Description
int stream_select
( array &$read
, array &$write
, array &$except
, int $tv_sec
[, int $tv_usec
] )
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.
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).
The streams listed in the write
array will be
watched to see if a write will not block.
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 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).
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).
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.
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 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 */
}
?>
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:
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.
See also
stream_set_blocking().
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