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.TH PerlIO 3 "2002-06-01" "perl v5.8.0" "Perl Programmers Reference Guide"
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 \f(CW\*(C`open\*(C'\fR or
\&\f(CW\*(C`binmode\*(C'\fR layer specification then C code performs the equivalent of:
The perl code in PerlIO.pm then attempts to locate a layer by doing
Otherwise the \f(CW\*(C`PerlIO\*(C'\fR package is a place holder for additional
PerlIO related functions.
The following layers are currently defined:
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.
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.
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
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.)
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.
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.
\& open(F, ">:utf8", "data.utf");
\& open(F, "<:utf8", "data.utf");
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
The \f(CW\*(C`:raw\*(C'\fR layer is \fIdefined\fR as being identical to calling
\&\f(CW\*(C`binmode($fh)\*(C'\fR \- the stream is made suitable for passing binary
data i.e. each byte is passed as\-is. The stream will still be
buffered. Unlike earlier versions of perl \f(CW\*(C`:raw\*(C'\fR is \fInot\fR just the
inverse of \f(CW\*(C`:crlf\*(C'\fR \- other layers which would affect the binary nature of
the stream are also removed or disabled.
The implementation of \f(CW\*(C`:raw\*(C'\fR is as a pseudo-layer which when \*(L"pushed\*(R"
pops itself and then any layers which do not declare themselves as suitable
for binary data. (Undoing :utf8 and :crlf are implemented by clearing
flags rather than poping layers but that is an implementation detail.)
As a consequence of the fact that \f(CW\*(C`:raw\*(C'\fR normally pops layers
it usually only makes sense to have it as the only or first element in a
layer specification. When used as the first element it provides
a known base on which to build e.g.
\& open($fh,":raw:utf8",...)
will construct a \*(L"binary\*(R" stream, but then enable \s-1UTF\-8\s0 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 \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:
\& binmode($fh,":encoding(...)"); # next chunk is encoded
\& binmode($fh,":pop"); # back to un-encocded
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:
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. \f(CW\*(C`:unix\*(C'\fR)
\& open($fh,"<:unix",$path)
.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 :
(The low level \*(L"unix\*(R" layer may be replaced by a platform specific low
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 :
Otherwise the default layers are
These defaults may change once perlio has been better tested and tuned.
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).
This can be used to see the effect of/bugs in the various layers e.g.
\& PERLIO=stdio ./perl harness
\& PERLIO=perlio ./perl harness
Nick Ing-Simmons <nick@ing\-simmons.net>
\&\*(L"binmode\*(R" in perlfunc, \*(L"open\*(R" in perlfunc, perlunicode, Encode
projects / OpenSPARC-T2-DV / blob