(recv() == 0) means disconnected or timed out? (sockets, Linux&Windows)

Question

I have set a timeout on my blocking socket ..

DWORD to = 1200;
if (setsockopt (soc, SOL_SOCKET, SO_RCVTIMEO, (char *)&to, sizeof(to))) {
    ...
}

In the case that recv () then returns zero, how can I tell this is link disconnected or read timed out? If it's t/o I would like to read more, if it's discon I would like to take other action. I realise I could just remove the t/o, then I would know it's discon, but I also need to routinely monitor how the read process is progressing.

Any help much appreciated. Cheers - Rich

Answer 1

From the SO_RCVTIMEO section of the man page for socket:

...if no data has been transferred and the time‐out has been reached,
then -1 is returned with errno set to EAGAIN or EWOULDBLOCK, or
EINPROGRESS (for connect(2)) just as if the socket was specified to
be non‐blocking.

From the man page for recv:

These calls return the number of bytes received, or -1 if an error
occurred.  In the event of an error, errno is set to indicate the
error.

When a stream socket peer has performed an orderly shutdown, the
return value will be 0 (the traditional "end-of-file" return).

Datagram sockets in various domains (e.g., the UNIX and Internet
domains) permit zero-length datagrams.  When such a datagram is
received, the return value is 0.

The value 0 may also be returned if the requested number of bytes to
receive from a stream socket was 0.

A call to recv will return 0 on disconnect, or if a zero-length datagram is received, or if the requested number of bytes is 0.

A call to recv will return -1 on any error, including a timeout. You need to examine errno to differentiate between a timeout versus some other error.