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Leon Timmermans


PerlIO::Layers - Querying your filehandle's capabilities


version 0.011


 use PerlIO::Layers qw/query_handle/;

 if (!query_handle(\*STDOUT, 'binary')) {


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.


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 wheter 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.


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.


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


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


    An error has occurred.


    Truncate file suggested by open mode.


    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.


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


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


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


    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.


    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.


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


Leon Timmermans <fawaka@gmail.com>


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.