# support platform-native and CRLF text files
open(my $fh, "<:crlf", "my.txt") or die "open failed: $!";
# append UTF-8 encoded text
open(my $fh, ">>:encoding(UTF-8)", "some.log")
or die "open failed: $!";
# portably open a binary file for reading
open(my $fh, "<", "his.jpg") or die "open failed: $!";
binmode($fh) or die "binmode failed: $!";
Shell:
PERLIO=:perlio perl ....
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.
There are also layers which actually just set flags on lower layers, or layers that modify the current stack but don't persist on the stack themselves; these are referred to as pseudo-layers.
When opening a handle, it will be opened with any layers specified explicitly in the open() call (or the platform defaults, if specified as a colon with no following layers).
If layers are not explicitly specified, the handle will be opened with the layers specified by the ${^OPEN} variable (usually set by using the open pragma for a lexical scope, or the "-C" command-line switch or "PERL_UNICODE" environment variable for the main program scope).
If layers are not specified in the open() call or "${^OPEN}" variable, the handle will be opened with the default layer stack configured for that architecture; see ``Defaults and how to override them''.
Some layers will automatically insert required lower level layers if not present; for example ":perlio" will insert ":unix" below itself for low level IO, and ":encoding" will insert the platform defaults for buffered IO.
The "binmode" function can be called on an opened handle to push additional layers onto the stack, which may also modify the existing layers. "binmode" called with no layers will remove or unset any existing layers which transform the byte stream, making the handle suitable for binary data.
The following layers are currently defined:
":perlio" will insert a ":unix" layer below itself to do low level IO.
It currently does not mimic MS-DOS as far as treating of Control-Z as being an end-of-file marker.
On DOS/Windows like architectures where this layer is part of the defaults, it also acts like the ":perlio" layer, and removing the CRLF translation (such as with ":raw") will only unset the CRLF translation flag. Since Perl 5.14, you can also apply another ":crlf" layer later, such as when the CRLF translation must occur after an encoding layer. On other architectures, it is a mundane CRLF translation layer and can be added and removed normally.
# translate CRLF after encoding on Perl 5.14 or newer
binmode $fh, ":raw:encoding(UTF-16LE):crlf"
or die "binmode failed: $!";
This layer (which actually sets a flag on the preceding layer, and is implicitly set by any ":encoding" layer) does not translate or validate byte sequences. It instead indicates that the byte stream will have been arranged by other layers to be provided in Perl's internal upgraded encoding, which Perl code (and correctly written XS code) will interpret as decoded Unicode characters.
CAUTION: Do not use this layer to translate from UTF-8 bytes, as invalid UTF-8 or binary data will result in malformed Perl strings. It is unlikely to produce invalid UTF-8 when used for output, though it will instead produce UTF-EBCDIC on EBCDIC systems. The ":encoding(UTF-8)" layer (hyphen is significant) is preferred as it will ensure translation between valid UTF-8 bytes and valid Unicode characters.
This is very dangerous to push on a handle using an ":encoding" layer, as such a layer assumes to be working with Perl's internal upgraded encoding, so you will likely get a mangled result. Instead use ":raw" or ":pop" to remove encoding layers.
In Perl 5.6 and some books the ":raw" layer 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, or open the handle explicitly with that layer, to replace the platform default of ":crlf".
The implementation of ":raw" is as a pseudo-layer which when ``pushed'' pops itself and then any layers which would modify the binary data stream. (Undoing ":utf8" and ":crlf" may be 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(my $fh,">:raw:encoding(UTF-8)",...)
or die "open failed: $!";
will construct a ``binary'' stream regardless of the platform defaults, but then enable UTF-8 translation.
open(my $fh,...) or die "open failed: $!";
...
binmode($fh,":encoding(...)") or die "binmode failed: $!";
# next chunk is encoded
...
binmode($fh,":pop") or die "binmode failed: $!";
# back to un-encoded
A more elegant (and safer) interface is needed.
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()". See PerlIO::mmap for more information.
open(my $fh,"<","whatever") or die "open failed: $!";
binmode($fh) or die "binmode failed: $!";
This has the advantage of being backward compatible with older versions of Perl that did not use PerlIO or where ":raw" was buggy (as it was before Perl 5.14).
To get an unbuffered stream specify an unbuffered layer (e.g. ":unix") in the open call:
open(my $fh,"<:unix",$path) or die "open failed: $!";
:unix:crlf
Otherwise if "Configure" found out how to do ``fast'' IO using the system's stdio (not common on modern architectures), then the default layers are:
:stdio
Otherwise the default layers are
:unix:perlio
Note that the ``default stack'' depends on the operating system and on the Perl version, and both the compile-time and runtime configurations of Perl. The default can be overridden by setting the environment variable PERLIO to a space or colon separated list of layers, however this cannot be used to set layers that require loading modules like ":encoding".
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
For the various values of PERLIO see ``PERLIO'' in perlrun.
The following table summarizes the default layers on UNIX-like and DOS-like platforms and depending on the setting of $ENV{PERLIO}:
PERLIO UNIX-like DOS-like ------ --------- -------- unset / "" :unix:perlio / :stdio [1] :unix:crlf :stdio :stdio :stdio :perlio :unix:perlio :unix:perlio # [1] ":stdio" if Configure found out how to do "fast stdio" (depends # on the stdio implementation) and in Perl 5.8, else ":unix:perlio"
my @layers = PerlIO::get_layers($fh); # Or FH, *FH, "FH".
The layers are returned in the order an open() or binmode() call would use them, and without colons.
By default the layers from the input side of the filehandle are returned; to get the output side, use the optional "output" argument:
my @layers = PerlIO::get_layers($fh, output => 1);
(Usually the layers are identical on either side of a filehandle but for example with sockets there may be differences.)
There is no set_layers(), nor does get_layers() return a tied array mirroring the stack, or anything fancy like that. This is not accidental or unintentional. The PerlIO layer stack is a bit more complicated than just a stack (see for example the behaviour of ":raw"). You are supposed to use open() and binmode() to manipulate the stack.
Implementation details follow, please close your eyes.
The arguments to layers are by default returned in parentheses after the name of the layer, and certain layers (like ":utf8") are not real layers but instead flags on real layers; to get all of these returned separately, use the optional "details" argument:
my @layer_and_args_and_flags = PerlIO::get_layers($fh, details => 1);
The result will be up to be three times the number of layers: the first element will be a name, the second element the arguments (unspecified arguments will be "undef"), the third element the flags, the fourth element a name again, and so forth.