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 (1)
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 (4)
这是另外一个ROT13 server的实现，使用select()
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 16384charrot13_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;
}
但是我们并没有就此完成所有东西。因为对于超大集合的fd，生成或者读取select()的内置的位数组需要按比例花费时间。当套接字的数量非常多的时候，select调用的扩展能力就非常差。
不同的操作系统对于此类问题都有不同的替代方案。包含有poll、epoll、kqueue、evports和/dev/poll。所以这些都比select的性能要好，但是当添加或者移除一个套接字时，poll的时间复杂度为O(1)。
不幸的是，这些高效的接口并没有普及并标准化。Linux有epoll，BSDs（包括Darwin)有kqueue、Solaris 有evports和/dev/poll，并且这些系统没有其它系统所具有的。因此如果你想写一个合适的高性能的异步的程序，你需要有一个抽象层来封装所有这些接口，并且提供最高效的一个选择。
libevent API为你所做的就是上面所说的最底层的封装。它为不同的select的替代品提供一个统一的接口，当它运行的时候使用针对你所要求运行的操作系统的最高效的可用的版本。