一个对异步IO的小小的介绍 (3)
一个对异步IO的小小的介绍 (1)
一个对异步IO的小小的介绍 (2)
一个对异步IO的小小的介绍 (4)
这里是另一个ROT13 server的异步实现。这次，它使用libevent 2 来替代select。注意fd_sets已经不再使用，取而代之的是: 我们使用一个event_base结构关联或者取消关联事件，它内部实现了select、poll、epoll、kqueue等。
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 16384void do_read(evutil_socket_t fd, short events, void *arg);void do_write(evutil_socket_t fd, short events, void *arg);charrot13_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));
f (!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;
}
（另外需要注意的地方：我们使用evutil_socket_t代替int来表示一个socket；使用evutil_make_socket_nonblocking代替fcntl(O_NONBLOCK)让套接字变成非阻塞方式，这些改变让我们的代码兼容Win32下的一些分散的网络API)
使用方便性如何？Windows系统下又如何？
你可能已经意识到我们的代码变得更加高效，同时它也变得更复杂。回到之前当我们forking时，我们不需要为每一个连接管理一个缓冲，我们仅仅为每一个进程带有一个单独的基于堆栈上的缓冲。我们不需要明确地去跟踪每一个套接字的读取或者写入事件，那已经在库的代码内部实现了。并且我们也不需要一个结构去跟踪每一个操作是否已经完成，我们仅使用循环和堆栈变量就可以了。
再者，如果你有windows下网络编程经验，你将会意识到，上面的例子中，libevent的使用可能没有带来性能的优化。在window中，实现高效异步IO的方式并不是使用像select这样的接口，而是IOCP（完成端口）。不像其他的一些高效的网络API，当套接字就绪可以操作时IOCP并不处理并告知你的应用程序需要进一步处理。取而代之的是，应用程序调用一个windows网络API去发起操作调用，稍后由IOCP通知应用程序操作已经完成了。
幸运的是，libevent2 的 bufferevents 接口解决了这些问题：它使编码更容易，并且提供了高效的实现在windows和Unix上的接口。