socket_recv not receiving full data

Question

I'm sending from browser through Websocket an image data of around 5000 bytes but this line is receiving total of 1394 bytes only:

while ($bytes = socket_recv($socket, $r_data, 4000, MSG_DONTWAIT)) {
    $data .= $r_data;
}

This is after handshake is done which is correctly being received. The json data is being cutoff after 1394 bytes. What could be the reason?

In the browser interface it is sending image as JSON:

websocket.send(JSON.stringify(request));

The browser interface is fine as it is working with other PHP websocket free programs I've tested.

Here is the full source code.

Answer 1

You have our socket set up as non-blocking by specifying MSG_DONTWAIT, so it will return EAGAIN after it reads the first chunk of data, rather than waiting for more data. Remove the MSG_DONTWAIT flag and use MSG_WAITALL instead, so that it waits for all the data to be received.

There are a few ways of knowing if you have received all the data you are expecting:

1. Send the length of the data. This is useful if you want to send multiple blocks of variable length content. For example if I want to send three strings, I might first send a "3" to tell the receiver how many string to expect, then for each one I would send the length of the string, followed by the string data.
2. Use fixed length messages. If you are expecting multiple messages but each one is the same size, then you can just read from the socket until you have at least that many bytes and then process the message. Note that you may receive more than one message (including partial messages) in a single recv() call.
3. Close the connection. If you are sending only one message, then you can half-close the connection. This works because TCP connections maintain separate states for sending and receiving, so the server and close the sending connection yet leave the receiving one open for the client's reply. In this case, the server sends all its data to the client and then calls socket_shutdown(1)

1 and 2 are useful if you want to process the data while receiving it - for example if you are writing a game, chat application, or something else where the socket stays open and multiple messages are passed back and forth. #3 is the easiest one, and is useful when you just want to receive all the data in one go, for example a file download.

Answer 2

Just my 2 cents on this. socket_recv can return false on an error. Where it can also receive zero (0) bytes in non-blocking IO.

Your check in your loop should be:

while(($bytes = socket_recv($resource, $r_data, 4000, MSG_DONTWAIT)) !== false) {}

Altough I would check the socket for errors also and add some usleep call to prevent "CPU burn".

$data = '';
$done = false;
while(!$done) {
    socket_clear_error($resource);
    $bytes = @socket_recv($resource, $r_data, 4000, MSG_DONTWAIT);

    $lastError = socket_last_error($resource);

    if ($lastError != 11 && $lastError > 0) {
        // something went wrong! do something
        $done = true;
    }
    else if ($bytes === false) {
        // something went wrong also! do something else
        $done = true;
    }
    else if (intval($bytes) > 0) {
        $data .= $r_data;
    }
    else {
        usleep(2000); // prevent "CPU burn"
    }
}