.\" Automatically generated by Pod::Man v1.34, Pod::Parser v1.13
.\"
.\" Standard preamble:
.\" ========================================================================
.de Sh \" Subsection heading
.br
.if t .Sp
.ne 5
.PP
\fB\\$1\fR
.PP
..
.de Sp \" Vertical space (when we can't use .PP)
.if t .sp .5v
.if n .sp
..
.de Vb \" Begin verbatim text
.ft CW
.nf
.ne \\$1
..
.de Ve \" End verbatim text
.ft R
.fi
..
.\" Set up some character translations and predefined strings.  \*(-- will
.\" give an unbreakable dash, \*(PI will give pi, \*(L" will give a left
.\" double quote, and \*(R" will give a right double quote.  | will give a
.\" real vertical bar.  \*(C+ will give a nicer C++.  Capital omega is used to
.\" do unbreakable dashes and therefore won't be available.  \*(C` and \*(C'
.\" expand to `' in nroff, nothing in troff, for use with C<>.
.tr \(*W-|\(bv\*(Tr
.ds C+ C\v'-.1v'\h'-1p'\s-2+\h'-1p'+\s0\v'.1v'\h'-1p'
.ie n \{\
.    ds -- \(*W-
.    ds PI pi
.    if (\n(.H=4u)&(1m=24u) .ds -- \(*W\h'-12u'\(*W\h'-12u'-\" diablo 10 pitch
.    if (\n(.H=4u)&(1m=20u) .ds -- \(*W\h'-12u'\(*W\h'-8u'-\"  diablo 12 pitch
.    ds L" ""
.    ds R" ""
.    ds C` ""
.    ds C' ""
'br\}
.el\{\
.    ds -- \|\(em\|
.    ds PI \(*p
.    ds L" ``
.    ds R" ''
'br\}
.\"
.\" If the F register is turned on, we'll generate index entries on stderr for
.\" titles (.TH), headers (.SH), subsections (.Sh), items (.Ip), and index
.\" entries marked with X<> in POD.  Of course, you'll have to process the
.\" output yourself in some meaningful fashion.
.if \nF \{\
.    de IX
.    tm Index:\\$1\t\\n%\t"\\$2"
..
.    nr % 0
.    rr F
.\}
.\"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.hy 0
.if n .na
.\"
.\" Accent mark definitions (@(#)ms.acc 1.5 88/02/08 SMI; from UCB 4.2).
.\" Fear.  Run.  Save yourself.  No user-serviceable parts.
.    \" fudge factors for nroff and troff
.if n \{\
.    ds #H 0
.    ds #V .8m
.    ds #F .3m
.    ds #[ \f1
.    ds #] \fP
.\}
.if t \{\
.    ds #H ((1u-(\\\\n(.fu%2u))*.13m)
.    ds #V .6m
.    ds #F 0
.    ds #[ \&
.    ds #] \&
.\}
.    \" simple accents for nroff and troff
.if n \{\
.    ds ' \&
.    ds ` \&
.    ds ^ \&
.    ds , \&
.    ds ~ ~
.    ds /
.\}
.if t \{\
.    ds ' \\k:\h'-(\\n(.wu*8/10-\*(#H)'\'\h"|\\n:u"
.    ds ` \\k:\h'-(\\n(.wu*8/10-\*(#H)'\`\h'|\\n:u'
.    ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'^\h'|\\n:u'
.    ds , \\k:\h'-(\\n(.wu*8/10)',\h'|\\n:u'
.    ds ~ \\k:\h'-(\\n(.wu-\*(#H-.1m)'~\h'|\\n:u'
.    ds / \\k:\h'-(\\n(.wu*8/10-\*(#H)'\z\(sl\h'|\\n:u'
.\}
.    \" troff and (daisy-wheel) nroff accents
.ds : \\k:\h'-(\\n(.wu*8/10-\*(#H+.1m+\*(#F)'\v'-\*(#V'\z.\h'.2m+\*(#F'.\h'|\\n:u'\v'\*(#V'
.ds 8 \h'\*(#H'\(*b\h'-\*(#H'
