Select() sometimes does not wait - c++

Question

I'm writing a chat program, and my receive function sometimes does not wait at all.. Here is the receiving code: The important parts are basically the first half, but i've added the whole function just in case. (Edit: the commenting is for myself, not notes to you guys reading! sorry!)
ReceiveStatus Server::Receive(PacketInternal*& packetInternalOut)
{
fd_set fds ;
int n ;
struct timeval tv ;
// Set up the file descriptor set.
FD_ZERO(&fds) ;
FD_SET(*p_socket, &fds) ;
// Set up the struct timeval for the timeout.
tv.tv_sec = NETWORKTIMEOUTSEC ;
tv.tv_usec = NETWORKTIMEOUTUSEC ;
// Wait until timeout or data received.
n = select ( *p_socket, &fds, NULL, NULL, &tv ) ;
if ( n == 0)
{
return ReceiveStatus::ReceiveTimeout;
}
else if( n == -1 )
{
return ReceiveStatus::ReceiveSocketError;
}
//need to make this more flexible so it can support others
sockaddr_in fromAddr;
int flags = 0;
int fromLength = sizeof(fromAddr);
char dataIn[TOTALPACKETSIZE];
int bytesIn = recvfrom(*p_socket, dataIn, TOTALPACKETSIZE, flags, (SOCKADDR*)&fromAddr, &fromLength);
// Convert fromAddr into ip, port
if(bytesIn == SOCKET_ERROR)
{
return ReceiveStatus::ReceiveSocketError;
}
if(bytesIn > 0)
{
memcpy(packetInternalOut,dataIn,bytesIn);
return ReceiveStatus::ReceiveSuccessful;
}
else
{
return ReceiveStatus::ReceiveEmpty;
}
}
Is there anything that could effect whether or not this works or doesn't work? my chat program can either be a server or a client. they both use this same code. The server, when waiting for a connection, sits on Select() for 100 seconds, as NETWORKTIMEOUTSEC = 100. But in the char program, whenever I want to send a message, i first send a transfer request, and then I wait for an acknowledgement (For the acknowledgement packet, i need to call receive again). Now this is the step that does not wait. my ReceiveAck function calls Receive(), and receive just runs straight over the entire code. I can test this by creating a client and no server. If i send a message where there is no server, it should wait 100 seconds for an acknowledgement, and then time out. But instead, as soon as i hit enter, it says it timed out.
i cant work out what would be making it skip this step. I have debugged my chat program in both its server and client states. The values of tv and fds are the same in both, yet the server will wait and the client wont...

Answer 1

The first parameter to select() is one greater than the last socket. So you need:
n = select ( *p_socket + 1, &fds, NULL, NULL, &tv ) ;

Answer 2

Select also returns early (i.e. without any of the sockets having data present) when your application is hit by a signal. So if your app uses a lot of usleep() and friends in a different thread, you might be in for a surprise.

Answer 3

select() should always be used in a loop. You must check its return for three conditions:
-1 (an error), which you must evaluate to determine if it is fatal. EINTR is an example of a non-fatal error.
a zero, in which case some indeterminate amount of time has passed and, if you care about how long its been, you need to check the time separately.
A positive value, in which case you should check all of the flagged descriptors and act on them.
In all cases, you should check whether any other conditions exist which might make you want to exit the loop, such as how much time as actually passed.
Note that the first parameter to select() should generally be the constant FD_SETSIZE. There is little to be gained in setting it to anything else.
Also note that just because you received a datagram doesn't mean you received the datagram you wanted. You need a way to check that you did not get some random datagram that happened to be floating around on the network (it happens). Along those lines, make sure TOTALPACKETSIZE is 65536, because that's theoretically (approximately) how big a random packet could be.