# 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.

> Note: 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).

Warning

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

> Note: 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
> $e = NULL;
> stream_select($r, $w, $e, 0);
> ?>`

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

> Note: 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.

> Note: 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().