Read constant UDP stream in php

Question

how do i go about reading data being sent almost constantly to my server. the protocol is udp. if i try to read in a while(1) loop, i dont get anything. it seems like the read will only echo once all the reading is done. so it waits till the loop is done reading which it will never be. i want the socket_read to echo immediately when it gets the data. here is the code that doesnt work. thanks in advance.

<?php
$sock = socket_create(AF_INET, SOCK_DGRAM, SOL_UDP);
socket_bind($sock, $local, $port) or die('Could not bind to address');

//this is where the reading loop should go.
while(1)
{
    echo socket_read($sock,1024);
}

socket_close($sock);
?>

Answer 1

Try calling flush() immediately after that echo statement.

Answer 2

Something like this might help:

do {
    echo socket_read($handle,1024);
    $status = socket_get_status($handle);
} while($status['unread_bytes']);

OR

while ( $buffer = @socket_read($sock,512,PHP_NORMAL_READ) )
    echo $buffer;

Answer 3

The PHP manual entry on socket_read() is a little vague when it comes to how much (if any) internal buffering it's doing. Given that you are passing 1024 in for the length, that specifies that it should return after receiving no more than 1024 bytes of data.

Disclaimer: the following is just speculation, as I have no knowledge of the internal implementation of socket_read().

If the socket_read() function is using its length parameter as a hint for an internal buffer size, you might see bad performance with small UDP packets. For example, if socket_read() waits for 1024 bytes of data regardless of the size of the packets, if you are constantly receiving 60 byte UDP packets it'll take a while for the buffer to fill and the function to return.

(Note: after looking up the "unread_bytes" field mentioned by Tim, it looks like PHP does keep internal buffers, but it makes no mention of how large or small those might be.)

In this case, socket_read() will return larger chunks of data once its buffers fill to reduce processing resource consumption, but at the expense of higher latency. If you need the packets as past as possible, perhaps setting a lower length field would work. That would force socket_read() to return sooner, albeit at the expense of executing your loop more often. Also if you set the length too low, your socket_read()'s might start returning incomplete packets, so you'll have to account for that in your code. (If that matters for your application, of course.)

Answer 4

I needed to call ob_flush();. Never even heard of it before. turns out my problem wasn't the loop, but the fact that php naturally waits till script is done before actually sending the internal buffer to the web browser. calling flush(); followed by ob_flush(); will force php to send whatever buffer it has stored to the browser immediately. This is needed for scripts that will not stop (infinite loops) and want to echo data to the browser. Sometimes flush() doesn't work as it didn't in this case. Hope that helps.