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PerlIO(3) |
Perl Programmers Reference Guide |
PerlIO(3) |
PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space
# 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 ....
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.
Generally speaking, PerlIO layers (previously sometimes referred to as
"disciplines") are an ordered stack applied to a filehandle
(specified as a space- or colon-separated list, conventionally written with a
leading colon). Each layer performs some operation on any input or output,
except when bypassed such as with "sysread"
or "syswrite". Read operations go through
the stack in the order they are set (left to right), and write operations in
the reverse order.
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:
- :unix
- Lowest level layer which provides basic PerlIO operations in terms of
UNIX/POSIX numeric file descriptor calls (open(), read(),
write(), lseek(), close()). It is used even on
non-Unix architectures, and most other layers operate on top of it.
- :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. This layer implements both low level IO and buffering, but is
rarely used on modern architectures.
- :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
will silently refuse to be pushed on top of itself.
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: $!";
- :utf8
- Pseudo-layer that declares that the stream accepts Perl's internal
upgraded encoding of characters, which is approximately UTF-8 on ASCII
machines, but UTF-EBCDIC on EBCDIC machines. This allows any character
Perl can represent to be read from or written to the stream.
This layer (which actually sets a flag on the preceding layer,
and is implicitly set by any
":encoding" layer) does not translate
or validate byte sequences. It instead indicates that the byte stream
will have been arranged by other layers to be provided in Perl's
internal upgraded encoding, which Perl code (and correctly written XS
code) will interpret as decoded Unicode characters.
CAUTION: Do not use this layer to translate from UTF-8
bytes, as invalid UTF-8 or binary data will result in malformed Perl
strings. It is unlikely to produce invalid UTF-8 when used for output,
though it will instead produce UTF-EBCDIC on EBCDIC systems. The
":encoding(UTF-8)" layer (hyphen is
significant) is preferred as it will ensure translation between valid
UTF-8 bytes and valid Unicode characters.
- :bytes
- This is the inverse of the ":utf8"
pseudo-layer. It turns off the flag on the layer below so that data read
from it is considered to be Perl's internal downgraded encoding, thus
interpreted as the native single-byte encoding of Latin-1 or EBCDIC.
Likewise on output Perl will warn if a "wide" character (a
codepoint not in the range 0..255) is written to a such a stream.
This is very dangerous to push on a handle using an
":encoding" layer, as such a layer
assumes to be working with Perl's internal upgraded encoding, so you
will likely get a mangled result. Instead use
":raw" or
":pop" to remove encoding layers.
- :raw
- The ":raw" pseudo-layer is
defined as being identical to calling
"binmode($fh)" - the stream is made
suitable for passing binary data, i.e. each byte is passed as-is. The
stream will still be buffered (but this was not always true before Perl
5.14).
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.
- :pop
- A pseudo-layer that removes the top-most layer. Gives Perl code a way to
manipulate the layer stack. Note that
":pop" only works on real layers and
will not undo the effects of pseudo-layers or flags like
":utf8". An example of a possible use
might be:
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.
- :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.
It is possible to write custom layers in addition to the above builtin ones,
both in C/XS and Perl, as a module named
"PerlIO::<layer name>". Some custom
layers come with the Perl distribution.
- :encoding
- Use ":encoding(ENCODING)" to
transparently do character set and encoding transformations, for example
from Shift-JIS to Unicode. Note that an
":encoding" also enables
":utf8". See PerlIO::encoding for more
information.
- :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()". See PerlIO::mmap for more
information.
- :via
- ":via(MODULE)" allows a transformation
to be applied by an arbitrary Perl module, for example compression /
decompression, encryption / decryption. See PerlIO::via for more
information.
- :scalar
- A layer implementing "in memory" files using scalar variables,
automatically used in place of the platform defaults for IO when opening
such a handle. As such, the scalar is expected to act like a file, only
containing or storing bytes. See PerlIO::scalar for more information.
To get a binary stream an alternate method is to use:
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: $!";
If the platform is MS-DOS like and normally does CRLF to "\n"
translation for text files then the default layers are:
: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
$ENV{PERLIO}:
PERLIO UNIX-like DOS-like
------ --------- --------
unset / "" :unix:perlio / :stdio [1] :unix:crlf
:stdio :stdio :stdio
:perlio :unix:perlio :unix:perlio
# [1] ":stdio" if Configure found out how to do "fast stdio" (depends
# on the stdio implementation) and in Perl 5.8, else ":unix:perlio"
The following returns the names of the PerlIO layers on a filehandle.
my @layers = PerlIO::get_layers($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.
You may open your eyes now.
Nick Ing-Simmons <nick@ing-simmons.net>
"binmode" in perlfunc, "open" in perlfunc, perlunicode,
perliol, Encode
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