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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.

> 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, $stream2); if (false === ($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 */ } ?>`

> 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 stream_select($r, $w, $e = NULL, 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 if (false === stream_select($r, $w, $e = NULL, 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().

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