PerlIO(3p) Perl Programmers Reference Guide PerlIO(3p) #

PerlIO(3p) Perl Programmers Reference Guide PerlIO(3p)

NNAAMMEE #

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

SSYYNNOOPPSSIISS #

   # 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 ....

DDEESSCCRRIIPPTTIIOONN #

 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.

LLaayyeerrss 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 ooppeenn(()) 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 ooppeenn(()) 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 (ooppeenn(()), rreeaadd(()), wwrriittee(()),
     llsseeeekk(()), cclloossee(())).  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 _n_o_t 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 _i_n_t_e_r_n_a_l
     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.

     CCAAUUTTIIOONN: 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 _d_e_f_i_n_e_d 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.

CCuussttoomm LLaayyeerrss 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::”. 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 _m_a_y 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.

AAlltteerrnnaattiivveess ttoo rraaww 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: $!";

DDeeffaauullttss aanndd hhooww ttoo oovveerrrriiddee tthheemm 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"

QQuueerryyiinngg tthhee llaayyeerrss ooff ffiilleehhaannddlleess The following returns the nnaammeess 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 ooppeenn(()) or bbiinnmmooddee(()) 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 sseett__llaayyeerrss(()), nor does ggeett__llaayyeerrss(()) 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 ooppeenn(()) and bbiinnmmooddee(()) to manipulate the stack.

 IImmpplleemmeennttaattiioonn ddeettaaiillss ffoollllooww,, pplleeaassee cclloossee yyoouurr eeyyeess..

 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.

 YYoouu mmaayy ooppeenn yyoouurr eeyyeess nnooww..

AAUUTTHHOORR #

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

SSEEEE AALLSSOO #

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

perl v5.36.3 2023-02-15 PerlIO(3p)