PHP: how to use socket_select() and socket_read() properly - php

Question

Edit: *Bug in test harness was causing me to misinterpret the results. socket_select() works exactly as you might expect: it really does wait until there is data ready on the socket. But it will report ready if you call it after the client closes the connection. Turns out it's all my fault, but I'll leave the question here in case anyone else ever suspects the behavior of socket_select().
I have a multi-threaded PHP application and I'm using sockets to communicate between the threads. The communication works pretty well, but I end up doing a lot of unnecessary reading of sockets that have no data ready. Perhaps there's something about socket programming that I'm missing.
The PHP doc for socket_select() says The sockets listed in the read array will be watched to see if a read will not block.
This is exactly the behavior I want: call socket_select() to wait until one of my child threads is trying to talk to me. But that only works for the very first time the thread writes, after I've accepted the connection. After that, socket_select() will forever say that the socket is ready, even after I've read all the data from it.
Is there some way I can mark that socket 'unready' so socket_select() won't report that it's ready until more data arrives? Or is it conventional to close that connection after I've read all the data, and await another connection request? I've done a lot of tutorial- and explanation-reading, but I haven't been able to figure out how to do this correctly. Maybe I'm missing something obvious?
In case it helps to see code, here's what I'm doing:
// Server side setup in the main thread
$this->mConnectionSocket = socket_create(AF_INET, SOCK_STREAM, 0);
$arrOpt = array('l_onoff' => 1, 'l_linger' => 0);
#socket_set_option($this->mConnectionSocket, SOL_SOCKET, SO_LINGER, $arrOpt);
#socket_set_option($this->mConnectionSocket, SOL_SOCKET, SO_REUSEADDR, true);
#socket_bind($this->mConnectionSocket, Hostname, $this->mPortNumber);
#socket_listen($this->mConnectionSocket);
#socket_set_block($this->mConnectionSocket);
.
.
.
// Then call listen(), which looks like this:
public function listen(&$outReadySockets) {
$null = null;
while(true) {
$readyArray = array_merge(array($this->mConnectionSocket), $this->mReceiverSockets);
socket_select($readyArray, $null, $null, $waitTime = null);
if(in_array($this->mConnectionSocket, $readyArray) === true) {
$this->acceptConnection();
$key = array_search($this->mConnectionSocket, $readyArray);
if($key === false) {
throw new IPCException("array_search() returned unexpected value");
} else {
unset($readyArray[$key]);
if(in_array($this->mConnectionSocket, $readyArray) === true) {
throw new IPCException("in_array() says the key is still there");
}
}
}
if(count($readyArray) > 0) {
$outReadySockets = array_merge($readyArray);
break;
}
}
}
// Client side setup in the child thread
$this->mSocket = #socket_create(AF_INET, SOCK_STREAM, 0);
#socket_set_block($this->mSocket);
#socket_connect($this->mSocket, Hostname, $this->mPortNumber);
.
.
.
#socket_write($this->mSocket, $inDataToWrite, $lengthToWrite);
// Main thread reads the socket until it's empty
$data = "";
$totalBytesRead = 0;
while($totalBytesRead < $inNumberOfBytesToRead) {
// Strange that even if we set the socket to block mode, socket_read()
// will not block. If there's nothing there, it will just return an
// empty string. This is documented in the PHP docs.
$tdata = socket_read($inSock, $inNumberOfBytesToRead);
if($tdata === false) {
throw new IPCException("socket_read() failed: " . socket_strerror(socket_last_error()));
} else {
$data .= $tdata;
$bytesReadThisPass = strlen($tdata);
if($bytesReadThisPass === 0) {
break;
}
}
$totalBytesRead += $bytesReadThisPass;
}
.
.
.
// Then calls listen() again
As I say, it works great, except that when I call listen() a second time, it tells me that the socket is still ready. That seems to be what the PHP doc is saying, but I don't want that. I want to know when there really is data there. Am I doing it wrong? Or just missing the point?

Answer 1

You are misusing the socket functions.
First of all, socket_read will return false on error; your code does not check for this and will treat error returns the same as empty strings (this is an artifact of the specific constructs you are using, i.e. string concatenation and strlen).
Another problem is that when a connection attempt is detected you are clearly violating the instructions for socket_select:
No socket resource must be added to any set if you do not intend to
check its result after the socket_select() call, and respond
appropriately. After socket_select() returns, all socket resources in
all arrays must be checked. Any socket resource that is available for
writing must be written to, and any socket resource available for
reading must be read from.
Instead of this, if a connection is detected the code accepts it and goes back to socket_select again; sockets ready for reading are not serviced.
Finally it looks like you are confused about what EOF means on a socket: it means that the client has closed its write end of the connection. Once socket_read returns the empty string (not false!) for the first time, it will never return anything meaningful again. Once that happens you can send data from the server's write end and/or simply close the connection entirely.