Avoid send to block when not using O_NONBLOCK

4
votes

I have to write a chat client-server for a class using unix sockets (without O_NONBLOCK) and select for asynchronous I/O on them. At the moment, on the server, i read 1024 bytes from the client, and directly handle it.

For example, in case of a message, i will receive a command formatted as MSG <msg> (representing a client sending a message), i will go through all the sockets of the connected clients and write the message on them.

This approach actually works but i recently found by reading the man of send that it can blocks if the socket buffer is full and is the flag O_NONBLOCK is not set on the socket.

I think this problem could happen when a client does not read for some reasons (crash, bugged etc.) and this would be critical for my server since it will basically blocks until this client read again.

So here is my question:

What is the correct approach on a potentially blocking socket to avoid send to block if the socket buffer is full?

I'm currently using select only to check if there is something to read on sockets but maybe i should use it also to see if i can write on a particular socket too? And also, can i know how many bytes i can read/write when select returns? For example, if select "tells" that i can write on this socket, how can i know how many bytes i can write at most before writing on this socket actually becomes blocking?

Thanks.

3
csocketsunixselect
Your question embodies a contrarian in terms. You want non-blocking behaviour, and yet you don't want to set the socket into non-blocking mode. Not a real question. - user207421
Use MSG_DONTWAIT flag for send() :) Combining it with select is a good idea, so you dont just busy-loop around the send when the buffer is full. - thuovila
This is exactly what O_NONBLOCK is for. Is there any reason you're not setting the descriptor to O_NONBLOCK ? - nos

3 Answers

1
votes

You could use setsockopt() together with SO_SNDTIMEO to set up a maximum amount of time send() will try to do its work.

See man setsockoptand man 7 socket for details.

0
votes

It might be horrible. If you don't go NONBLOCK-ing mode and calling select(), which internally puts the process on sleep for specific timeout value. That means, fd will be blocked for that specific time period.

0
votes

This approach actually works but i recently found by reading the man of send that it can blocks if the socket buffer is full and is the flag O_NONBLOCK is not set on the socket.

This is why you use select, but it still isn't reliable, as man select states:

Under Linux, select() may report a socket file descriptor as "ready for reading", while nevertheless a subsequent read blocks. This could for example happen when data has arrived but upon examination has wrong checksum and is discarded. There may be other circumstances in which a file descriptor is spuriously reported as ready. Thus it may be safer to use O_NONBLOCK on sockets that should not block.