How to handle all possible errors on async socket? - linux

Question

I have decided to use async io for my project and simply do a single threaded loop where I try to read some data each frame from each open socket. This worked quite well and overall I'm happy with it for now. The problem is weird problems I'm having with the async sockets.
I have code like this:
accept a connection...
fcntl(O_NONBLOCK) on the client socket...
int rc;
if((rc = recv(socket))>0)
process data
if rc == 0
close socket and cleanup
The problem is that I get rc == 0 sometimes even though I know that the connection is not closed. If I don't clean up then my app works as normal. But if I do cleanup then the client receives a disconnect before the connection is even established.
So my question is: Do I have to check somehow whether the socket is ready before doing a recv in order to get the correct return value from it?
Most of the information I have been able to find was inconclusive. I found a references to select() but it seems to block until there is a status change on the socket - but I need the socket to be nonblocking.
What I'm looking for is just the intuitive behavior that if there is data, it is read to the buffer and recv returns number of bytes read, if there is no data it returns -1 and if the socket is disconnected then it should return 0.
Do I have to do anything else to the socket before calling recv to make it work as expected?

Answer 1

First, taking on the heavy lifting of going "all asynchronous" with a socket server is a good start for a design and will enable scalability very easily.
As for your question.
recv() will return the following values:
A postive value returned by recv() indicates the number of bytes
copied to your buffer.(i.e you actually received these bytes)
recv() will return 0 when the socket was closed by the remote side.
For async sockets, recv() will return -1 and set errno to either
EAGAIN or EWOULDBLOCK if the connection is still valid, but there's
no new data to be consumed. Call select() or poll() on the socket to
wait for data.
Otherwise, any general connection failure will result in -1 being returned by recv(). (And the only thing you can do is close the socket).
So when you say, "rc == 0 sometimes even though I know that the connection is not closed", I suspect your pseudocode is not checking the return value, but instead checking the result of (rc > 0).
This is closer to the logic you want:
int rc;
rc = recv(s, buf, buffersize, 0);
if (rc == 0)
{
/* socket closed by remote end */
close(s); s=-1;
}
else if ((rc == -1) && ((errno == EAGAIN) || (errno == EWOULDBLOCK)) )
{
// need to wait. Call select() or poll()
}
else if (rc == -1)
{
close(s); s=-1;
}
else
{
ProcessNewData(s, buffer, rc);
}