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Perl Documentation


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


open($fh, "<:crlf", "my.txt"); # support platform-native and 
                               # CRLF text files
open($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 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:

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.

Alternatives to raw

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


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

To get an unbuffered 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 the 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 values 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 $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
# [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 are 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 parentheses 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 details 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 <>


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