PerlIO(3pm) Perl Programmers Reference Guide PerlIO(3pm)
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
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
open(F, ">:utf8", "data.utf");
print F $out;
open(F, "<:utf8", "data.utf");
$in = <F>;
This is the inverse of ":utf8" layer. It turns off the flag on the layer below so that
data read from it is considered to be "octets" i.e. characters in range 0..255 only.
Likewise on output perl will warn if a "wide" character is written to a such a stream.
raw The ":raw" layer is defined as being identical to calling "binmode($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 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 poping 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.
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 ":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.
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 :
(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 :
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 PERLIO to a space sepa-
rated 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 <email@example.com>
"binmode" in perlfunc, "open" in perlfunc, perlunicode, perliol, Encode
perl v5.8.0 2002-06-01 PerlIO(3pm)