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Jarkko Hietaniemi


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


  open($fh,"<:crlf", "my.txt"); # portably open a text file for reading

  open($fh,"<","his.jpg");      # portably open a binary file for reading

    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. (It currently does not mimic MS-DOS as far as treating of Control-Z as being an end-of-file marker.)


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>;

This is the inverse of :utf8 layer. It turns off the flag on the layer below so that data read from it is considered to be "octets" i.e. characters in range 0..255 only. Likewise on output perl will warn if a "wide" character is written to a such a stream.


The :raw layer is defined as being identical to calling binmode($fh) - the stream is made suitable for passing binary data i.e. each byte is passed as-is. The stream will still be buffered. Unlike in the earlier versions of Perl :raw is not just the inverse of :crlf - other layers which would affect the binary nature of the stream are also removed or disabled.

The implementation of :raw is as a pseudo-layer which when "pushed" pops itself and then any layers which do not declare themselves as suitable for binary data. (Undoing :utf8 and :crlf are implemented by clearing flags rather than popping layers but that is an implementation detail.)

As a consequence of the fact that :raw normally pops layers it usually only makes sense to have it as the only or first element in a layer specification. When used as the first element it provides a known base on which to build e.g.


will construct a "binary" stream, but then enable UTF-8 translation.


A pseudo layer that removes the top-most layer. Gives perl code a way to manipulate the layer stack. Should be considered as experimental. Note that :pop only works on real layers and will not undo the effects of pseudo layers like :utf8. An example of a possible use might be:

    binmode($fh,":encoding(...)");  # next chunk is encoded
    binmode($fh,":pop");            # back to un-encocded

A more elegant (and safer) interface is needed.

Custom Layers

It is possible to write custom layers in addition to the above builtin ones, both in C/XS and Perl. Two such layers (and one example written in Perl using the latter) come with the Perl distribution.


Use :encoding(ENCODING) either in open() or binmode() to install a layer that does transparently character set and encoding transformations, for example from Shift-JIS to Unicode. Note that under stdio an :encoding also enables :utf8. See PerlIO::encoding for more information.


Use :via(MODULE) either in open() or binmode() to install a layer that does whatever transformation (for example compression / decompression, encryption / decryption) to the filehandle. See PerlIO::via for more information.

Alternatives to raw

To get a binary stream an alternate method is to use:


this has advantage of being backward compatible with how such things have had to be coded on some platforms for years.

To get an un-buffered stream specify an unbuffered layer (e.g. :unix) in the open call:


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

For the various value of PERLIO see "PERLIO" in perlrun.

Querying the layers of filehandles

The following returns the names of the PerlIO layers on a filehandle.

   my @layers = PerlIO::get_layers($fh); # Or FH, *FH, "FH".

The layers are returned in the order an open() or binmode() call would use them. Note that the "default stack" depends on the operating system and on the Perl version, and both the compile-time and runtime configurations of Perl.

The following table summarizes the default layers on UNIX-like and DOS-like platforms and depending on the setting of the $ENV{PERLIO}:

 PERLIO     UNIX-like                   DOS-like
 unset / "" unix perlio / stdio [1]     unix crlf
 stdio      unix perlio / stdio [1]     stdio
 perlio     unix perlio                 unix perlio
 mmap       unix mmap                   unix mmap

 # [1] "stdio" if Configure found out how to do "fast stdio" (depends
 # on the stdio implementation) and in Perl 5.8, otherwise "unix perlio"

By default the layers from the input side of the filehandle is returned, to get the output side use the optional output argument:

   my @layers = PerlIO::get_layers($fh, output => 1);

(Usually the layers are identical on either side of a filehandle but for example with sockets there may be differences, or if you have been using the open pragma.)

There is no set_layers(), nor does get_layers() return a tied array mirroring the stack, or anything fancy like that. This is not accidental or unintentional. The PerlIO layer stack is a bit more complicated than just a stack (see for example the behaviour of :raw). You are supposed to use open() and binmode() to manipulate the stack.

Implementation details follow, please close your eyes.

The arguments to layers are by default returned in parenthesis after the name of the layer, and certain layers (like utf8) are not real layers but instead flags on real layers: to get all of these returned separately use the optional separate argument:

   my @layer_and_args_and_flags = PerlIO::get_layers($fh, details => 1);

The result will be up to be three times the number of layers: the first element will be a name, the second element the arguments (unspecified arguments will be undef), the third element the flags, the fourth element a name again, and so forth.

You may open your eyes now.


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


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