刚刚学习libevent这个开源网络库时,在研究libevent-book examples_01 目录下有一例子,使用select()实现rto13 server.这个select()的基本框架在并发连接非常少时可以用来做一个简易服务器,可以在此基础上进行相应修改即可。
此server的功能基本上就是一个 echo server. 我比较关注的是select()监听读写事件的数据处理存在一个bug,如果向 rto13 server 连续发送数据,前提是两次发送数据间隔很短,那么两次的数据会被合并,执行一次写事件。先附上源码在分析:libevent-book/examples_01/01_r0t13_server_select.c
获取源码地址 git clone git://github.com/nmathewson/libevent-book.git
/* 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;
}
正常情况测试如下:
上面是rto13_server 监听 40713端口
下面是使用 nc 来作为客户端向server 发送数据,数字是不做处理原样返回的
先分析流程:
1.服务器启动,绑定端口,设置监听事件,然后阻塞在select处
if (select(maxfd+1, &readset, &writeset, &exset, NULL) < 0) {
perror("select");
return;
}
2.客户端连接上 server时,select()会返回已经准备好的描述符(为正值),即连接事件
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*/
}
}
此时fd 是在套接口listener接受的一个连接,后续的读写数据就通过此fd,此fd是唯一确定的,一般取值是系统空闲文件描述符最小值,我们记此fd 为 FD;
此FD会在下一个while 中被设置监听读事件,然后继续阻塞在select()处
3.当客户端发送 “123456\n",server 中 select()监听到FD读事件触发,会继续往下执行:
if (FD_ISSET(i, &readset)) {
// printf("read--%d\n", i);
r = do_read(i, state[i]);
}
通过 do_read 将“123456\n”读取出来并存放在缓冲区,设置 写标志
state->writing = 1
do_read()
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;
}
}
当再次回到while处就会设置 写事件:
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);
}
}
}
执行到select()处立即返回,后续执行
do_write() 返回给客户端
接着 发送“987654\n” 做相同动作。
不过在此之间,如果客户端两次数据间隔比较短,那么server的执行就不是如此,会一次性把两次收到的数据返回给客户端。
下面模拟此种情况:
先在 select()之前 加上一段延时, 如下:
sleep(3);
if (select(maxfd+1, &readset, &writeset, &exset, NULL) < 0) {
perror("select");
return;
}
do_read 添加打印信息
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;
}
}
}
printf("do_read:\n");
printf("state->buffer_used=%d\nstate->write_upto=%d\nstate->writing=%d\n", state->buffer_used, state->write_upto, state->writing);
if (result == 0) {
return 1;
} else if (result < 0) {
if (errno == EAGAIN)
return 0;
return -1;
}
return 0;
}
do_write添加打印信息:
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;
}
printf("do_write:\n");
printf("state->buffer_used=%d\nstate->write_upto=%d\nstate->n_written=%d\n", state->buffer_used, state->write_upto, state->n_written);
if (state->n_written == state->buffer_used)
state->n_written = state->write_upto = state->buffer_used = 0;
state->writing = 0;
return 0;
}
测试:
可以看到,我们在3秒内连续输入两行 “1234\n” “12315\n”
server 接收两条信息,但是却用一次返回给客户端,造成这个的原因是select()接收第二条信息 "12315\n"的时候同时也有写事件 ,但是是先处理读事件,再处理写事件,并且第二次的收到的"12315\n直接拼接在第一条信息 “1234\n”的后面,一起作为第一次的返回响应,这个在逻辑上存在一点问题,第二条信息的响应怎么和第一条合在一起了,这个对于输入缓冲带“\n”没有问题,但是如果将此程序移植作为他用,是存在一定问题的,收发响应需一一对应
改善,先处理写事件,再处理读事件,如下:
for (i=0; i < maxfd+1; ++i) {
int r = 0;
if (i == listener)
continue;
if (r == 0 && FD_ISSET(i, &writeset)) {
r = do_write(i, state[i]);
}
if (FD_ISSET(i, &readset)) {
r = do_read(i, state[i]);
}
if (r) {
free_fd_state(state[i]);
state[i] = NULL;
close(i);
}
}
继续上次测试:
此时符合收发一致。
除了select,还有 poll epoll处理事件的原理基本上差不多,不过在监听效率上要比select好的多(此处是在高并发的前提下)