官方示例

/* 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;
}
 

上一節，我們介紹了從開始，建立event base，到最後分發訊息。這一節我們介紹一下，具體的，收到監聽，傳送訊息和接收訊息。

回撥流程

寫服務端程式，肯定是先建立監聽socket，然後等待連結進來，連結進來後，我們儲存起來，進行收發訊息。對於libevent封裝後，我們看一下，連結是怎麼進來的。

首先在上面的程式中，還是先建立socket，繫結埠，開啟監聽，然後建立event，與socket繫結，新增到event base中，開啟分發。

建立event的時候，在event_new中，綁定了event base，event，socket，監聽事件，回撥介面和回撥引數。

呼叫event_base_dispatch分發事件。然後就沒有額外的程式碼了，這就說明event_base_dispatch是一個迴圈，等待事件的到來，我們跟蹤一下如何在呼叫了event_base_dispatch之後，到達事件後，可以回撥到傳入的回撥函式，比如上面的do_accept do_read do_write。

event_base_dispatch->event_base_loop->event_process_active->event_process_active_single_queue

上面就是事件回撥呼叫堆疊，先是event_base_dispatch，然後具體呼叫在event_base_loop，獲得事件後，呼叫event_process_active，做了一些處理，最終呼叫event_process_active_single_queue，在這個函式中，根據不同的事件，呼叫儲存的對應的回撥函式。

整體流程就是先建立一個event base，event base根據系統的不同選擇不同的IO模型，linux下選擇epoll，也就是呼叫epoll_wait。然後建立監聽socket，繫結一個event，新增到主迴圈，呼叫epoll_ctl。在event_base_dispatch中，呼叫epoll_wait等待事件到達。到達後，根據傳入的回撥函式，執行回撥。