提问人：Nick 提问时间：6/14/2020 更新时间：6/14/2020 访问量：48
使用 select（） 的套接字多路复用无法按预期工作
Socket multiplexing with select() does not work as expecting
问：
我正在尝试创建一个服务器，该服务器侦听两个不同的端口，从相应的套接字中读取消息并将其打印到 stdout。当我第一次连接到任何可用端口时，程序会按预期工作。然后，当我尝试连接到第二个端口时，connect（） 根本不会取消阻止。
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int main(int argc, char** argv)
{
struct sockaddr_in addr1;
struct sockaddr_in addr2;
int srv_fd1;
int srv_fd2;
int srv_fd;
int accept_fd;
fd_set srv_fds;
char mem[1024];
if ((srv_fd1 = socket(AF_INET, SOCK_STREAM, 0)) == -1 ||
(srv_fd2 = socket(AF_INET, SOCK_STREAM, 0)) == -1)
{
fprintf(stderr, "socket(): %s\n", strerror(errno));
exit(EXIT_FAILURE);
}
addr1.sin_family = AF_INET;
addr1.sin_port = htons(16000);
addr1.sin_addr.s_addr = htonl(INADDR_ANY);
addr2.sin_family = AF_INET;
addr2.sin_port = htons(16001);
addr2.sin_addr.s_addr = htonl(INADDR_ANY);
if (bind(srv_fd1, (struct sockaddr *)&addr1, (socklen_t)sizeof(addr1)) == -1 ||
bind(srv_fd2, (struct sockaddr *)&addr2, (socklen_t)sizeof(addr2)) == -1)
{
fprintf(stderr, "bind(): %s\n", strerror(errno));
exit(EXIT_FAILURE);
}
if (listen(srv_fd1, 128) == -1 || listen(srv_fd2, 128) == -1) {
fprintf(stderr, "listen(): %s\n", strerror(errno));
exit(EXIT_FAILURE);
}
FD_ZERO(&srv_fds);
FD_SET(srv_fd1, &srv_fds);
FD_SET(srv_fd2, &srv_fds);
while (select(srv_fd2 +1, &srv_fds, NULL, NULL, NULL) != -1) {
printf("Inside loop\n");
fflush(stdout);
memset(mem, 0, 1024);
if (FD_ISSET(srv_fd1, &srv_fds))
srv_fd = srv_fd1;
else if (FD_ISSET(srv_fd2, &srv_fds))
srv_fd = srv_fd2;
accept_fd = accept(srv_fd, NULL, NULL);
if (accept_fd == -1) {
fprintf(stderr, "accept(): %s\n", strerror(errno));
exit(EXIT_FAILURE);
}
read(accept_fd, mem, 1023);
printf("%s", mem);
close(accept_fd);
}
close(srv_fd1);
close(srv_fd2);
return 0;
}
telnet问题的演示：
$ telnet 127.0.0.1 16000
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
FIRST ATTEMPT
Connection closed by foreign host.
$ telnet 127.0.0.1 16001
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
SECOND ATTEMPT
^]
telnet> quit
Connection closed.
服务器响应：
$ ./test
Inside loop
FIRST ATTEMPT
^C
答：
3赞
bruno
6/14/2020
#1
你需要做的
FD_ZERO(&srv_fds);
FD_SET(srv_fd1, &srv_fds);
FD_SET(srv_fd2, &srv_fds);
每次在调用 select 之前，因为该 fd 会重置以指示哪个 fd 有东西。在您的情况下，取消的第一个调用，因此仅管理第二个调用srv_fdsselectFD_SET(srv_fd2, &srv_fds);selectsrv_fd1
请注意，您还假设这可能是错误的。srv_fd2 +1srv_fd2 > srv_fd1
你还假设你只有一个现成的事情
if (FD_ISSET(srv_fd1, &srv_fds))
srv_fd = srv_fd1;
else if (FD_ISSET(srv_fd2, &srv_fds))
srv_fd = srv_fd2;
...
