异步IO简介

最近想学习一下libevent,就先翻译一下libevent的官方文档吧.

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

大部分编程初学者都是从阻塞IO开始的。何谓阻塞IO？,即你进行一个IO调用时,除非这个操作完成,或者超时网络协议栈放弃了，否则这个调用是不返回的.比如你对TCP连接调用“connect()”时,你的操作系统将发送一个SYN包给TCP连接的对端,除非收到对端发送的SYN ACK包或者是超时了,否则connect()将不会返回。

这里是一个简单的使用阻塞网络调用的客户端例子.客户端连接到www.google.com,发起一个HTTP请求,把响应打印到标准输出.

/* 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[];

    /* 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 ;
    }
    if (h->h_addrtype != AF_INET) {
        fprintf(stderr, "No ipv6 support, sorry.");
        return ;
    }

    /* Allocate a new socket */
    fd = socket(AF_INET, SOCK_STREAM, );
    if (fd < ) {
        perror("socket");
        return ;
    }

    /* Connect to the remote host. */
    sin.sin_family = AF_INET;
    sin.sin_port = htons();
    sin.sin_addr = *(struct in_addr*)h->h_addr;
    if (connect(fd, (struct sockaddr*) &sin, sizeof(sin))) {
        perror("connect");
        close(fd);
        return ;
    }

    /* Write the query. */
    /* XXX Can send succeed partially? */
    cp = query;
    remaining = strlen(query);
    while (remaining) {
      n_written = send(fd, cp, remaining, );
      if (n_written <= ) {
        perror("send");
        return ;
      }
      remaining -= n_written;
      cp += n_written;
    }

    /* Get an answer back. */
    while () {
        ssize_t result = recv(fd, buf, sizeof(buf), );
        if (result == ) {
            break;
        } else if (result < ) {
            perror("recv");
            close(fd);
            return ;
        }
        fwrite(buf, , result, stdout);
    }

    close(fd);
    return ;
}

上面代码中所有的网络调用都是阻塞的,gethostbyname()在解析www.google.com成功或失败前不会返回,connect()在链路建立链接之前不会返回,recv()在接收到数据或是链接关闭请求之前不会返回,send()在数据发送到内核的写缓冲区之前不会返回.

阻塞IO也不是完全有害.如果你的程序在上述函数阻塞期间没啥想干的,用阻塞IO也没啥问题.但是想象一下,你现在需要写一个同时处理多个链接的程序,比如你要从2条链接里读数据,但是你并不知道哪条链接会先来数据.你可以写这样一个程序：

Bad Example

 /* This won't work. */
 char buf[];
 int i, n;
 while (i_still_want_to_read()) {
     for (i=; i<n_sockets; ++i) {
         n = recv(fd[i], buf, sizeof(buf), );
         if (n==)
             handle_close(fd[i]);
         else if (n<)
             handle_error(fd[i], errno);
         else
             handle_input(fd[i], buf, n);
     }
 }

为什么说这个程序很不好呢？因为如果fd[2]上数据先来了,程序在fd[0]和fd[1]上数据来了并处理完成之前,根本就不会去尝试从fd[2]读数据,因为这时候还阻塞在recv(fd[0], buf, sizeof(buf),0)这里呢。

有时候我们通过多线程或者多进程解决这个问题.最简单的一种处理方式就是每一个链接用一个进程(或线程)来处理.由于每条链接都有自己的进程,所以一条链接上的阻塞IO阻塞了并不会影响到别的链接处理进程.

下面是另一个例子。这是一个比较繁琐的服务器程序,在端口40713上等待tcp链接,从到来的数据中每次读一行,并将这一行的ROT13加密(其实就是简单的字符变换,比如把'a'变成'n',‘b’变成'o')数据输出.程序用了UNIX下的fork()来为每一条链接创建一个进程.

