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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

说明

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.

参数

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.

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.

tv_sec

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).

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.

tv_usec

See tv_sec description.

返回值

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).

范例

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
 
 $read    = array( $stream1 ,  $stream2 );
 $write   =  NULL ;
 $except  =  NULL ;
if ( false  === ( $num_changed_streams  =  stream_select ( $read ,  $write ,  $except ,  0 ))) {
     
 } elseif ( $num_changed_streams  >  0 ) {
     
 }
 ?>

注释

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: 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.

参见

stream_set_blocking() - 为资源流设置阻塞或者阻塞模式

用户评论:

[#1] doingitwrong at mailismagic dot com [2015-06-17 22:35:33]

If you use stream_select() with a blocking stream, you are doing it wrong!

Just because this function returns something in one or more of the arrays does NOT mean that a future read or write operation will not block.

The above sentence is the most important sentence you will ever read regarding stream manipulation.  Using stream_select() with blocking streams is a very common amateur mistake and causes major headaches when tracking down usage of this and similar select() system functions.  PHP (and really the underlying OS) should verify that the supplied stream set is not blocking and throw an error/exception if any socket is set to block so that people are forced to fix their code.  The documentation for stream_select() is, at best, misleading.

If you want a non-blocking stream, then set the stream to not block.  Otherwise, live with the blocking stream.  That is, after all, the whole point of blocking - to block indefinitely until the operation completes.  select() is built for non-blocking streams ONLY.  Any other use will result in very hard to track down bugs.

I got the above lecture many years ago after encountering the very bugs I mention.  I fixed my code and now correct similar mistakes when I run into the issue elsewhere.  Writing code for non-blocking streams is simpler than trying to write hacks for blocking streams with select() functions and ending up with application bugs.

[#2] rob at associatedtechs dot com [2014-07-18 12:26:19]

If you try to use stream_select() with fread(), you may get bit by a combination of bugs (https://bugs.php.net/bug.php?id=52602 and https://bugs.php.net/bug.php?id=51056). As of PHP 5.5.10, fread() and stream_select() don't reliably play well together.

If you need stream_select() and you don't need an encrypted connection (e.g. TLS), use stream_socket_recvfrom() instead of fread().

I can't find a way to reliably handle an encrypted connection with blocking functions in PHP; non-blocking might be the only way to do it.

[#3] mark2xv at gmail dot com [2013-08-20 16:52:21]

If you're getting unexplainable problems with nonblocking sockets using stream_select, disable the buffers using:

stream_set_read_buffer($socket, 0);
stream_set_write_buffer($socket, 0);

For some reason when writing (in total) ~256k, sockets start returning FALSE when reading, yet always appear in the stream_select arrays. This fixed that problem. (for us.)

[#4] aks at esoft dot dk [2010-11-10 07:56:21]

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.

[#5] lynch at php dot net [2010-03-15 12:48:32]

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

[#6] jerrywilborn at gmail dot com [2009-07-27 10:55:09]

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.

[#7] php at richardneill dot org [2009-07-04 19:45:55]

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.

[#8] Maxdamantus [2007-09-29 22:28:11]

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"; } ?>

[#9] bluej100@gmail [2007-08-21 16:31:09]

@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.

[#10] Ben [2007-02-01 17:35:10]

You can key on file descriptors just fine by casting them to an int or a string, which returns what you would expect.

[#11] maartenwolzak at gmail dot com [2007-01-25 01:36:00]

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; } ?>

[#12] aidan at aidans dot org [2006-12-07 19:53:39]

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!

[#13] phpdoc at shemesh dot biz [2005-03-14 01:33:10]

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.

[#14] aidan at php dot net [2004-06-22 11:30:23]

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; } ?>