{
    "content": [
        {
            "type": "text",
            "text": "# PerlIO (man)\n\n## NAME\n\nPerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space\n\n## SYNOPSIS\n\n# support platform-native and CRLF text files\nopen(my $fh, \"<:crlf\", \"my.txt\") or die \"open failed: $!\";\n# append UTF-8 encoded text\nopen(my $fh, \">>:encoding(UTF-8)\", \"some.log\")\nor die \"open failed: $!\";\n# portably open a binary file for reading\nopen(my $fh, \"<\", \"his.jpg\") or die \"open failed: $!\";\nbinmode($fh) or die \"binmode failed: $!\";\nShell:\nPERLIO=:perlio perl ....\n\n## DESCRIPTION\n\nWhen an undefined layer 'foo' is encountered in an \"open\" or \"binmode\" layer specification\nthen C code performs the equivalent of:\n\n## Sections\n\n- **NAME**\n- **SYNOPSIS**\n- **DESCRIPTION** (7 subsections)\n- **AUTHOR**\n- **SEE ALSO**\n\nUse structuredContent.sections for detailed options, examples, and full documentation.\n"
        }
    ],
    "structuredContent": {
        "command": "PerlIO",
        "section": "",
        "mode": "man",
        "summary": "PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space",
        "synopsis": "# support platform-native and CRLF text files\nopen(my $fh, \"<:crlf\", \"my.txt\") or die \"open failed: $!\";\n# append UTF-8 encoded text\nopen(my $fh, \">>:encoding(UTF-8)\", \"some.log\")\nor die \"open failed: $!\";\n# portably open a binary file for reading\nopen(my $fh, \"<\", \"his.jpg\") or die \"open failed: $!\";\nbinmode($fh) or die \"binmode failed: $!\";\nShell:\nPERLIO=:perlio perl ....",
        "tldr_summary": null,
        "tldr_examples": [],
        "tldr_source": null,
        "flags": [],
        "examples": [],
        "see_also": [],
        "section_outline": [
            {
                "name": "NAME",
                "lines": 2,
                "subsections": []
            },
            {
                "name": "SYNOPSIS",
                "lines": 14,
                "subsections": []
            },
            {
                "name": "DESCRIPTION",
                "lines": 11,
                "subsections": [
                    {
                        "name": "Layers",
                        "lines": 141
                    },
                    {
                        "name": "Custom Layers",
                        "lines": 33
                    },
                    {
                        "name": "Alternatives to raw",
                        "lines": 12
                    },
                    {
                        "name": "Defaults and how to override them",
                        "lines": 39
                    },
                    {
                        "name": "Querying the layers of filehandles",
                        "lines": 20
                    },
                    {
                        "name": "Implementation details follow, please close your eyes.",
                        "lines": 10
                    },
                    {
                        "name": "You may open your eyes now.",
                        "lines": 1
                    }
                ]
            },
            {
                "name": "AUTHOR",
                "lines": 2,
                "subsections": []
            },
            {
                "name": "SEE ALSO",
                "lines": 3,
                "subsections": []
            }
        ],
        "sections": {
            "NAME": {
                "content": "PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space\n",
                "subsections": []
            },
            "SYNOPSIS": {
                "content": "# support platform-native and CRLF text files\nopen(my $fh, \"<:crlf\", \"my.txt\") or die \"open failed: $!\";\n\n# append UTF-8 encoded text\nopen(my $fh, \">>:encoding(UTF-8)\", \"some.log\")\nor die \"open failed: $!\";\n\n# portably open a binary file for reading\nopen(my $fh, \"<\", \"his.jpg\") or die \"open failed: $!\";\nbinmode($fh) or die \"binmode failed: $!\";\n\nShell:\nPERLIO=:perlio perl ....\n",
                "subsections": []
            },
            "DESCRIPTION": {
                "content": "When an undefined layer 'foo' is encountered in an \"open\" or \"binmode\" layer specification\nthen C code performs the equivalent of:\n\nuse PerlIO 'foo';\n\nThe Perl code in PerlIO.pm then attempts to locate a layer by doing\n\nrequire PerlIO::foo;\n\nOtherwise the \"PerlIO\" package is a place holder for additional PerlIO related functions.\n",
                "subsections": [
                    {
                        "name": "Layers",
