如何设置connect超时时间

当网络设备不可用时，connect的链接会非常的耗时。

下面这篇文章解决了这个问题。

来源：https://www.codeproject.com/Tips/168704/How-to-set-a-socket-connection-timeout

文章名：How to set a socket connection timeout

Resolve long timeout
when connection
target is unavailable

Sometimes,
the connect time-out
can take too much time when
the target is unavailable. To resolve this issue, we can use non-blocking socket mode to select the timeout.

bool connect(char *host,int port, int timeout)
{
TIMEVAL Timeout;
Timeout.tv_sec = timeout;
Timeout.tv_usec = 0;
struct sockaddr_in address; /* the libc network address data structure */
 
sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
 
address.sin_addr.s_addr = inet_addr(host); /* assign the address */
address.sin_port = htons(port); /* translate int2port num */
address.sin_family = AF_INET;
 
//set the socket in non-blocking
unsigned long iMode = 1;
int iResult = ioctlsocket(sock, FIONBIO, &iMode);
if (iResult != NO_ERROR)
{
printf("ioctlsocket failed with error: %ld\n", iResult);
}

if(connect(sock,(struct sockaddr *)&address,sizeof(address))==false)
{
return false;
}
 
// restart the socket mode
iMode = 0;
iResult = ioctlsocket(sock, FIONBIO, &iMode);
if (iResult != NO_ERROR)
{
printf("ioctlsocket failed with error: %ld\n", iResult);
}
 
fd_set Write, Err;
FD_ZERO(&Write);
FD_ZERO(&Err);
FD_SET(sock, &Write);
FD_SET(sock, &Err);
 
// check if the socket is ready
select(0,NULL,&Write,&Err,&Timeout);
if(FD_ISSET(sock, &Write))
{
return true;
}
 
return false;
}