Runs the equivalent of the select() system call on the given arrays of streams with a timeout specified by seconds and microseconds

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- PHP Manual
- Stream Functions
- Runs the equivalent of the select() system call on the given arrays of streams with a timeout specified by seconds and microseconds

# stream_select

(PHP 4 >= 4.3.0, PHP 5, PHP 7, PHP 8)

stream_select — Runs the equivalent of the select() system call on the given arrays of streams with a timeout specified by seconds and microseconds

### Description

stream_select(
?array `&$read`,
?array `&$write`,
?array `&$except`,
?int `$seconds`,
?int `$microseconds` = `null`
): int|false

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.

### Parameters

`read`

  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).

`write`

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

`except`

  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 original keys of the arrays are preserved.

`seconds`

  The `seconds` and `microseconds` together form the timeout parameter, `seconds` specifies the number of seconds while `microseconds` the number of microseconds. The `timeout` is an upper bound on the amount of time that stream_select() will wait before it returns. If `seconds` and `microseconds` 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 `seconds` 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).

  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.

`microseconds`

  See `seconds` description.

### Return Values

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).

### Changelog

Version | Description
--- | ---
8.1.0 | `microseconds` is now nullable.

### Examples

Example #1 stream_select() Example

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 */}?>

### Notes

> 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:

> Some streams (like `zlib`) cannot be selected by this function.

> Note: Windows compatibility

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

> `STDIN` from a console changes status as soon as any input events are available, but reading from the stream may still block.

### See Also

- stream_set_blocking() - Set blocking/non-blocking mode on a stream