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.\" ========================================================================
.\"
.IX Title "PerlIO 3pm"
.TH PerlIO 3pm 2023-11-28 "perl v5.38.2" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH NAME
PerlIO \- On demand loader for PerlIO layers and root of PerlIO::* name space
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 2
\&  # support platform\-native and CRLF text files
\&  open(my $fh, "<:crlf", "my.txt") or die "open failed: $!";
\&
\&  # append UTF\-8 encoded text
\&  open(my $fh, ">>:encoding(UTF\-8)", "some.log")
\&    or die "open failed: $!";
\&
\&  # portably open a binary file for reading
\&  open(my $fh, "<", "his.jpg") or die "open failed: $!";
\&  binmode($fh) or die "binmode failed: $!";
\&
\&  Shell:
\&    PERLIO=:perlio perl ....
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
When an undefined layer 'foo' is encountered in an \f(CW\*(C`open\*(C'\fR or
\&\f(CW\*(C`binmode\*(C'\fR layer specification then C code performs the equivalent of:
.PP
.Vb 1
\&  use PerlIO \*(Aqfoo\*(Aq;
.Ve
.PP
The Perl code in PerlIO.pm then attempts to locate a layer by doing
.PP
.Vb 1
\&  require PerlIO::foo;
.Ve
.PP
Otherwise the \f(CW\*(C`PerlIO\*(C'\fR package is a place holder for additional
PerlIO related functions.
.SS Layers
.IX Subsection "Layers"
Generally speaking, PerlIO layers (previously sometimes referred to as
"disciplines") are an ordered stack applied to a filehandle (specified as
a space\- or colon-separated list, conventionally written with a leading
colon).  Each layer performs some operation on any input or output, except
when bypassed such as with \f(CW\*(C`sysread\*(C'\fR or \f(CW\*(C`syswrite\*(C'\fR.  Read operations go
through the stack in the order they are set (left to right), and write
operations in the reverse order.
.PP
There are also layers which actually just set flags on lower layers, or
layers that modify the current stack but don't persist on the stack
themselves; these are referred to as pseudo-layers.
.PP
When opening a handle, it will be opened with any layers specified
explicitly in the \fBopen()\fR call (or the platform defaults, if specified as
a colon with no following layers).
.PP
If layers are not explicitly specified, the handle will be opened with the
layers specified by the ${^OPEN} variable (usually
set by using the open pragma for a lexical scope, or the \f(CW\*(C`\-C\*(C'\fR
command-line switch or \f(CW\*(C`PERL_UNICODE\*(C'\fR environment variable for the main
program scope).
.PP
If layers are not specified in the \fBopen()\fR call or \f(CW\*(C`${^OPEN}\*(C'\fR variable,
the handle will be opened with the default layer stack configured for that
architecture; see "Defaults and how to override them".
.PP
Some layers will automatically insert required lower level layers if not
present; for example \f(CW\*(C`:perlio\*(C'\fR will insert \f(CW\*(C`:unix\*(C'\fR below itself for low
level IO, and \f(CW\*(C`:encoding\*(C'\fR will insert the platform defaults for buffered
IO.
.PP
The \f(CW\*(C`binmode\*(C'\fR function can be called on an opened handle to push
additional layers onto the stack, which may also modify the existing
layers.  \f(CW\*(C`binmode\*(C'\fR called with no layers will remove or unset any
existing layers which transform the byte stream, making the handle
suitable for binary data.
.PP
The following layers are currently defined:
.IP :unix 4
.IX Item ":unix"
Lowest level layer which provides basic PerlIO operations in terms of
UNIX/POSIX numeric file descriptor calls
(\fBopen()\fR, \fBread()\fR, \fBwrite()\fR, \fBlseek()\fR, \fBclose()\fR).
It is used even on non-Unix architectures, and most other layers operate on
top of it.
.IP :stdio 4
.IX Item ":stdio"
Layer which calls \f(CW\*(C`fread\*(C'\fR, \f(CW\*(C`fwrite\*(C'\fR and \f(CW\*(C`fseek\*(C'\fR/\f(CW\*(C`ftell\*(C'\fR etc.  Note
that as this is "real" stdio it will ignore any layers beneath it and
go straight to the operating system via the C library as usual.
