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
int stream_select
( array &$read
, array &$write
, array &$except
, int $tv_sec
[, int $tv_usec = 0
] )
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.
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.
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 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.