(PHP 4 >= 4.1.0, PHP 5)
socket_select — Runs the select() system call on the given arrays of sockets with a specified timeout
&$read
, array &$write
, array &$except
, int $tv_sec
[, int $tv_usec = 0
] )socket_select() 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.
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() will return a zero length string).
write
The sockets listed in the write array will be
watched to see if a write will not block.
except
The sockets listed in the except array will be
watched for exceptions.
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() return.
tv_sec may be zero , causing
socket_select() to return immediately. This is useful
for polling. If tv_sec is NULL (no timeout),
socket_select() can block indefinitely.
tv_usec
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(). 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() returns.
Note:
Due a limitation in the current Zend Engine it is not possible to pass a constant modifier like
NULLdirectly 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
NULLwith socket_select()<?php
$e = NULL;
socket_select($r, $w, $e, 0);
?>
On success socket_select() 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().
Note:
Be sure to use the === operator when checking for an error. Since the socket_select() may return 0 the comparison with == would evaluate to
TRUE:Example #2 Understanding socket_select()'s result
<?php
$e = NULL;
if (false === socket_select($r, $w, $e, 0)) {
echo "socket_select() failed, reason: " .
socket_strerror(socket_last_error()) . "\n";
}
?>
Example #3 socket_select() 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 */
}
?>
Note:
Be aware that some socket implementations need to be handled very carefully. A few basic rules:
- You should always try to use socket_select() 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.
- No socket resource must be added to any set if you do not intend to check its result after the socket_select() call, and respond appropriately. After socket_select() 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.
- 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.
- It's common to most socket implementations that the only exception caught with the
exceptarray is out-of-bound data received on a socket.