---
name: "PerlIO::Layers"
section: "3pm"
platform: "linux"
package: "libperlio-layers-perl"
version: "0.012-3+b3"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/PerlIO::Layers"
---

# PerlIO::Layers(3pm)

Querying your filehandle's capabilities

## NAME

PerlIO::Layers - Querying your filehandle's capabilities

## VERSION

version 0.012

## SYNOPSIS

```
 use PerlIO::Layers qw/query_handle/;
 if (!query_handle(\*STDOUT, 'binary')) {
     ...
 }
```

## DESCRIPTION

Perl's filehandles are implemented as a stack of layers, with the bottom-most usually doing the actual IO and the higher ones doing buffering, encoding/decoding or transformations. PerlIO::Layers allows you to query the filehandle's properties concerning these layers.

## FUNCTIONS

### query\_handle($fh, $query\_name \[, $argument\])

This query a filehandle for some information. All queries can take an optional argument, that will test for that layer's properties instead of all layers of the handle. Currently supported queries include:

- •

  layer

  Check the presence of a certain layer. Unlike most other properties `$argument` is mandatory for this query.

- •

  utf8

  Check whether the filehandle/layer handles unicode

- •

  crlf

  Check whether the filehandle/layer does crlf translation

- •

  binary

  Check whether the filehandle/layer is binary. This test is pessimistic (for unknown layers it will assume it's not binary).

- •

  mappable

  Checks whether the filehandle/layer is memory mappable. It is the same as binary, except that the `"utf8"` layer is accepted.

- •

  buffered

  Check whether the filehandle/layer is buffered.

- •

  readable

  Check whether the filehandle/layer is readable.

- •

  writeable

  Check whether the filehandle/layer is writeable.

- •

  open

  Check whether the filehandle/layer is open.

- •

  temp

  Check whether the filehandle/layer refers to a temporary file.

- •

  can\_crlf

  Checks whether layer `$argument` (or any layer if `$argument` it not given) can do crlf translation.

- •

  line\_buffered

  Check whether the filehandle is in line-buffering mode.

- •

  autoflush

  Checks whether the filehandle is in unbuffering mode. Note that this is not the opposite of buffering, but more similar to autoflush, hence the name of this test.

- •

  buffer\_size

  Check whether the buffer size is equal to `$argument`.

### get\_layers($fh)

Gets information on the layers of a filehandle. It's a list with whose entries have 3 elements: the name of the layer, the arguments of the layer (may be undef) and an arrayref with the flags of the layer as strings. The flags array can contain any of these values:

- •

  EOF

  End of file has been reached.

- •

  CANWRITE

  Writes are permitted, i.e. opened as "\>" or "+\<" or "\>\>", etc.

- •

  CANREAD

  Reads are permitted i.e. opened "\<" or "+\>".

- •

  ERROR

  An error has occurred.

- •

  TRUNCATE

  Truncate file suggested by open mode.

- •

  APPEND

  All writes should be appends.

- •

  CRLF

  Layer is performing Win32-like "\\n" mapped to CR,LF for output and CR,LF mapped to "\\n" for input. Normally the provided "crlf" layer is the only layer that need bother about this. `"binmode"` will mess with this flag rather than add/remove layers if the PERLIO\_K\_CANCRLF bit is set for the layers class.

- •

  UTF8

  Data written to this layer should be UTF-8 encoded; data provided by this layer should be considered UTF-8 encoded. Can be set on any layer by ":utf8" dummy layer. Also set on ":encoding" layer.

- •

  UNBUF

  Layer is unbuffered - i.e. write to next layer down should occur for each write to this layer.

- •

  WRBUF

  The buffer for this layer currently holds data written to it but not sent to next layer.

- •

  RDBUF

  The buffer for this layer currently holds unconsumed data read from layer below.

- •

  LINEBUF

  Layer is line buffered. Write data should be passed to next layer down whenever a "\\n" is seen. Any data beyond the "\\n" should then be processed.

- •

  TEMP

  File has been **unlink()**ed, or should be deleted on **close()**.

- •

  OPEN

  Handle is open.

- •

  FASTGETS

  This instance of this layer supports the "fast gets" interface. Normally set based on PERLIO\_K\_FASTGETS for the class and by the existence of the function(s) in the table. However a class that normally provides that interface may need to avoid it on a particular instance. The "pending" layer needs to do this when it is pushed above a layer which does not support the interface.

`"query_handle"` provides a more high level interface to this, you should probably use that when you can.

### get\_buffer\_sizes($fh)

Returns a list of buffer sizes for all buffered layers. Unbuffered layers are skipped.

## AUTHOR

Leon Timmermans \<fawaka@gmail.com\>

## COPYRIGHT AND LICENSE

This software is copyright (c) 2010 by Leon Timmermans.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.