blocking sockets timeout on receive issues

Question

I'm writing a MFC\C++ program that needs to create a UDP socket to receive datagrams on. I am using blocking socket (for performance reasons) and have some errors (or misunderstanding) when trying to set timeout for receive calls.

When I set the receive timeout to 100mili using setsockopt() and the receive does timeout - it timeouts after about 600mili.

When I set the receive timeout to 1000mili using setsockopt() and the receive does timeout - it timeouts after about 1600mili.

Why is this ? Am I doing something wrong ?

My code goes something like this:

WSADATA              wsaData;
SOCKET               ReceivingSocket;
SOCKADDR_IN          ReceiverAddr;
int                  Port = 2345;
char                 ReceiveBuf[1024];
int                  BufLength = 1024;
SOCKADDR_IN          SenderAddr;
int                  SenderAddrSize = sizeof(SenderAddr);
int                  Ret;

// 100mili timeout
int RecvTimeout = 100 ;

// Initialize Winsock version 2.2
if ((Ret = WSAStartup(MAKEWORD(2,2), &wsaData)) != 0)
{
    TRACE("ERROR: WSAStartup failed with error %d\n", Ret);
    return;
}

// Create a new socket to receive datagrams
if ((ReceivingSocket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))
    == INVALID_SOCKET)
{
    TRACE("ERROR: socket failed with error %d\n", WSAGetLastError());
    WSACleanup();
    return;
} 

// receive datagrams from all interfaces using port 2345
ReceiverAddr.sin_family = AF_INET;
ReceiverAddr.sin_port = htons(Port);    
ReceiverAddr.sin_addr.s_addr = htonl(INADDR_ANY);

// bind
if (bind(ReceivingSocket, (SOCKADDR *)&ReceiverAddr, sizeof(ReceiverAddr))
    == SOCKET_ERROR)
{
    TRACE("ERROR: bind failed with error %d\n", WSAGetLastError());
    closesocket(ReceivingSocket);
    WSACleanup();
    return;
}

// set receive timeout
if ( setsockopt (
    ReceivingSocket,
    SOL_SOCKET,
    SO_RCVTIMEO,
    (const char*)&RecvTimeout,
    sizeof(RecvTimeout) ) == SOCKET_ERROR )
{
    TRACE("Error using setsockopt\n") ;
    closesocket(ReceivingSocket);
    WSACleanup();
    return;
}

// Receive 10 messages - here the timeout comes to life...
for(int i = 0 ; i < 10 ; i++)
{
    TRACE("Before Recv, Ticks=%d\n", GetTickCount()) ;
    if ((Ret = recvfrom(ReceivingSocket, ReceiveBuf, BufLength, 0,
        (SOCKADDR *)&SenderAddr, &SenderAddrSize)) == SOCKET_ERROR)
    {
        if(WSAETIMEDOUT == WSAGetLastError())
            TRACE("After Recv, Ticks=%d\n\n", GetTickCount()) ;
        else
        {
            TRACE("ERROR: receive failed with error %d\n", WSAGetLastError());
            closesocket(ReceivingSocket);
            WSACleanup();
            return;
        }
    }
}

closesocket(ReceivingSocket);
WSACleanup();

and the output I get is this:

Before Recv, Ticks=1476485406
After Recv, Ticks=1476486031

Before Recv, Ticks=1476486031
After Recv, Ticks=1476486656

Before Recv, Ticks=1476486656
After Recv, Ticks=1476487281
.
.
.

In addition, when I looked at the MSDN to find out more about SO_RCVTIMEO I noticed the following:

If a send or receive operation times out on a socket, the socket state is indeterminate, and should not be used..."

So basically using SO_RCVTIMEO seems like a bad idea if I do get timeouts. Am I missing something ?

FYI when I run your code using VS2012 on Win8 I get the roughly correct timings, 94-109ms per wait. — mark
MSDN is incorrect or at least imprecise. After a read timeout the socket is still connected and can still be used for further I/O. After a write timeout the socket almost certainly is dead and will provoke ECONNRESET if used again. — user207421

Kshitij Mayekar Kshitij Mayekar · Accepted Answer · 2016-07-28T10:59:36

Use 'timeval' stuct to initialize timeout value.

struct timeval timeout;
timeout.tv_sec = SOCKET_READ_TIMEOUT_SEC; // in milli secs
timeout.tv_usec = 0;

blocking sockets timeout on receive issues

1 Answers