但你可以让他们都准备好。
一种方法可以是：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#define NSOCK 2
#define FIRST_PORT 16000
/* returns the max descriptor */
int do_fd_set(int * srv_fd, fd_set * srv_fds)
{
int r = 0;
int i;
FD_ZERO(srv_fds);
for (i = 0; i != NSOCK; ++i) {
FD_SET(srv_fd[i], srv_fds);
if (srv_fd[i] > r)
r = srv_fd[i];
}
return r;
}
int main(int argc, char** argv)
{
int srv_fd[NSOCK];
fd_set srv_fds;
int i;
for (i = 0; i != NSOCK; ++i) {
struct sockaddr_in addr;
if ((srv_fd[i] = socket(AF_INET, SOCK_STREAM, 0)) == -1)
{
fprintf(stderr, "socket(): %s\n", strerror(errno));
exit(EXIT_FAILURE);
}
addr.sin_family = AF_INET;
addr.sin_port = htons(FIRST_PORT + i);
addr.sin_addr.s_addr = htonl(INADDR_ANY);
if (bind(srv_fd[i], (struct sockaddr *)&addr, (socklen_t)sizeof(addr)) == -1)
{
fprintf(stderr, "bind(): %s\n", strerror(errno));
exit(EXIT_FAILURE);
}
if (listen(srv_fd[i], 128) == -1) {
fprintf(stderr, "listen(): %s\n", strerror(errno));
exit(EXIT_FAILURE);
}
}
while (select(do_fd_set(srv_fd, &srv_fds) + 1, &srv_fds, NULL, NULL, NULL) != -1) {
printf("Inside loop\n");
for (int i = 0; i != NSOCK; ++i) {
if (FD_ISSET(srv_fd[i], &srv_fds)) {
char mem[1024];
int accept_fd;
memset(mem, 0, 1024);
accept_fd = accept(srv_fd[i], NULL, NULL);
if (accept_fd == -1) {
fprintf(stderr, "accept(): %s\n", strerror(errno));
exit(EXIT_FAILURE);
}
read(accept_fd, mem, sizeof(mem) - 1);
printf("from port %d : %s\n", FIRST_PORT + i, mem);
close(accept_fd);
}
}
}
/* probably never reach that code */
for (i = 0; i != NSOCK; ++i)
close(srv_fd[i]);
return 0;
}
编译和执行：
pi@raspberrypi:/tmp $ gcc -Wall so.c
pi@raspberrypi:/tmp $ ./a.out
Inside loop
from port 16000 : aze
Inside loop
from port 16000 : qsd
Inside loop
from port 16001 : wxc
^C
pi@raspberrypi:/tmp $
在其他终端执行操作：
pi@raspberrypi:/tmp $ telnet localhost 16000
Trying ::1...
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
aze
Connection closed by foreign host.
pi@raspberrypi:/tmp $ telnet localhost 16000
Trying ::1...
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
qsd
Connection closed by foreign host.
pi@raspberrypi:/tmp $ telnet localhost 16001
Trying ::1...
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
wxc
Connection closed by foreign host.
评论
0赞
m0hithreddy
6/14/2020
我想提到srv_fd2 > srv_fd1部分，但是在简单的 OP 所示示例中，由于保证套接字始终返回最低的未使用描述符，因此srv_fd2 > srv_fd1
1赞
bruno
6/14/2020
@MohithReddy我想警告OP，这并不总是正确的
0赞
Nick
6/14/2020
因此，如果两个描述符都准备好了，select（） 不会在下一次迭代时立即再次返回吗？也就是说，我一次处理一个......
0赞
bruno
6/14/2020
@Nick，如果它们同时准备好，你将只管理其中一个，但由于重置下一回合的设置，你永远不会管理第二个。您需要在同一回合中管理所有准备好的 fd。提示：除了数组之外，不要对所有变量使用分隔变量，并使用函数来执行FD_SET当所有操作完成后也返回最大 id 或 0
1赞
bruno
6/14/2020
@Nick我编辑了我的答案以添加代码和执行的建议
2赞
Jeremy Friesner
6/14/2020
#2
布鲁诺的回答正确地诊断了问题。作为补充，以下是相关代码的工作版本：
while(1) {
FD_ZERO(&srv_fds);
FD_SET(srv_fd1, &srv_fds);
FD_SET(srv_fd2, &srv_fds);
int maxFD = (srv_fd1 > srv_fd2) ? srv_fd1 : srv_fd2;
if (select(maxFD+1, &srv_fds, NULL, NULL, NULL) == -1) break;
[...]
评论
srv_fds