NAME¶

PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space

SYNOPSIS¶

  open($fh,"<:crlf", "my.txt"); # support platform-native and CRLF text files
  open($fh,"<","his.jpg");      # portably open a binary file for reading
  binmode($fh);
  Shell:
    PERLIO=perlio perl ....

DESCRIPTION¶

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:

:unix

Lowest level layer which provides basic PerlIO operations in terms of UNIX/POSIX numeric file descriptor calls (open(), read(), write(), lseek(), close()).

:stdio

Layer which calls "fread", "fwrite" and "fseek"/"ftell" 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.

:perlio

A from scratch implementation of buffering for PerlIO. Provides fast access to the buffer for "sv_gets" which implements perl's readline/<> and in general attempts to minimize data copying.

":perlio" will insert a ":unix" layer below itself to do low level IO.

:crlf

A layer that implements DOS/Windows like CRLF line endings. On read converts pairs of CR,LF to a single "\n" newline character. On write converts each "\n" to a CR,LF pair. Note that this layer likes to be one of its kind: it silently ignores attempts to be pushed into the layer stack more than once.

It currently does not mimic MS-DOS as far as treating of Control-Z as being an end-of-file marker.

(Gory details follow) To be more exact what happens is this: after pushing itself to the stack, the ":crlf" layer checks all the layers below itself to find the first layer that is capable of being a CRLF layer but is not yet enabled to be a CRLF layer. If it finds such a layer, it enables the CRLFness of that other deeper layer, and then pops itself off the stack. If not, fine, use the one we just pushed.

The end result is that a ":crlf" means "please enable the first CRLF layer you can find, and if you can't find one, here would be a good spot to place a new one."

Based on the ":perlio" layer.

:mmap

A layer which implements "reading" of files by using "mmap()" to make a (whole) file appear in the process's address space, and then using that as PerlIO's "buffer". This may be faster in certain circumstances for large files, and may result in less physical memory use when multiple processes are reading the same file.

Files which are not "mmap()"-able revert to behaving like the ":perlio" layer. Writes also behave like the ":perlio" layer, as "mmap()" for write needs extra house-keeping (to extend the file) which negates any advantage.

The ":mmap" layer will not exist if the platform does not support "mmap()".

:utf8

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 back in.

        open(F, ">:utf8", "data.utf");
        print F $out;
        close(F);
        open(F, "<:utf8", "data.utf");
        $in = <F>;
        close(F);
    

Note that this layer does not validate byte sequences. For reading input, using ":encoding(utf8)" instead of bare ":utf8" is strongly recommended.

:bytes

This is the inverse of the ":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 the 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.

In Perl 5.6 and some books the ":raw" layer (previously sometimes also referred to as a "discipline") is documented as the inverse of the ":crlf" 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 ":perlio" to the PERLIO environment variable.

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 popping 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

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:

    open($fh,...)
    ...
    binmode($fh,":encoding(...)");  # next chunk is encoded
    ...
    binmode($fh,":pop");            # back to un-encoded
    

A more elegant (and safer) interface is needed.

:win32

On Win32 platforms this experimental layer uses the native "handle" IO rather than the unix-like numeric file descriptor layer. Known to be buggy as of perl 5.8.2.

    open($fh,"whatever")
    binmode($fh);

    open($fh,"<:unix",$path)

  unix crlf

  unix stdio

  unix perlio

  cd .../perl/t
  PERLIO=stdio  ./perl harness
  PERLIO=perlio ./perl harness

   my @layers = PerlIO::get_layers($fh); # Or FH, *FH, "FH".

 PERLIO     UNIX-like                   DOS-like
 ------     ---------                   --------
 unset / "" unix perlio / stdio [1]     unix crlf
 stdio      unix perlio / stdio [1]     stdio
 perlio     unix perlio                 unix perlio
 mmap       unix mmap                   unix mmap
 # [1] "stdio" if Configure found out how to do "fast stdio" (depends
 # on the stdio implementation) and in Perl 5.8, otherwise "unix perlio"

   my @layers = PerlIO::get_layers($fh, output => 1);

   my @layer_and_args_and_flags = PerlIO::get_layers($fh, details => 1);

AUTHOR¶

Nick Ing-Simmons <nick@ing-simmons.net>

SEE ALSO¶

"binmode" in perlfunc, "open" in perlfunc, perlunicode, perliol, Encode