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stream_select
(PHP 4 >= 4.3.0, PHP 5, PHP 7)
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
&$read
, array &$write
, array &$except
, int $tv_sec
[, int $tv_usec = 0
] ) : intThe 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
readarray 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
writearray will be watched to see if a write will not block. -
except -
The streams listed in the
exceptarray will be watched for high priority exceptional ("out-of-band") data arriving.
You do not need to pass every array to stream_select(). You can leave it out and use an empty array orNote:
When stream_select() returns, the arrays
read,writeandexceptare modified to indicate which stream resource(s) actually changed status.NULLinstead. Also do not forget that those arrays are passed by reference and will be modified after stream_select() returns. -
tv_sec -
The
tv_secandtv_usectogether form the timeout parameter,tv_secspecifies the number of seconds whiletv_usecthe number of microseconds. Thetimeoutis an upper bound on the amount of time that stream_select() will wait before it returns. Iftv_secandtv_usecare 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_secisNULLstream_select() can block indefinitely, returning only when an event on one of the watched streams occurs (or if a signal interrupts the system call).WarningUsing 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.
-
tv_usec -
See
tv_secdescription.
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).
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
NULLdirectly 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: 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
FALSEunder Windows.
English translation
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Document created the 30/01/2003, last modified the 26/10/2018
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References
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