This layer implements both low level IO and buffering, but is rarely used
on modern architectures.
.IP :perlio 4
.IX Item ":perlio"
A from scratch implementation of buffering for PerlIO. Provides fast
access to the buffer for \f(CW\*(C`sv_gets\*(C'\fR which implements Perl's readline/<>
and in general attempts to minimize data copying.
.Sp
\&\f(CW\*(C`:perlio\*(C'\fR will insert a \f(CW\*(C`:unix\*(C'\fR layer below itself to do low level IO.
.IP :crlf 4
.IX Item ":crlf"
A layer that implements DOS/Windows like CRLF line endings.  On read
converts pairs of CR,LF to a single "\en" newline character.  On write
converts each "\en" to a CR,LF pair.  Note that this layer will silently
refuse to be pushed on top of itself.
.Sp
It currently does \fInot\fR mimic MS-DOS as far as treating of Control-Z
as being an end-of-file marker.
.Sp
On DOS/Windows like architectures where this layer is part of the defaults,
it also acts like the \f(CW\*(C`:perlio\*(C'\fR layer, and removing the CRLF translation
(such as with \f(CW\*(C`:raw\*(C'\fR) will only unset the CRLF translation flag.  Since
Perl 5.14, you can also apply another \f(CW\*(C`:crlf\*(C'\fR layer later, such as when
the CRLF translation must occur after an encoding layer.  On other
architectures, it is a mundane CRLF translation layer and can be added and
removed normally.
.Sp
.Vb 3
\&    # translate CRLF after encoding on Perl 5.14 or newer
\&    binmode $fh, ":raw:encoding(UTF\-16LE):crlf"
\&      or die "binmode failed: $!";
.Ve
.IP :utf8 4
.IX Item ":utf8"
Pseudo-layer that declares that the stream accepts Perl's \fIinternal\fR
upgraded encoding of characters, which is approximately UTF\-8 on ASCII
machines, but UTF-EBCDIC on EBCDIC machines.  This allows any character
Perl can represent to be read from or written to the stream.
.Sp
This layer (which actually sets a flag on the preceding layer, and is
implicitly set by any \f(CW\*(C`:encoding\*(C'\fR layer) does not translate or validate
byte sequences.  It instead indicates that the byte stream will have been
arranged by other layers to be provided in Perl's internal upgraded
encoding, which Perl code (and correctly written XS code) will interpret
as decoded Unicode characters.
.Sp
\&\fBCAUTION\fR: Do not use this layer to translate from UTF\-8 bytes, as
invalid UTF\-8 or binary data will result in malformed Perl strings.  It is
unlikely to produce invalid UTF\-8 when used for output, though it will
instead produce UTF-EBCDIC on EBCDIC systems.  The \f(CW:encoding(UTF\-8)\fR
layer (hyphen is significant) is preferred as it will ensure translation
between valid UTF\-8 bytes and valid Unicode characters.
.IP :bytes 4
.IX Item ":bytes"
This is the inverse of the \f(CW\*(C`:utf8\*(C'\fR pseudo-layer.  It turns off the flag
on the layer below so that data read from it is considered to
be Perl's internal downgraded encoding, thus interpreted as the native
single-byte encoding of Latin\-1 or EBCDIC.  Likewise on output Perl will
warn if a "wide" character (a codepoint not in the range 0..255) is
written to a such a stream.
.Sp
This is very dangerous to push on a handle using an \f(CW\*(C`:encoding\*(C'\fR layer,
as such a layer assumes to be working with Perl's internal upgraded
encoding, so you will likely get a mangled result.  Instead use \f(CW\*(C`:raw\*(C'\fR or
\&\f(CW\*(C`:pop\*(C'\fR to remove encoding layers.
.IP :raw 4
.IX Item ":raw"
The \f(CW\*(C`:raw\*(C'\fR pseudo-layer is \fIdefined\fR as being identical to calling
\&\f(CWbinmode($fh)\fR \- the stream is made suitable for passing binary data,
i.e. each byte is passed as-is. The stream will still be buffered
(but this was not always true before Perl 5.14).
