connect与select函数用法

最新推荐文章于 2022-04-23 13:38:10 发布

原创最新推荐文章于 2022-04-23 13:38:10 发布 · 1.8k 阅读

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CC 4.0 BY-SA版权

文章标签：

#socket #struct #null #stream #include #测试

本文介绍了一个使用非阻塞套接字进行连接的C程序实例，该程序通过select机制监测连接是否完成，并通过getsockopt检查连接状态。适用于Linux环境。

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <errno.h>
#include <time.h>
#include <arpa/inet.h>

int main(int argc, char *argv[])
{
int fd, retval;
struct sockaddr_in addr;
struct timeval timeo = {3, 0};
socklen_t len = sizeof(timeo);
fd_set set;

fd = socket(AF_INET, SOCK_STREAM, 0);
if (argc == 4) timeo.tv_sec = atoi(argv[3]);
int savefl = fcntl(fd,F_GETFL);
fcntl(fd, F_SETFL, savefl | O_NONBLOCK);
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr(argv[1]);
addr.sin_port = htons(atoi(argv[2]));
printf( "%d/n ", time(NULL));
if (connect(fd, (struct sockaddr*)&addr, sizeof(addr)) == 0)
{
close(fd);
printf( "connected..1/n ");
return 0;
}

if (errno != EINPROGRESS)
{
close(fd);
perror( "connect..2 ");
return -1;
}
FD_ZERO(&set);
FD_SET(fd, &set);
retval = select(fd + 1, NULL, &set, NULL, &timeo);
if (retval == -1)
{
close(fd);
perror( "select ");
return -1;
}
else if(retval == 0)
{
close(fd);
fprintf(stderr, "timeout/n ");
printf( "%d/n ", time(NULL));
return 0;
}
printf( "connected .. 3/n ");
fcntl(fd, F_SETFL,savefl);
close(fd);
return 0;
}

1. OS:linux kenerl 2.6 (fedora.unix-center.net)
2. 测试连接 (202.108.22.43:80,202.108.22.43:8000,) 百度，程序正常，其他机器也正常
3. 但是测试 127.0.0.1 任何端口都说能连接上，非常奇怪

今天仔细看了 man connect,明白了错误的原因:
EINPROGRESS
The socket is non-blocking and the connection
cannot be completed immediately. It is possible
to select(2) or poll(2) for completion by
selecting the socket for writing. After
select(2) indicates writability, use getsock-
opt(2) to read the SO_ERROR option at level
SOL_SOCKET to determine whether connect() com-
pleted successfully (SO_ERROR is zero) or unsuc-
cessfully (SO_ERROR is one of the usual error
codes listed here, explaining the reason for the
failure).
修改如下：
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <errno.h>
#include <time.h>
#include <arpa/inet.h>

int main(int argc, char *argv[])
{
int fd, retval;
struct sockaddr_in addr;
struct timeval timeo = {3, 0};
socklen_t len = sizeof(timeo);
fd_set set;

fd = socket(AF_INET, SOCK_STREAM, 0);
if (argc == 4) timeo.tv_sec = atoi(argv[3]);
int savefl = fcntl(fd,F_GETFL);
fcntl(fd, F_SETFL, savefl | O_NONBLOCK);
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr(argv[1]);
addr.sin_port = htons(atoi(argv[2]));
printf( "%d/n ", time(NULL));
if (connect(fd, (struct sockaddr*)&addr, sizeof(addr)) == 0)
{
close(fd);
printf( "connected..1/n ");
return 0;
}

if (errno != EINPROGRESS)
{
close(fd);
perror( "connect..2 ");
return -1;
}
FD_ZERO(&set);
FD_SET(fd, &set);
retval = select(fd + 1, NULL, &set, NULL, &timeo);
if (retval == -1)
{
close(fd);
perror( "select ");
return -1;
}
else if(retval == 0)
{
close(fd);
fprintf(stderr, "timeout/n ");
printf( "%d/n ", time(NULL));
return 0;
}

if(FD_ISSET (fd,&set)) //|| FD_ISSET(SockFd,&wset))
{
int error = 0;
socklen_t len = sizeof (error);
if(getsockopt(fd, SOL_SOCKET, SO_ERROR, &error, &len) < 0)
{
printf ( "getsockopt fail,connected fail/n ");
return -1;
}

if (error == ETIMEDOUT)
{
printf ( "connected timeout/n ");
}

if(error == ECONNREFUSED)
{
printf( "No one listening on the remote address./n ");
return -1;
}
}
printf ( "connected .. 3/n ");
fcntl(fd, F_SETFL, savefl);
close (fd);

return 0;
}