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
intfalse stream_select(
arraynull &$read
,
arraynull &$write
,
arraynull &$except
,
intnull $seconds
,
intnull $microseconds
= null
)
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).
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.