/bufferevent.h>

#include<assert.h>

#include<unistd.h>

#include<string.h>

#include<stdlib.h>

#include<stdio.h>

#include<errno.h>

#defineMAX_LINE 16384

void do_read(evutil_socket_tfd, short events, void *arg);

voiddo_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 consideredalphabetical. */

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;

}

void  readcb(struct bufferevent *bev, void *ctx)

{

struct evbuffer *input, *output;

char *line;

size_t n;

int i;

input = bufferevent_get_input(bev);

output = bufferevent_get_output(bev);

while ((line = evbuffer_readln(input, &n,EVBUFFER_EOL_LF))) {

for (i = 0; i < n; ++i)

line[i] = rot13_char(line[i]);

evbuffer_add(output, line, n);

evbuffer_add(output, "\n",1);

free(line);

}

if (evbuffer_get_length(input) >=MAX_LINE) {

/* Too long; just process what there isand go on so that the buffer

* doesn't grow infinitely long. */

char buf[1024];

while (evbuffer_get_length(input)) {

int n = evbuffer_remove(input, buf,sizeof(buf));

for (i = 0; i < n; ++i)

buf[i] = rot13_char(buf[i]);

evbuffer_add(output, buf, n);

}

evbuffer_add(output, "\n",1);

}

}

void  errorcb(struct bufferevent *bev, short error,void *ctx)

{

if (error & BEV_EVENT_EOF) {

/* connection has been closed, do anyclean up here */

/* ... */

} else if (error & BEV_EVENT_ERROR) {

/* check errno to see what erroroccurred */

/* ... */

} else if (error & BEV_EVENT_TIMEOUT) {

/* must be a timeout event handle,handle it */

/* ... */

}

bufferevent_free(bev);

}

void  do_accept(evutil_socket_t listener, shortevent, void *arg)

{

struct event_base *base = arg;

struct sockaddr_storage ss;

socklen_t slen = sizeof(ss);

int fd = accept(listener, (structsockaddr*)&ss, &slen);

if (fd < 0) {

perror("accept");

} else if (fd > FD_SETSIZE) {

close(fd);

} else {

struct bufferevent *bev;

evutil_make_socket_nonblocking(fd);

bev = bufferevent_socket_new(base, fd,BEV_OPT_CLOSE_ON_FREE);

bufferevent_setcb(bev, readcb, NULL,errorcb, NULL);

bufferevent_setwatermark(bev, EV_READ,0, MAX_LINE);

bufferevent_enable(bev,EV_READ|EV_WRITE);

}

}

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);

#ifndefWIN32

{

int one = 1;

setsockopt(listener, SOL_SOCKET,SO_REUSEADDR, &one, sizeof(one));

}

#endif

if (bind(listener, (structsockaddr*)&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(intc, char **v)

{

setvbuf(stdout, NULL, _IONBF, 0);

run();

return 0;

}

原文：http://www.wangafu.net/~nickm/libevent-book/01_intro.html