.Sp
In Perl 5.6 and some books the \f(CW\*(C`:raw\*(C'\fR layer is documented as the inverse
of the \f(CW\*(C`:crlf\*(C'\fR layer. That is no longer the case \- other layers which
would alter the binary nature of the stream are also disabled.  If you
want UNIX line endings on a platform that normally does CRLF translation,
but still want UTF\-8 or encoding defaults, the appropriate thing to do is
to add \f(CW\*(C`:perlio\*(C'\fR to the PERLIO environment variable, or open the handle
explicitly with that layer, to replace the platform default of \f(CW\*(C`:crlf\*(C'\fR.
.Sp
The implementation of \f(CW\*(C`:raw\*(C'\fR is as a pseudo-layer which when "pushed"
pops itself and then any layers which would modify the binary data stream.
(Undoing \f(CW\*(C`:utf8\*(C'\fR and \f(CW\*(C`:crlf\*(C'\fR may be implemented by clearing flags
rather than popping layers but that is an implementation detail.)
.Sp
As a consequence of the fact that \f(CW\*(C`:raw\*(C'\fR 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.
.Sp
.Vb 2
\&    open(my $fh,">:raw:encoding(UTF\-8)",...)
\&      or die "open failed: $!";
.Ve
.Sp
will construct a "binary" stream regardless of the platform defaults,
but then enable UTF\-8 translation.
.IP :pop 4
.IX Item ":pop"
A pseudo-layer that removes the top-most layer. Gives Perl code a
way to manipulate the layer stack.  Note that \f(CW\*(C`:pop\*(C'\fR only works on
real layers and will not undo the effects of pseudo-layers or flags
like \f(CW\*(C`:utf8\*(C'\fR.  An example of a possible use might be:
.Sp
.Vb 7
\&    open(my $fh,...) or die "open failed: $!";
\&    ...
\&    binmode($fh,":encoding(...)") or die "binmode failed: $!";
\&    # next chunk is encoded
\&    ...
\&    binmode($fh,":pop") or die "binmode failed: $!";
\&    # back to un\-encoded
.Ve
.Sp
A more elegant (and safer) interface is needed.
.SS "Custom Layers"
.IX Subsection "Custom Layers"
It is possible to write custom layers in addition to the above builtin
ones, both in C/XS and Perl, as a module named \f(CW\*(C`PerlIO::<layer name>\*(C'\fR.
Some custom layers come with the Perl distribution.
.IP :encoding 4
.IX Item ":encoding"
Use \f(CW:encoding(ENCODING)\fR to transparently do character set and encoding
transformations, for example from Shift-JIS to Unicode.  Note that an
\&\f(CW\*(C`:encoding\*(C'\fR also enables \f(CW\*(C`:utf8\*(C'\fR.  See PerlIO::encoding for more
information.
.IP :mmap 4
.IX Item ":mmap"
A layer which implements "reading" of files by using \f(CWmmap()\fR to
make a (whole) file appear in the process's address space, and then
using that as PerlIO's "buffer". This \fImay\fR be faster in certain
circumstances for large files, and may result in less physical memory
use when multiple processes are reading the same file.
.Sp
Files which are not \f(CWmmap()\fR\-able revert to behaving like the \f(CW\*(C`:perlio\*(C'\fR
layer. Writes also behave like the \f(CW\*(C`:perlio\*(C'\fR layer, as \f(CWmmap()\fR for write
needs extra house-keeping (to extend the file) which negates any advantage.
.Sp
The \f(CW\*(C`:mmap\*(C'\fR layer will not exist if the platform does not support \f(CWmmap()\fR.
See PerlIO::mmap for more information.
.IP :via 4
.IX Item ":via"
\&\f(CW:via(MODULE)\fR allows a transformation to be applied by an arbitrary Perl
module, for example compression / decompression, encryption / decryption.
See PerlIO::via for more information.
.IP :scalar 4
.IX Item ":scalar"
A layer implementing "in memory" files using scalar variables,
automatically used in place of the platform defaults for IO when opening
such a handle.  As such, the scalar is expected to act like a file, only
containing or storing bytes.  See PerlIO::scalar for more information.
.SS "Alternatives to raw"
.IX Subsection "Alternatives to raw"
To get a binary stream an alternate method is to use:
.PP
.Vb 2
\&    open(my $fh,"<","whatever") or die "open failed: $!";
\&    binmode($fh) or die "binmode failed: $!";
.Ve
.PP
This has the advantage of being backward compatible with older versions
of Perl that did not use PerlIO or where \f(CW\*(C`:raw\*(C'\fR was buggy (as it was
before Perl 5.14).