Example: Forking ROT13 serve

/* 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)
{
    /* 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 + ;
    else if ((c >= 'n' && c <= 'z') || (c >= 'N' && c <= 'Z'))
        return c - ;
    else
        return c;
}

void
child(int fd)
{
    char outbuf[MAX_LINE+];
    size_t outbuf_used = ;
    ssize_t result;

    while () {
        char ch;
        result = recv(fd, &ch, , );
        if (result == ) {
            break;
        } else if (result == -) {
            perror("read");
            break;
        }

        /* We do this test to keep the user from overflowing the buffer. */
        if (outbuf_used < sizeof(outbuf)) {
            outbuf[outbuf_used++] = rot13_char(ch);
        }

        if (ch == '\n') {
            send(fd, outbuf, outbuf_used, );
            outbuf_used = ;
            continue;
        }
    }
}

void
run(void)
{
    int listener;
    struct sockaddr_in sin;

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

    listener = socket(AF_INET, SOCK_STREAM, );

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

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

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

    while () {
        struct sockaddr_storage ss;
        socklen_t slen = sizeof(ss);
        int fd = accept(listener, (struct sockaddr*)&ss, &slen);
        if (fd < ) {
            perror("accept");
        } else {
            if (fork() == ) {
                child(fd);
                exit();
            }
        }
    }
}

int
main(int c, char **v)
{
    run();
    return ;
}

那么我们现在对多条链接的处理有了完美的解决方案吗？我是不是可以不写这本书而去干点别的了呢？不是这样的。首先，创建进程（线程）在某些平台上会是一笔不小的开销。在实际应用中，你可能更想使用一个线程池来代替它。但是从根本上讲，多线程并不能够达到你所期望的那种扩展性。如果你程序需要同时处理成千上万个连接，对于每个CPU仅能处理很少的线程的情况，处理成千上万个线程效率并不高。

如果线程不是处理多条链接的答案,那什么才是呢？在Unix范例中,你可以设置socket为noblocking(非阻塞)。Unix中完成这个设置的调用如下：

fcntl(fd, F_SETFL, O_NONBLOCK);

fd是代表socket的文件描述符.一旦你设置fd(也就是socket)为非阻塞的,你在fd上进行的网络调用要么立刻完成,要么返回一个特定的错误码,告诉你“我现在没法处理,再试一遍吧”.基于此,我们的处理两条socket的程序可以这样写：
Bad Example: busy-polling all sockets

/* This will work, but the performance will be unforgivably 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 have any data for us to read. */
            else
                 handle_error(fd[i], errno);
         } else {
            handle_input(fd[i], buf, n);
         }
    }
}

现在我们使用了非阻塞socket，上述代码可以工作...但也仅限于此了。上面代码性能很差,有两点原因：一.如果两个连接上都没有数据,那么就相当于在执行一个死循环,占据了所有的cpu。二.如果你通过这种方法处理多条以上的链接,那么对每一条链接来说,都需要执行内核调用(译者注：也就是上述代码中的recv),不管链接上有没有数据.所以说我们需要的是这样一种机制来告诉内核：“一直等着,直到某条链接上有数据了就告诉我是哪条链接上来数据了”。

下面是一个使用select的例子：
Example: Using select

/* If you only have a couple dozen fds, this version won't be awful */
fd_set readset;
int i, n;
char buf[1024];

while (i_still_want_to_read()) {
    int maxfd = -1;
    FD_ZERO(&readset);

    /* Add all of the interesting fds to readset */
    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 still set 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't have any data for us to read. */
                else
                     handle_error(fd[i], errno);
             } else {
                handle_input(fd[i], buf, n);
             }
        }
    }
}

 下面是一个使用select()重新实现ROT13 server的例子
Example: select()-based ROT13 server

/* For sockaddr_in */
#include <netinet/in.h>
/* For socket functions */
#include <sys/socket.h>
/* For fcntl */
#include <fcntl.h>
/* for select */
#include <sys/select.h>

