stream_select() looks deceptively like a simple wrapper around POSIX select(2).
But beware: while select(2) allows you to pass no file descriptors and use it as a "portable subsecond sleep", PHP will complain with "Warning: stream_select(): No stream arrays were passed in ****" if all arrays are empty or null, and it WONT sleep, it will return immediately. So... if the number of file descriptors you have isn't static, you have to deal with the special case yourself.
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 = 0
] )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
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.
See Also
- stream_set_blocking() - Set blocking/non-blocking mode on a stream
add a note
User Contributed Notes 12 notes
aks at esoft dot dk ¶
2 years ago
Ben ¶
6 years ago
You can key on file descriptors just fine by casting them to an int or a string, which returns what you would expect.
aidan at aidans dot org ¶
6 years ago
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);
// $key will be "sock_1", "sock_2", "sock_3", etc.
}
}
?>
Hope that helps someone out there!
lynch at php dot net ¶
3 years ago
Note: At least one of the input arrays must be non-empty, or you get an E_WARNING message:
PHP Warning: stream_select(): unable to select [9]: Bad file descriptor (max_fd=0) in
jerrywilborn at gmail dot com ¶
3 years ago
http://bugs.php.net/bug.php?id=42682
This function will not return the number of changed streams under certain systems with older versions, but instead it will return '0'. Be careful.
php at richardneill dot org ¶
3 years ago
Note that reading from a regular file which is on end-of-file will *not* block. You'll get a non-blocking, zero-byte read. However, stream_select *will* block if the input is a pipe, and there is no more data to be had.
Maxdamantus ¶
5 years ago
Simple stream_select wrapper.. Returns the first stream in the array, and sets parameter 2 to the key (So that it is easy to identify what received data):
<?php
function select($array, &$vkey, $timeout=0){
$select = array();
$null = NULL;
foreach($array as $key => $sock){
$x = count($select);
$select[$x] = $sock;
$keys[$x] = $key;
}
if(stream_select($select, $null, $null, $timeout)){
foreach($keys as $key){
if($array[$key] == $select[0]){
$vkey = $key;
return($select[0]);
}
}
}
}
$streams = array("foo" => $stream_one, "bar" => $stream_two); // Create an array of two (already existant) streams.
if($new = select($streams, $key, 60)){ //Sets $new to the resource that next gets new data, and $key to either "foo", or "bar", depending which one it is.
echo $key.":".stream_get_line($new, 2048)."\n";
}
?>
bluej100@gmail ¶
5 years ago
@mbaynton at gmail dot com
A handy syntactic trick:
<?php
$r = Array($stream1, $stream2);
stream_select($r, $w = null, $x = null, 1337);
?>
I've seen it recommended elsewhere in the documentation for clarifying magic arguments so maintainers don't have to go check the function itself, but it also solves your problem here.
maartenwolzak at gmail dot com ¶
6 years ago
Note that you should change the calctimeout function below to divide the outcome by 1.000.000 otherwise you'll be waiting for two years instead of one minute for the socket to timeout...
<?php
// The function to calculate the timeout
function calctimeout($maxtime, $starttime)
{
return ($maxtime - ((microtime(true) - $starttime) * 1000000))/1000000;
}
?>
phpdoc at shemesh dot biz ¶
8 years ago
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 ¶
8 years ago
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;
}
?>
mbaynton at gmail dot com ¶
5 years ago
The documentation states that:
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 appears to be incorrect, at least in PHP 5.1.6, because PHP sees these parameters are passed by reference and thus insists on a bona fide variable for the reference to refer to:
Fatal error: Only variables can be passed by reference
is what you get if you pass NULL or anything else literally.