.ds o \\k:\h'-(\\n(.wu+\w'\(de'u-\*(#H)/2u'\v'-.3n'\*(#[\z\(de\v'.3n'\h'|\\n:u'\*(#]
.ds d- \h'\*(#H'\(pd\h'-\w'~'u'\v'-.25m'\f2\(hy\fP\v'.25m'\h'-\*(#H'
.ds D- D\\k:\h'-\w'D'u'\v'-.11m'\z\(hy\v'.11m'\h'|\\n:u'
.ds th \*(#[\v'.3m'\s+1I\s-1\v'-.3m'\h'-(\w'I'u*2/3)'\s-1o\s+1\*(#]
.ds Th \*(#[\s+2I\s-2\h'-\w'I'u*3/5'\v'-.3m'o\v'.3m'\*(#]
.ds ae a\h'-(\w'a'u*4/10)'e
.ds Ae A\h'-(\w'A'u*4/10)'E
.    \" corrections for vroff
.if v .ds ~ \\k:\h'-(\\n(.wu*9/10-\*(#H)'\s-2\u~\d\s+2\h'|\\n:u'
.if v .ds ^ \\k:\h'-(\\n(.wu*10/11-\*(#H)'\v'-.4m'^\v'.4m'\h'|\\n:u'
.    \" for low resolution devices (crt and lpr)
.if \n(.H>23 .if \n(.V>19 \
\{\
.    ds : e
.    ds 8 ss
.    ds o a
.    ds d- d\h'-1'\(ga
.    ds D- D\h'-1'\(hy
.    ds th \o'bp'
.    ds Th \o'LP'
.    ds ae ae
.    ds Ae AE
.\}
.rm #[ #] #H #V #F C
.\" ========================================================================
.\"
.IX Title "PerlIO 3"
.TH PerlIO 3 "2002-06-01" "perl v5.8.0" "Perl Programmers Reference Guide"
.SH "NAME"
PerlIO \- On demand loader for PerlIO layers and root of PerlIO::* name space
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&  open($fh,"<:crlf", "my.txt"); # portably open a text file for reading
.Ve
.PP
.Vb 2
\&  open($fh,"<","his.jpg");      # portably open a binary file for reading
\&  binmode($fh);
.Ve
.PP
.Vb 2
\&  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 'foo';
.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.
.PP
The following layers are currently defined:
.IP "unix" 4
.IX Item "unix"
Low level layer which calls \f(CW\*(C`read\*(C'\fR, \f(CW\*(C`write\*(C'\fR and \f(CW\*(C`lseek\*(C'\fR etc.
.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 \*(L"real\*(R" stdio it will ignore any layers beneath it and
got straight to the operating system via the C library as usual.
.IP "perlio" 4
.IX Item "perlio"
This is a re-implementation of \*(L"stdio\-like\*(R" buffering written as a
PerlIO \*(L"layer\*(R".  As such it will call whatever layer is below it for
its operations.
.IP "crlf" 4
.IX Item "crlf"
A layer which does \s-1CRLF\s0 to \*(L"\en\*(R" translation distinguishing \*(L"text\*(R" and
\&\*(L"binary\*(R" files in the manner of MS-DOS and similar operating systems.
(It currently does \fInot\fR mimic MS-DOS as far as treating of Control-Z
as being an end-of-file marker.)
.IP "utf8" 4
.IX Item "utf8"
Declares that the stream accepts perl's internal encoding of
characters.  (Which really is \s-1UTF\-8\s0 on \s-1ASCII\s0 machines, but is
UTF-EBCDIC on \s-1EBCDIC\s0 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.
.Sp
Here is how to write your native data out using \s-1UTF\-8\s0 (or \s-1UTF\-EBCDIC\s0)
and then read it back in.
