---
name: "IO::Async::Loop::Select"
section: "3pm"
platform: "linux"
package: "libio-async-perl"
version: "0.804-2"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/IO::Async::Loop::Select"
---

# IO::Async::Loop::Select(3pm)

use IO::Async with "select(2)"

## NAME

"IO::Async::Loop::Select" - use IO::Async with "[select(2)](/select/2.md)"

## SYNOPSIS

Normally an instance of this class would not be directly constructed by a program. It may however, be useful for runinng IO::Async with an existing program already using a `"select"` call.

```
   use IO::Async::Loop::Select;
   my $loop = IO::Async::Loop::Select->new;
   $loop->add( ... );
   while(1) {
      my ( $rvec, $wvec, $evec ) = ('') x 3;
      my $timeout;
      $loop->pre_select( \$rvec, \$wvec, \$evec, \$timeout );
      ...
      my $ret = select( $rvec, $wvec, $evec, $timeout );
      ...
      $loop->post_select( $rvec, $evec, $wvec );
   }
```

## DESCRIPTION

This subclass of IO::Async::Loop uses the `select(2)` syscall to perform read-ready and write-ready tests.

To integrate with an existing `"select"`-based event loop, a pair of methods `"pre_select"` and `"post_select"` can be called immediately before and after a `"select"` call. The relevant bits in the read-ready, write-ready and exceptional-state bitvectors are set by the `"pre_select"` method, and tested by the `"post_select"` method to pick which event callbacks to invoke.

## CONSTRUCTOR

### new

```
   $loop = IO::Async::Loop::Select->new;
```

This function returns a new instance of a `"IO::Async::Loop::Select"` object. It takes no special arguments.

## METHODS

### pre\_select

```
   $loop->pre_select( \$readvec, \$writevec, \$exceptvec, \$timeout );
```

This method prepares the bitvectors for a `"select"` call, setting the bits that the Loop is interested in. It will also adjust the `$timeout` value if appropriate, reducing it if the next event timeout the Loop requires is sooner than the current value.

- \\$readvec

  - \\$writevec

  - \\$exceptvec

  Scalar references to the reading, writing and exception bitvectors

- \\$timeout

  Scalar reference to the timeout value

### post\_select

```
   $loop->post_select( $readvec, $writevec, $exceptvec );
```

This method checks the returned bitvectors from a `"select"` call, and calls any of the callbacks that are appropriate.

- $readvec

  - $writevec

  - $exceptvec

  Scalars containing the read-ready, write-ready and exception bitvectors

### loop\_once

```
   $count = $loop->loop_once( $timeout );
```

This method calls the `"pre_select"` method to prepare the bitvectors for a `"select"` syscall, performs it, then calls `"post_select"` to process the result. It returns the total number of callbacks invoked by the `"post_select"` method, or `"undef"` if the underlying `select(2)` syscall returned an error.

## SEE ALSO

- •

  IO::Select - OO interface to select system call

## AUTHOR

Paul Evans \<leonerd@leonerd.org.uk\>