SOCKET_SELECT(3) 1 SOCKET_SELECT(3)
socket_select - Runs the select() system call on the given arrays of sockets with a specified timeout
SYNOPSIS
int socket_select (array &$read, array &$write, array &$except, int $tv_sec, [int $tv_usec])
DESCRIPTION
socket_select(3) accepts arrays of sockets and waits for them to change status. Those coming with BSD sockets background will recognize
that those socket resource arrays are in fact the so-called file descriptor sets. Three independent arrays of socket resources are watched.
PARAMETERS
o $read
- The sockets 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 socket resource is also ready on end-of-file, in which case a socket_read(3) will
return a zero length string).
o $write
- The sockets listed in the $write array will be watched to see if a write will not block.
o $except
- The sockets listed in the $except array will be watched for exceptions.
o $tv_sec
- The $tv_sec and $tv_usec together form the timeout parameter. The timeout is an upper bound on the amount of time elapsed before
socket_select(3) return. $tv_sec may be zero , causing socket_select(3) to return immediately. This is useful for polling. If
$tv_sec is NULL (no timeout), socket_select(3) can block indefinitely.
o $tv_usec
-
Warning
On exit, the arrays are modified to indicate which socket resource actually changed status.
You do not need to pass every array to socket_select(3). 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 socket_select(3) returns.
Note
Due 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:
Example #1
Using NULL with socket_select(3)
<?php
$e = NULL;
socket_select($r, $w, $e, 0);
?>
RETURN VALUES
On success socket_select(3) returns the number of socket resources contained in the modified arrays, which may be zero if the timeout
expires before anything interesting happens. On error FALSE is returned. The error code can be retrieved with socket_last_error(3).
Note
Be sure to use the === operator when checking for an error. Since the socket_select(3) may return 0 the comparison with == would
evaluate to TRUE:
Example #2
Understanding socket_select(3)'s result
<?php
$e = NULL;
if (false === socket_select($r, $w, $e, 0)) {
echo "socket_select() failed, reason: " .
socket_strerror(socket_last_error()) . "
";
}
?>
EXAMPLES
Example #3
socket_select(3) example
<?php
/* Prepare the read array */
$read = array($socket1, $socket2);
$write = NULL;
$except = NULL;
$num_changed_sockets = socket_select($read, $write, $except, 0);
if ($num_changed_sockets === false) {
/* Error handling */
} else if ($num_changed_sockets > 0) {
/* At least at one of the sockets something interesting happened */
}
?>
NOTES
Note
Be aware that some socket implementations need to be handled very carefully. A few basic rules:
o You should always try to use socket_select(3) without timeout. Your program should have nothing to do if there is no data
available. Code that depends on timeouts is not usually portable and difficult to debug.
o No socket resource must be added to any set if you do not intend to check its result after the socket_select(3) call, and
respond appropriately. After socket_select(3) returns, all socket resources in all arrays must be checked. Any socket
resource that is available for writing must be written to, and any socket resource available for reading must be read from.
o If you read/write to a socket returns 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.
o It's common to most socket implementations that the only exception caught with the $except array is out-of-bound data
received on a socket.
SEE ALSO
socket_read(3), socket_write(3), socket_last_error(3), socket_strerror(3).
PHP Documentation Group SOCKET_SELECT(3)