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Jarkko Hietaniemi


PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space



    PERLIO=perlio perl ....


When an undefined layer 'foo' is encountered in an open or binmode layer specification then C code performs the equivalent of:

  use PerlIO 'foo';

The perl code in PerlIO.pm then attempts to locate a layer by doing

  require PerlIO::foo;

Otherwise the PerlIO package is a place holder for additional PerlIO related functions.

The following layers are currently defined:


Low level layer which calls read, write and lseek etc.


Layer which calls fread, fwrite and fseek/ftell etc. Note that as this is "real" stdio it will ignore any layers beneath it and got straight to the operating system via the C library as usual.


This is a re-implementation of "stdio-like" buffering written as a PerlIO "layer". As such it will call whatever layer is below it for its operations.


A layer which does CRLF to "\n" translation distinguishing "text" and "binary" files in the manner of MS-DOS and similar operating systems.


Declares that the stream accepts perl's internal encoding of characters. (Which really is UTF-8 on ASCII machines, but is UTF-EBCDIC on EBCDIC machines.) This allows any character perl can represent to be read from or written to the stream. The UTF-X encoding is chosen to render simple text parts (i.e. non-accented letters, digits and common punctuation) human readable in the encoded file.

Here is how to write your native data out using UTF-8 (or UTF-EBCDIC) and then read it back in.

        open(F, ">:utf8", "data.utf");
        print F $out;

        open(F, "<:utf8", "data.utf");
        $in = <F>;

A pseudo-layer which performs two functions (which is messy, but necessary to maintain compatibility with non-PerlIO builds of Perl and their way things have been documented elsewhere).

Firstly it forces the file handle to be considered binary at that point in the layer stack,

Secondly in prevents the IO system seaching back before it in the layer specification. Thus:


Forces the use of perlio layer even if the platform default, or use open default is something else (such as ":encoding(iso-8859-7)") (the :encoding requires use Encode) which would interfere with binary nature of the stream.

Defaults and how to override them

If the platform is MS-DOS like and normally does CRLF to "\n" translation for text files then the default layers are :

  unix crlf

(The low level "unix" layer may be replaced by a platform specific low level layer.)

Otherwise if Configure found out how to do "fast" IO using system's stdio, then the default layers are :

  unix stdio

Otherwise the default layers are

  unix perlio

These defaults may change once perlio has been better tested and tuned.

The default can be overridden by setting the environment variable PERLIO to a space separated list of layers (unix or platform low level layer is always pushed first).

This can be used to see the effect of/bugs in the various layers e.g.

  cd .../perl/t
  PERLIO=stdio  ./perl harness
  PERLIO=perlio ./perl harness


Nick Ing-Simmons <nick@ing-simmons.net>


"binmode" in perlfunc, "open" in perlfunc, perlunicode, Encode