socket_select

Runs the select() system call on the given arrays of sockets with a specified timeout

&reftitle.description; `int` `socket_select` `array` `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.

&reftitle.parameters;

`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.

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:

#### Using &null; with `socket_select`

    <?php
    $e = NULL;
    socket_select($r, $w, $e, 0);
    ?>

&reftitle.returnvalues;

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`.

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;:

#### 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";
    }
    ?>

&reftitle.examples;

### `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 */
    }
    ?>

&reftitle.notes;

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 `except` array is out-of-bound data received on a socket.

&reftitle.seealso;

- `socket_read`
- `socket_write`
- `socket_last_error`
- `socket_strerror`