                        "content": "Generally speaking, PerlIO layers (previously sometimes referred to as \"disciplines\") are an\nordered stack applied to a filehandle (specified as a space- or colon-separated list,\nconventionally written with a leading colon).  Each layer performs some operation on any\ninput or output, except when bypassed such as with \"sysread\" or \"syswrite\".  Read operations\ngo through the stack in the order they are set (left to right), and write operations in the\nreverse order.\n\nThere are also layers which actually just set flags on lower layers, or layers that modify\nthe current stack but don't persist on the stack themselves; these are referred to as pseudo-\nlayers.\n\nWhen opening a handle, it will be opened with any layers specified explicitly in the open()\ncall (or the platform defaults, if specified as a colon with no following layers).\n\nIf layers are not explicitly specified, the handle will be opened with the layers specified\nby the ${^OPEN} variable (usually set by using the open pragma for a lexical scope, or the\n\"-C\" command-line switch or \"PERLUNICODE\" environment variable for the main program scope).\n\nIf layers are not specified in the open() call or \"${^OPEN}\" variable, the handle will be\nopened with the default layer stack configured for that architecture; see \"Defaults and how\nto override them\".\n\nSome layers will automatically insert required lower level layers if not present; for example\n\":perlio\" will insert \":unix\" below itself for low level IO, and \":encoding\" will insert the\nplatform defaults for buffered IO.\n\nThe \"binmode\" function can be called on an opened handle to push additional layers onto the\nstack, which may also modify the existing layers.  \"binmode\" called with no layers will\nremove or unset any existing layers which transform the byte stream, making the handle\nsuitable for binary data.\n\nThe following layers are currently defined:\n\n:unix\nLowest  level layer which provides basic PerlIO operations in terms of UNIX/POSIX numeric\nfile descriptor calls (open(), read(), write(), lseek(), close()).  It is  used  even  on\nnon-Unix architectures, and most other layers operate on top of it.\n\n:stdio\nLayer which calls \"fread\", \"fwrite\" and \"fseek\"/\"ftell\" etc.  Note that as this is \"real\"\nstdio  it  will  ignore any layers beneath it and go straight to the operating system via\nthe C library as usual.  This layer implements both low level IO and  buffering,  but  is\nrarely used on modern architectures.\n\n:perlio\nA from scratch implementation of buffering for PerlIO. Provides fast access to the buffer\nfor  \"svgets\"  which  implements  Perl's readline/<> and in general attempts to minimize\ndata copying.\n\n\":perlio\" will insert a \":unix\" layer below itself to do low level IO.\n\n:crlf\nA layer that implements DOS/Windows like CRLF line endings.  On read  converts  pairs  of\nCR,LF  to  a single \"\\n\" newline character.  On write converts each \"\\n\" to a CR,LF pair.\nNote that this layer will silently refuse to be pushed on top of itself.\n\nIt currently does not mimic MS-DOS as far as treating of Control-Z as  being  an  end-of-\nfile marker.\n\nOn  DOS/Windows like architectures where this layer is part of the defaults, it also acts\nlike the \":perlio\" layer, and removing the CRLF translation (such as  with  \":raw\")  will\nonly  unset  the  CRLF  translation  flag.   Since  Perl 5.14, you can also apply another\n\":crlf\" layer later, such as when the CRLF  translation  must  occur  after  an  encoding\nlayer.   On  other architectures, it is a mundane CRLF translation layer and can be added\nand removed normally.\n\n# translate CRLF after encoding on Perl 5.14 or newer\nbinmode $fh, \":raw:encoding(UTF-16LE):crlf\"\nor die \"binmode failed: $!\";\n\n:utf8\nPseudo-layer that declares that the stream accepts Perl's internal upgraded  encoding  of\ncharacters,  which  is  approximately  UTF-8  on ASCII machines, but UTF-EBCDIC on EBCDIC\nmachines.  This allows any character Perl can represent to be read from or written to the\nstream.\n\nThis layer (which actually sets a flag on the preceding layer, and is implicitly  set  by\nany  \":encoding\"  layer)  does  not  translate  or  validate  byte sequences.  It instead\nindicates that the byte stream will have been arranged by other layers to be provided  in\nPerl's  internal  upgraded encoding, which Perl code (and correctly written XS code) will\ninterpret as decoded Unicode characters.\n\nCAUTION: Do not use this layer to translate from UTF-8 bytes, as invalid UTF-8 or  binary\ndata will result in malformed Perl strings.  It is unlikely to produce invalid UTF-8 when\nused  for  output,  though  it  will  instead  produce UTF-EBCDIC on EBCDIC systems.  