What determines how blocking a blocking socket read is?

[johanafm]

Running the code below yields different results on OSX and Ubuntu, and I'd like to know more about what happens, why one or the other way might be implemented, and if there could be other differences on other platforms etc.

When sending signals (e.g. SIGUSR1, SIGINT) to the process while it is reading from socket in blocking mode
OSX: remains blocked the entire specified timeout.
Ubuntu: immediately aborts socket read and resumes program execution with

PHP Warning: socket_recvfrom(): unable to recvfrom [4]: Interrupted system call on line 161

Is one way considered correct and the other incorrect?

Why does behaviour differ? Is it because there are different implementations of sockets (and if so, could the socket implementations even be different on two systems of the same kind)? Or do the kernels have different approaches in dealing with process signals?

I personally find Ubuntu's handling superior, because that allows me to use high timeouts, which means the process will use no resources until either the socket recieves data or the process recieves a signal. On OSX, I cannot use more than a one-second timeout, because execution must resume in order for pending signals to be handled.

Can I trust Ubuntu to keep doing things the same way? Do other systems handle things in other ways than those described here?

And in case the implementing code is relevant…

Socket is created and set to a long enough timeout to notice what happens

$this->socketRecieveTimeout = 10;

$this->socketReceive = socket_create(AF_INET, SOCK_DGRAM, SOL_UDP);
socket_set_option(
    $this->socketReceive,
    SOL_SOCKET,
    SO_RCVTIMEO,
    ['sec' => $this->socketRecieveTimeout, 'usec' => 0]
);

Main program loop

while (true) {
    pcntl_signal_dispatch();
    $bytes = socket_recvfrom($this->socketReceive, $buffer, $bufferSize, 0, $name, $this->port);
    usleep($sleepTime);
}

[sneakyimp]

I'm not sure I'll be much use here, but I have written a multi-client socket server in PHP. I haven't looked at it in years, but the source code of the main loop might be helpful. My experience has been that almost no one knows about sockets, especially in PHP, so it's a bit of a black art.

I did have some issues where I couldn't figure out why data was being sent from my script, but was building up and not being sent across the network. The problem was Nagle's Algorithm or something like that which tries to optimize traffic across a network by waiting for decent-sized packets to optimize network throughput.

I'm not sure exactly why a socket read operation might block and expect there could be many answers. I do recall that sometimes socket data payloads don't get sent until you send some kind of delimiting char: \r, \n, or \0.

I cannot recall the exact details, but I believe I got my socket server to work in blocking mode (i.e., no timeout) on both osx and ubuntu.

Sorry I'm not more helpful. I was trying to find a thread where I was told I had to be prepared to read even a single byte from a socket -- this lead me to start using a buffer for each connected client. I could do a socket read, cram the data in the buffer if it wasn't complete, and move on to the next socket.