.Sp
.Vb 3
\&        open(F, ">:utf8", "data.utf");
\&        print F $out;
\&        close(F);
.Ve
.Sp
.Vb 3
\&        open(F, "<:utf8", "data.utf");
\&        $in = <F>;
\&        close(F);
.Ve
.IP "bytes" 4
.IX Item "bytes"
This is the inverse of \f(CW\*(C`:utf8\*(C'\fR layer. It turns off the flag
on the layer below so that data read from it is considered to
be \*(L"octets\*(R" i.e. characters in range 0..255 only. Likewise
on output perl will warn if a \*(L"wide\*(R" character is written
to a such a stream.
.IP "raw" 4
.IX Item "raw"
The \f(CW\*(C`:raw\*(C'\fR layer is \fIdefined\fR as being identical to calling
\&\f(CW\*(C`binmode($fh)\*(C'\fR \- the stream is made suitable for passing binary
data i.e. each byte is passed as\-is. The stream will still be
buffered. Unlike earlier versions of perl \f(CW\*(C`:raw\*(C'\fR is \fInot\fR just the
inverse of \f(CW\*(C`:crlf\*(C'\fR \- other layers which would affect the binary nature of
the stream are also removed or disabled.
.Sp
The implementation of \f(CW\*(C`:raw\*(C'\fR is as a pseudo-layer which when \*(L"pushed\*(R"
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 poping 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 1
\&    open($fh,":raw:utf8",...)
.Ve
.Sp
will construct a \*(L"binary\*(R" stream, but then enable \s-1UTF\-8\s0 translation.
.IP "pop" 4
.IX Item "pop"
A pseudo layer that removes the top-most layer. Gives perl code
a way to manipulate the layer stack. Should be considered
as experimental. Note that \f(CW\*(C`:pop\*(C'\fR only works on real layers
and will not undo the effects of pseudo layers like \f(CW\*(C`:utf8\*(C'\fR.
An example of a possible use might be:
.Sp
.Vb 5
\&    open($fh,...)
\&    ...
\&    binmode($fh,":encoding(...)");  # next chunk is encoded
\&    ...
\&    binmode($fh,":pop");            # back to un-encocded
.Ve
.Sp
A more elegant (and safer) interface is needed.
.Sh "Alternatives to raw"
.IX Subsection "Alternatives to raw"
To get a binary stream an alternate method is to use:
.PP
.Vb 2
\&    open($fh,"whatever")
\&    binmode($fh);
.Ve
.PP
this has advantage of being backward compatible with how such things have
had to be coded on some platforms for years.
.PP
To get an un-buffered stream specify an unbuffered layer (e.g. \f(CW\*(C`:unix\*(C'\fR)
in the open call:
.PP
.Vb 1
\&    open($fh,"<:unix",$path)
.Ve
.Sh "Defaults and how to override them"
.IX Subsection "Defaults and how to override them"
If the platform is MS-DOS like and normally does \s-1CRLF\s0 to \*(L"\en\*(R"
translation for text files then the default layers are :
.PP
.Vb 1
\&  unix crlf
.Ve
.PP
(The low level \*(L"unix\*(R" layer may be replaced by a platform specific low
level layer.)
.PP
Otherwise if \f(CW\*(C`Configure\*(C'\fR found out how to do \*(L"fast\*(R" \s-1IO\s0 using system's
stdio, then the default layers are :
.PP
.Vb 1
\&  unix stdio
.Ve
.PP
Otherwise the default layers are
.PP
.Vb 1
\&  unix perlio
.Ve
.PP
These defaults may change once perlio has been better tested and tuned.
.PP
The default can be overridden by setting the environment variable
\&\s-1PERLIO\s0 to a space separated list of layers (unix or platform low level
layer is always pushed first).
.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
.SH "AUTHOR"
.IX Header "AUTHOR"
Nick Ing-Simmons <nick@ing\-simmons.net>
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\*(L"binmode\*(R" in perlfunc, \*(L"open\*(R" in perlfunc, perlunicode, Encode