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Last updated: Sat, 27 Jan 2007

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# stream_select

(PHP 4 >= 4.3.0, PHP 5)

stream_select -- Runs the equivalent of the select() system call on the given arrays of streams with a timeout specified by tv_sec and tv_usec

## Description

int stream_select ( array &read, array &write, array &except, int tv_sec [, int tv_usec] )

The stream_select() function accepts arrays of streams and waits for them to change status. Its operation is equivalent to that of the socket_select() function except in that it acts on streams.

The streams listed in the `read` array will be watched to see if characters become available for reading (more precisely, to see if a read will not block - in particular, a stream resource is also ready on end-of-file, in which case an fread() will return a zero length string).

The streams listed in the `write` array will be watched to see if a write will not block.

The streams listed in the `except` array will be watched for high priority exceptional ("out-of-band") data arriving.

> Замечание: When stream_select() returns, the arrays `read`, `write` and `except` are modified to indicate which stream resource(s) actually changed status.

The `tv_sec` and `tv_usec` together form the timeout parameter, `tv_sec` specifies the number of seconds while `tv_usec` the number of microseconds. The timeout is an upper bound on the amount of time that stream_select() will wait before it returns. If `tv_sec` and `tv_usec` are both set to `0`, stream_select() will not wait for data - instead it will return immediately, indicating the current status of the streams. If `tv_sec` is `NULL` stream_select() can block indefinitely, returning only when an event on one of the watched streams occurs (or if a signal interrupts the system call).

On success stream_select() returns the number of stream resources contained in the modified arrays, which may be zero if the timeout expires before anything interesting happens. On error `FALSE` is returned and a warning raised (this can happen if the system call is interrupted by an incoming signal).

Внимание

Using a timeout value of `0` allows you to instantaneously poll the status of the streams, however, it is NOT a good idea to use a `0` timeout value in a loop as it will cause your script to consume too much CPU time.

It is much better to specify a timeout value of a few seconds, although if you need to be checking and running other code concurrently, using a timeout value of at least `200000` microseconds will help reduce the CPU usage of your script.

Remember that the timeout value is the maximum time that will elapse; stream_select() will return as soon as the requested streams are ready for use.

You do not need to pass every array to stream_select(). You can leave it out and use an empty array or `NULL` instead. Also do not forget that those arrays are passed by reference and will be modified after stream_select() returns.

This example checks to see if data has arrived for reading on either `$stream1` or `$stream2`. Since the timeout value is `0` it will return immediately:

`<?php
/* Prepare the read array */
$read = array($stream1, $stream2);
if (false === ($num_changed_streams = stream_select($read, $write = NULL, $except = NULL, 0))) {
/* Error handling */
} elseif ($num_changed_streams > 0) {
/* At least on one of the streams something interesting happened */
}
?>`

> Замечание: Due to a limitation in the current Zend Engine it is not possible to pass a constant modifier like `NULL` directly as a parameter to a function which expects this parameter to be passed by reference. Instead use a temporary variable or an expression with the leftmost member being a temporary variable:

> `<?php stream_select($r, $w, $e = NULL, 0); ?>`

> Замечание: Be sure to use the `===` operator when checking for an error. Since the stream_select() may return 0 the comparison with `==` would evaluate to `TRUE`:

> `<?php if (false === stream_select($r, $w, $e = NULL, 0)) { echo "stream_select() failed\n"; } ?>`

> Замечание: If you read/write to a stream returned in the arrays be aware that they do not necessarily read/write the full amount of data you have requested. Be prepared to even only be able to read/write a single byte.

> Замечание: Windows compatibility: stream_select() used on a pipe returned from proc_open() may cause data loss under Windows 98.

> Use of stream_select() on file descriptors returned by proc_open() will fail and return `FALSE` under Windows.

See also stream_set_blocking().

add a note User Contributed Notes
stream_select
Ben
01-Feb-2007 05:35
`You can key on file descriptors just fine by casting them to an int or a string, which returns what you would expect.`
maartenwolzak at gmail dot com
25-Jan-2007 01:36
`Note that you should change the calctimeout function below to divide the outcome by 1.000.000 otherwise you'll be waiting for two years instead of one minute for the socket to timeout...
<?php
// The function to calculate the timeout
function calctimeout($maxtime, $starttime)
{
return ($maxtime - ((microtime(true) - $starttime) * 1000000))/1000000;
}
?>`
aidan at aidans dot org
07-Dec-2006 07:53
`Note that contrary to what the previous poster said, one is not able to use a stream resource as a key for an array. Rather, if you want to know which socket you are dealing with, consider using code similar to this:
<?php
$sockets = array("sock_1" => $sock1, "sock_2" => $sock2, "sock_3" => $sock_3);
$read = $write = $error = $sockets;
$num = stream_select($read, $write, $error, 10);
if ($n > 0) {
foreach ($read as $r) {
$key = array_search($r, $sockets);
// $key will be "sock_1", "sock_2", "sock_3", etc.
}
}
?>
Hope that helps someone out there!`
phpdoc at shemesh dot biz
14-Mar-2005 01:33
`Please note that, on return, the key of "read" will be zero based, serially numbered according to the streams for which there is read data ready only. In other words, if you want to know which of the original streams placed in "read" is ready, there is no immediate way of knowing that.
If you want to know which of the original stream is which, you can either use "==", or possibly set a reverse map array, in which the stream is the key, and the key to the original "read" array is the data.`
aidan at php dot net
22-Jun-2004 11:30
`If you want to set an absolute maximum execution time for stream_select in a loop, it's important to decrement the max_time value passed to stream_select.
<?php
// The maximum time for execution in milliseconds
$maxtime = 200000;
// The time the loop started
$starttime = microtime(true);
// Original array of sockets
$r = $orig_sockets;
// The function to calculate the timeout
function calctimeout($maxtime, $starttime)
{
return $maxtime - ((microtime(true) - $starttime) * 1000000);
}
while (stream_select($r, $w = null, $e = null, 0, calctimeout($maxtime, $starttime)) !== 0)
{
// loop through the sockets that showed activity
foreach ($r as $socket) {
// $socket talked
}
// stream_select modifies the contents of $r
// in a loop we should replace it with the original
$r = $orig_sockets;
}
?>`
add a note
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Last updated: Sat, 27 Jan 2007
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