Sockets: my select() function logic has undefined behavior - c++

Question

I implemented a program that receives from one socket and sends/receives from the other socket.
For this i use polling of select(), in socket 1, i receive data at a high data rate, while in the other socket i receive periodic message and requests to receive data from the first socket.
When there is no request "from socket 2" to delegate the data from socket 1 to socket2 , i receive data from socket 1 normal and with no problem. However, say i received two requests "socket 2" while data is being received in socket 1, the second request breaks the the data reception as if it could no longer keep up with rate "rate isn't high really is only 150 Hz".
The pseudo code i do in the main():
fd_set readfds, rd_fds, writefds, wr_fds;
struct timeval tv;
do
{
do
{
rd_fds = readfds;
wr_fds = writefds;
FD_ZERO (&rd_fds);
FD_SET (sock1, &rd_fds);
FD_SET (sock2, &rd_fds);
FD_SET (sock1, &wr_fds);
tv.tv_sec = 0;
tv.tv_usec = 20;
int ls = sock2 + 1;
rslt = select (ls, &rd_fds, &wr_fds, NULL, &tv);
}
while (rslt == -1 && errno == EINTR);
if (FD_ISSET (sock1, &rd_fds))
{
rs1 = recvfrom (sock1, buff, size of the buff, ....);
if (rs1 > 0)
{
if (rs1 = alive message)
{
/* system is alive; */
}
else if (rs1 == request message)
{
/* store Request info (list or vector) */
}
else {}
}
}
if (FD_ISSET (StructArg.sock2, &rd_fds))
{
rs2 = recv (sock2, ..., 0);
if (rs2 > 0)
{
if ( /* Message (high rate) is from sock 2 */ )
{
/* process this message and do some computation */
int sp1 = sendto (sock1, .....);
if (sp1 < 0)
{
perror ("Failed data transmission ");
}
else
{
/* increase some counters */
}
}
}
}
if (FD_ISSET (sock1, &wr_fds))
{
/*
if there info stored in the list
do some calculaitons then send to sock 1
*/
if (sendto (sock1, ... ...) < 0)
{
perror ("Failed data transmission");
}
else
{
/* increase counter */
}
}
FD_CLR (sock1, &rd_fds);
FD_CLR (sock2, &rd_fds);
}
while (1);
Again, the question is, why does receiving from sock1 is interrupted if a request is received from sock2, while i am receiving from sock1 (fast messages), i expect interleaved messages in the output based on the timestamps in the message.

Answer 1

Note that nearly all socket functions can block execution unless you've created the socket with the O_NONBLOCK option:
http://pubs.opengroup.org/onlinepubs/009695399/functions/sendto.html
And you'll also have to handle the case where recvfrom only gives you a partial read - unless you use MSG_WAITALL:
http://pubs.opengroup.org/onlinepubs/009695399/functions/recvfrom.html
Personally, I'd use a multi-threaded implementation which can have threads just sit and wait for data on each socket.
As to your final question:
why does receiving from sock1 is interrupted if a request is received from sock2, while i am receiving from sock1 (fast messages), i expect interleaved messages in the output based on the timestamps in the message.
You are slave to the network stack's implementation and there are nearly no guarantees about the sending or receiving of data on one socket relative to another. You are only guaranteed that the data within a socket is properly ordered.

Answer 2

I expect interleaved messages in the output based on the timestamps in the message.
Your expectation is without foundation. If there is data in either socket receive buffer, select() will fire. That's all you can rely on. You don't have any guarantee about timestamps being observed and ordered as between multiple sockets.