Problem with blocking unix domain sockets

Question

I'm writing an application that is split into two parts for Mac OS X - a daemon and an agent. I'm using a standard unix socket to communicate between the daemon and the agents. That is, the socket is created with PF_UNIX and SOCK_STREAM.

When agents are created (whenever a user logs in), one of the first things it does is to connect to the socket. This seems to work perfectly for the first agent. However, when the second agent connects, the daemon experiences the following issue:

I'm using select() to check for data that can be read. The select() call succeeds, and indicates that there is data to be read. However, when i call recv() it returns with -1, and errno is set to 35, or "Resource temporarily unavailable".

Now, I would expect this for a non-blocking socket, but I have triple-checked - I never set the socket to be non-blocking.

As far as I can tell, this only happens when a second agent connects to the same unix socket. If I limit myself to one daemon and one agent then everything seems to work perfectly. What could be causing this odd behaviour?

Answer 1

It sounds a bit like you're trying to read from the wrong client fd. It's hard to tell without seeing your code, but it also sounds a bit that way from your description.

So just in case, here's how it works. Your server is ending up with three file descriptors, the socket it first starts listening on, and then one file descriptor for each connected client. When there's something to read on the original socket, that means there's a new client; it sounds like you have this part right. Each connected client then gives you its own independent fd to read/write from. Calling select() will return if any of these is ready to read; you then have to check each fd in the readfds variable from select with FD_ISSET() to see if it actually has data to read.

You can see a basic example of this type of code here.