The\n:encoding(UTF-8) layer (hyphen is significant) is preferred as it will ensure translation\nbetween valid UTF-8 bytes and valid Unicode characters.\n\n:bytes\nThis is the inverse of the \":utf8\" pseudo-layer.  It turns off  the  flag  on  the  layer\nbelow  so that data read from it is considered to be Perl's internal downgraded encoding,\nthus interpreted as the native single-byte encoding of Latin-1 or  EBCDIC.   Likewise  on\noutput  Perl  will  warn  if  a \"wide\" character (a codepoint not in the range 0..255) is\nwritten to a such a stream.\n\nThis is very dangerous to push on a handle using an \":encoding\" layer, as  such  a  layer\nassumes  to  be  working with Perl's internal upgraded encoding, so you will likely get a\nmangled result.  Instead use \":raw\" or \":pop\" to remove encoding layers.\n\n:raw\nThe \":raw\" pseudo-layer is defined as being  identical  to  calling  binmode($fh)  -  the\nstream  is  made  suitable  for  passing binary data, i.e. each byte is passed as-is. The\nstream will still be buffered (but this was not always true before Perl 5.14).\n\nIn Perl 5.6 and some books the \":raw\" layer is documented as the inverse of  the  \":crlf\"\nlayer.  That  is no longer the case - other layers which would alter the binary nature of\nthe stream are also disabled.  If you want UNIX line endings on a platform that  normally\ndoes  CRLF  translation, but still want UTF-8 or encoding defaults, the appropriate thing\nto do is to add \":perlio\"  to  the  PERLIO  environment  variable,  or  open  the  handle\nexplicitly with that layer, to replace the platform default of \":crlf\".\n\nThe  implementation  of  \":raw\"  is as a pseudo-layer which when \"pushed\" pops itself and\nthen any layers which would modify the binary data stream.  (Undoing \":utf8\" and  \":crlf\"\nmay  be  implemented  by  clearing  flags  rather  than  popping  layers  but  that is an\nimplementation detail.)\n\nAs a consequence of the fact that \":raw\" normally pops  layers,  it  usually  only  makes\nsense to have it as the only or first element in a layer specification.  When used as the\nfirst element it provides a known base on which to build e.g.\n\nopen(my $fh,\">:raw:encoding(UTF-8)\",...)\nor die \"open failed: $!\";\n\nwill  construct  a  \"binary\"  stream regardless of the platform defaults, but then enable\nUTF-8 translation.\n\n:pop\nA pseudo-layer that removes the top-most layer. Gives Perl code a way to  manipulate  the\nlayer stack.  Note that \":pop\" only works on real layers and will not undo the effects of\npseudo-layers or flags like \":utf8\".  An example of a possible use might be:\n\nopen(my $fh,...) or die \"open failed: $!\";\n...\nbinmode($fh,\":encoding(...)\") or die \"binmode failed: $!\";\n# next chunk is encoded\n...\nbinmode($fh,\":pop\") or die \"binmode failed: $!\";\n# back to un-encoded\n\nA more elegant (and safer) interface is needed.\n"
                    },
                    {
                        "name": "Custom Layers",
                        "content": "It is possible to write custom layers in addition to the above builtin ones, both in C/XS and\nPerl,  as  a  module  named  \"PerlIO::<layer  name>\".   Some custom layers come with the Perl\ndistribution.\n\n:encoding\nUse :encoding(ENCODING) to transparently do character set and  encoding  transformations,\nfor  example  from  Shift-JIS to Unicode.  Note that an \":encoding\" also enables \":utf8\".\nSee PerlIO::encoding for more information.\n\n:mmap\nA layer which implements \"reading\" of files by using mmap() to make a (whole) file appear\nin the process's address space, and then using that as PerlIO's  \"buffer\".  This  may  be\nfaster  in  certain circumstances for large files, and may result in less physical memory\nuse when multiple processes are reading the same file.\n\nFiles which are not mmap()-able revert to behaving like the \":perlio\" layer. Writes  also\nbehave like the \":perlio\" layer, as mmap() for write needs extra house-keeping (to extend\nthe file) which negates any advantage.\n\nThe  \":mmap\"  layer  will  not  exist  if  the  platform  does  not  support mmap().  See\nPerlIO::mmap for more information.\n\n:via\n:via(MODULE) allows a transformation to be applied  by  an  arbitrary  Perl  module,  for\nexample  compression  / decompression, encryption / decryption.  See PerlIO::via for more\ninformation.\n\n:scalar\nA layer implementing \"in memory\" files using  scalar  variables,  automatically  used  in\nplace of the platform defaults for IO when opening such a handle.  As such, the scalar is\nexpected  to  act  like a file, only containing or storing bytes.  See PerlIO::scalar for\nmore information.\n"
                    },