#include <assert.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>

#define MAX_LINE 16384

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;

    int writing;
    size_t n_written;
    size_t write_upto;
};

struct fd_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(struct fd_state *state)
{
    free(state);
}

void
make_nonblocking(int fd)
{
    fcntl(fd, F_SETFL, O_NONBLOCK);
}

int
do_read(int fd, 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(int fd, 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);

#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;
    }

    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, (struct sockaddr*)&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(int c, char **v)
{
    setvbuf(stdout, NULL, _IONBF, 0);

    run();
    return 0;
}

用select()就解决了我们之前提到的问题吗？不，还没完呢.当socket的数量很大时,select()调用的性能会很差.因为生成与读取select()的位数组的时间正比于你向select()提供的最大的fd值。

不同的操作系统提供了不同的select()的替代函数。包括poll()，epoll(),kqueue(),evports以及/dev/poll.上述所有函数都比select()的性能要好，并且除了poll(),对增加一条socket,移除一条socket,通知一条socket已经做好IO准备而言,这些函数的时间复杂度都是O(1)。

然而不幸地是,这些更为有效的接口并没有一个统一的标准.Linux有epoll(),BSDs(包括Darwin)有kqueue()，Solaris有evports和/dev/poll...而且这些操作系统没一个自身之外的系统的上述接口. 所以你想写一个可移植(跨平台)的高性能异步应用程序的话,你就需要一个包含上述所有接口的抽象。

这正是Libevent API可以为你提供的最基本的功能.Libevent根据你的系统，选择最高效的select()的替代函数,并提供统一的接口.

下面是异步ROT13 server的另一个版本.这一次我们使用Libevent 2替代select().请注意现在fd_sets没啦,取而代之的是,我们使用结构event_base与events进行关联以及解除关联.event_base根据select()，poll(),epoll()，kqueue()等实现.

Example: A low-level ROT13 server with Libevent

/* For sockaddr_in */
#include <netinet/in.h>
/* For socket functions */
#include <sys/socket.h>
/* For fcntl */
#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;
}

(代码中需要注意的:我们没把socket定义为"int"型,我们用了evutil_socket_t.我们没有使用fcntl(O_NONBLOCK)去设置socket为非阻塞的,我们使用了evutil_make_socket_nonblocking.上述这些改变使得我们的代码兼容Win32的网络API)

What about convenience? (and what about Windows?)

你可能注意到了,在我们的代码变得更高效的同时,也变得更复杂了.回顾一下我们使用fork的版本,我们不必为每一条链接都管理一个缓冲区：我们为每一个进程都有一个独立的栈上分配的缓冲区.

我们不必精确地知道哪一个socket在读或者写：这是隐含在代码中的.(that was implicit in our location in the code).我们也不需要一个结构来跟踪每一个操作都完成了多少:我们使用循环和栈变量就好了.

此外,如果你对windows网络编程非常有经验的话,你会意识到,我们在上述例子中对libevent的用法并不会有最好的性能.在windows上,最快的异步IO方式使用的不是select()-like的接口：它用的是IOCP(IO Completion Ports:IO完成端口) API.不像别的高效的网络API,IOCP并不在一个socket已经对你的程序需要做的某些操作做好准备的时候就通知你的程序.取而代之的是,你的程序告诉windows网络栈开始网络操作,当操作完成的时候IOCP再通知程序.

幸运的是,Libevent2的"bufferevents"接口解决了上述这些问题：这使得我们的程序更易于编写,并且可以高效地在Windows和Unix下运行.

下面是我们使用bufferevents API的最新的ROT13 server。

Example: A simpler ROT13 server with Libevent

/* For sockaddr_in */
#include <netinet/in.h>
/* For socket functions */
#include <sys/socket.h>
/* For fcntl */
#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>

#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;
}

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 is and 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 any clean up here */
        /* ... */
    } else if (error & BEV_EVENT_ERROR) {
        /* check errno to see what error occurred */
        /* ... */
    } else if (error & BEV_EVENT_TIMEOUT) {
        /* must be a timeout event handle, handle it */
        /* ... */
    }
    bufferevent_free(bev);
}

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) {
        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);

#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;
}