phpman > man > PerlIO(3perl)

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

SYNOPSIS
      # 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 ....

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.

  Layers
    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.

  Custom Layers
    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.

  Alternatives to raw
    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: $!";

  Defaults and how to override them
    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"

  Querying the layers of filehandles
    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.

AUTHOR
    Nick Ing-Simmons <nick AT ing-simmons.net>

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