PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space |
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 binmode($fh);
Shell: 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
require PerlIO::foo;
Otherwise the PerlIO
package is a place holder for additional
PerlIO related functions.
The following layers are currently defined:
read
, write
and lseek
etc.
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.
Here is how to write your native data out using UTF-8 (or UTF-EBCDIC) and then read it back in.
open(F, ">:utf8", "data.utf"); print F $out; close(F);
open(F, "<:utf8", "data.utf"); $in = <F>; close(F);
:utf8
layer. It turns off the flag
on the layer below so that data read from it is considered to
be ``octets'' i.e. characters in range 0..255 only. Likewise
on output perl will warn if a ``wide'' character is written
to a such a stream.
:raw
layer is defined as being identical to calling
binmode($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.
open($fh,":raw:utf8",...)
will construct a ``binary'' stream, but then enable UTF-8 translation.
:pop
only works on real layers
and will not undo the effects of pseudo layers like :utf8
.
An example of a possible use might be:
open($fh,...) ... binmode($fh,":encoding(...)"); # next chunk is encoded ... binmode($fh,":pop"); # back to un-encocded
A more elegant (and safer) interface is needed.
To get a binary stream an alternate method is to use:
open($fh,"whatever") binmode($fh);
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:
open($fh,"<:unix",$path)
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 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
Nick Ing-Simmons <nick@ing-simmons.net>
binmode in the perlfunc manpage, open in the perlfunc manpage, the perlunicode manpage, the perliol manpage, the Encode manpage
PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space |