                    {
                        "name": "Alternatives to raw",
                        "content": "To get a binary stream an alternate method is to use:\n\nopen(my $fh,\"<\",\"whatever\") or die \"open failed: $!\";\nbinmode($fh) or die \"binmode failed: $!\";\n\nThis has the advantage of being backward compatible with older versions of Perl that did  not\nuse PerlIO or where \":raw\" was buggy (as it was before Perl 5.14).\n\nTo get an unbuffered stream specify an unbuffered layer (e.g. \":unix\") in the open call:\n\nopen(my $fh,\"<:unix\",$path) or die \"open failed: $!\";\n"
                    },
                    {
                        "name": "Defaults and how to override them",
                        "content": "If the platform is MS-DOS like and normally does CRLF to \"\\n\" translation for text files then\nthe default layers are:\n\n:unix:crlf\n\nOtherwise  if  \"Configure\" found out how to do \"fast\" IO using the system's stdio (not common\non modern architectures), then the default layers are:\n\n:stdio\n\nOtherwise the default layers are\n\n:unix:perlio\n\nNote that the \"default stack\" depends on the operating system and on the  Perl  version,  and\nboth  the  compile-time and runtime configurations of Perl.  The default can be overridden by\nsetting the environment variable PERLIO to a space or colon separated list of layers, however\nthis cannot be used to set layers that require loading modules like \":encoding\".\n\nThis can be used to see the effect of/bugs in the various layers e.g.\n\ncd .../perl/t\nPERLIO=:stdio  ./perl harness\nPERLIO=:perlio ./perl harness\n\nFor the various values of PERLIO see \"PERLIO\" in perlrun.\n\nThe following table summarizes the default layers on UNIX-like  and  DOS-like  platforms  and\ndepending on the setting of $ENV{PERLIO}:\n\nPERLIO     UNIX-like                   DOS-like\n------     ---------                   --------\nunset / \"\" :unix:perlio / :stdio [1]   :unix:crlf\n:stdio     :stdio                      :stdio\n:perlio    :unix:perlio                :unix:perlio\n\n# [1] \":stdio\" if Configure found out how to do \"fast stdio\" (depends\n# on the stdio implementation) and in Perl 5.8, else \":unix:perlio\"\n"
                    },
                    {
                        "name": "Querying the layers of filehandles",
                        "content": "The following returns the names of the PerlIO layers on a filehandle.\n\nmy @layers = PerlIO::getlayers($fh); # Or FH, *FH, \"FH\".\n\nThe  layers are returned in the order an open() or binmode() call would use them, and without\ncolons.\n\nBy default the layers from the input side of the filehandle are returned; to get  the  output\nside, use the optional \"output\" argument:\n\nmy @layers = PerlIO::getlayers($fh, output => 1);\n\n(Usually the layers are identical on either side of a filehandle but for example with sockets\nthere may be differences.)\n\nThere  is  no setlayers(), nor does getlayers() return a tied array mirroring the stack, or\nanything fancy like that.  This is not accidental or unintentional.  The PerlIO  layer  stack\nis  a  bit more complicated than just a stack (see for example the behaviour of \":raw\").  You\nare supposed to use open() and binmode() to manipulate the stack.\n"
                    },
                    {
                        "name": "Implementation details follow, please close your eyes.",
                        "content": "The arguments to layers are by default returned in parentheses after the name of  the  layer,\nand  certain  layers  (like \":utf8\") are not real layers but instead flags on real layers; to\nget all of these returned separately, use the optional \"details\" argument:\n\nmy @layerandargsandflags = PerlIO::getlayers($fh, details => 1);\n\nThe result will be up to be three times the number of layers: the first  element  will  be  a\nname,  the  second  element  the arguments (unspecified arguments will be \"undef\"), the third\nelement the flags, the fourth element a name again, and so forth.\n"
                    },
                    {
                        "name": "You may open your eyes now.",
                        "content": ""
                    }
                ]
            },
            "AUTHOR": {
                "content": "Nick Ing-Simmons <nick@ing-simmons.net>\n",
                "subsections": []
            },
            "SEE ALSO": {
                "content": "\"binmode\" in perlfunc, \"open\" in perlfunc, perlunicode, perliol, Encode\n\nperl v5.38.2                                 2026-06-12                                PerlIO(3perl)",
                "subsections": []
            }
        }
    }
}