Re: select() always blocks non-blocking socket
- From: "Olaf Lahl" <olaf.lahl@xxxxxxxxxxxxxxxxxx>
- Date: Mon, 5 May 2008 22:43:09 +0200
Thank you, Ben. You are right, so I moved initialization of the fd into the loop, but still I always have to wait for the entire timeout period. By the way, actually I'd prefer a blocking socket, since this code runs in its own thread anyway. But then the thread gets stuck inside recv() forever as soon as the news server stops data transmission. Any idea?
Thank you, Olaf
for ( int cbBytes = 0, nRet = SOCKET_ERROR; ; )
{
fd_set fdRead;
FD_ZERO( &fdRead );
FD_SET( m_Socket, &fdRead );
nRet = select( 1, &fdRead, NULL, NULL, &tvTimeout );
// ...
"Ben Voigt [C++ MVP]" <rbv@xxxxxxxxxxxxx> wrote in message news:OcxnzqurIHA.1952@xxxxxxxxxxxxxxxxxxxxxxx
Olaf Lahl wrote:Hi,
I have simple app which sends NNTP commands like LIST or GROUP to a
news server and attempts to read the server's response. My problem
is, that select() will always wait for the entire timeout period
event though I created a non-blocking socket. If I specify a small
timeout value, select() will frequently return without any data, if I
supply a large value like 5 or 10 sec, it will always wait for
exactly that amount of time! Isn't the timeout parameter supposed to
be a maximum value? Here is the code that does it (error checking and
other code ommitted for brevity). Any idea what I'm doing wrong?
You're not filling in fdRead on each loop. The first three parameters to select are in/out and get overwritten. So the first time select times out, it clears fdRead and from then on it isn't even checking for activity.
Thanks in advance, Olaf
m_Socket = socket( AF_INET, SOCK_STREAM, IPPROTO_TCP );
connect( m_Socket, ( sockaddr* )&saServer, sizeof( saServer ) );
u_long ulIoctlBlock = 1;
ioctlsocket( m_Socket, FIONBIO, &ulIoctlBlock ); // <-- Create
non-blocking socket
fd_set fdRead;
FD_ZERO( &fdRead );
FD_SET( m_Socket, &fdRead );
timeval tvTimeout;
tvTimeout.tv_sec = 5; // <-- Maximum (?) timeout value
tvTimeout.tv_usec = 0;
for ( int cbBytes = 0, nRet = SOCKET_ERROR; ; )
{
nRet = select( 1, &fdRead, NULL, NULL, &tvTimeout ); <-- Will
always wait for the entire timeout period!
if ( nRet == SOCKET_ERROR )
{
break;
}
if ( nRet == 0 )
{
continue;
}
if ( ( cbBytes = recv( m_Socket, chBuffer, ulBufferSize, 0 ) ) <=
0 ) // <-- May reach this line without having read any data if
timeout period is zero or too short
{
break;
}
}
.
- References:
- select() always blocks non-blocking socket
- From: Olaf Lahl
- Re: select() always blocks non-blocking socket
- From: Ben Voigt [C++ MVP]
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