.PP
To get an unbuffered stream specify an unbuffered layer (e.g. \f(CW\*(C`:unix\*(C'\fR)
in the open call:
.PP
.Vb 1
\&    open(my $fh,"<:unix",$path) or die "open failed: $!";
.Ve
.SS "Defaults and how to override them"
.IX Subsection "Defaults and how to override them"
If the platform is MS-DOS like and normally does CRLF to "\en"
translation for text files then the default layers are:
.PP
.Vb 1
\&  :unix:crlf
.Ve
.PP
Otherwise if \f(CW\*(C`Configure\*(C'\fR found out how to do "fast" IO using the system's
stdio (not common on modern architectures), then the default layers are:
.PP
.Vb 1
\&  :stdio
.Ve
.PP
Otherwise the default layers are
.PP
.Vb 1
\&  :unix:perlio
.Ve
.PP
Note that the "default stack" depends on the operating system and on the
Perl version, and both the compile-time and runtime configurations of
Perl.  The default can be overridden by setting the environment variable
PERLIO to a space or colon separated list of layers, however this cannot
be used to set layers that require loading modules like \f(CW\*(C`:encoding\*(C'\fR.
.PP
This can be used to see the effect of/bugs in the various layers e.g.
.PP
.Vb 3
\&  cd .../perl/t
\&  PERLIO=:stdio  ./perl harness
\&  PERLIO=:perlio ./perl harness
.Ve
.PP
For the various values of PERLIO see "PERLIO" in perlrun.
.PP
The following table summarizes the default layers on UNIX-like and
DOS-like platforms and depending on the setting of \f(CW$ENV{PERLIO}\fR:
.PP
.Vb 5
\& PERLIO     UNIX\-like                   DOS\-like
\& \-\-\-\-\-\-     \-\-\-\-\-\-\-\-\-                   \-\-\-\-\-\-\-\-
\& unset / "" :unix:perlio / :stdio [1]   :unix:crlf
\& :stdio     :stdio                      :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, else ":unix:perlio"
.Ve
.SS "Querying the layers of filehandles"
.IX Subsection "Querying the layers of filehandles"
The following returns the \fBnames\fR of the PerlIO layers on a filehandle.
.PP
.Vb 1
\&   my @layers = PerlIO::get_layers($fh); # Or FH, *FH, "FH".
.Ve
.PP
The layers are returned in the order an \fBopen()\fR or \fBbinmode()\fR call would
use them, and without colons.
.PP
By default the layers from the input side of the filehandle are
returned; to get the output side, use the optional \f(CW\*(C`output\*(C'\fR argument:
.PP
.Vb 1
\&   my @layers = PerlIO::get_layers($fh, output => 1);
.Ve
.PP
(Usually the layers are identical on either side of a filehandle but
for example with sockets there may be differences.)
.PP
There is no \fBset_layers()\fR, nor does \fBget_layers()\fR 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 \f(CW\*(C`:raw\*(C'\fR).
You are supposed to use \fBopen()\fR and \fBbinmode()\fR to manipulate the stack.
.PP
\&\fBImplementation details follow, please close your eyes.\fR
.PP
The arguments to layers are by default returned in parentheses after
the name of the layer, and certain layers (like \f(CW\*(C`:utf8\*(C'\fR) are not real
layers but instead flags on real layers; to get all of these returned
separately, use the optional \f(CW\*(C`details\*(C'\fR argument:
.PP
.Vb 1
\&   my @layer_and_args_and_flags = PerlIO::get_layers($fh, details => 1);
.Ve
.PP
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 \f(CW\*(C`undef\*(C'\fR), the third element the flags,
the fourth element a name again, and so forth.
.PP
\&\fBYou may open your eyes now.\fR
.SH AUTHOR
.IX Header "AUTHOR"
Nick Ing-Simmons <nick@ing\-simmons.net>
.SH "SEE ALSO"
.IX Header "SEE ALSO"
"binmode" in perlfunc, "open" in perlfunc, perlunicode, perliol,
Encode