{
    "mode": "perldoc",
    "parameter": "PerlIO",
    "section": "",
    "url": "https://www.chedong.com/phpMan.php/perldoc/PerlIO/json",
    "generated": "2026-06-14T07:26:18Z",
    "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 ....",
    "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 then\nC 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 input or\noutput, except when bypassed such as with \"sysread\" or \"syswrite\". Read operations go through\nthe stack in the order they are set (left to right), and write operations in the reverse order.\n\nThere are also layers which actually just set flags on lower layers, or layers that modify the\ncurrent stack but don't persist on the stack themselves; these are referred to as pseudo-layers.\n\nWhen opening a handle, it will be opened with any layers specified explicitly in the open() call\n(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 by\nthe ${^OPEN} variable (usually set by using the open pragma for a lexical scope, or the \"-C\"\ncommand-line switch or \"PERLUNICODE\" environment variable for the main program scope).\n\nIf layers are not specified in the open() call or \"${^OPEN}\" variable, the handle will be opened\nwith the default layer stack configured for that architecture; see \"Defaults and how to override\nthem\".\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 remove or\nunset any existing layers which transform the byte stream, making the handle suitable for binary\ndata.\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 the C\nlibrary as usual. This layer implements both low level IO and buffering, but is rarely used\non 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 data\ncopying.\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 CR,LF\nto a single \"\\n\" newline character. On write converts each \"\\n\" to a CR,LF pair. Note that\nthis 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-file\nmarker.\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 only\nunset the CRLF translation flag. Since Perl 5.14, you can also apply another \":crlf\" layer\nlater, such as when the CRLF translation must occur after an encoding layer. On other\narchitectures, it is a mundane CRLF translation layer and can be added and 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: $!\";\n\n:utf8\nPseudo-layer that declares that the stream accepts Perl's *internal* upgraded encoding of\ncharacters, which is approximately UTF-8 on ASCII machines, but UTF-EBCDIC on EBCDIC\nmachines. This allows any character Perl can represent to be read from or written to the\nstream.\n\nThis layer (which actually sets a flag on the preceding layer, and is implicitly set by any\n\":encoding\" layer) does not translate or validate byte sequences. It instead indicates that\nthe byte stream will have been arranged by other layers to be provided in Perl's internal\nupgraded encoding, which Perl code (and correctly written XS code) will interpret as decoded\nUnicode characters.\n\nCAUTION: Do not use this layer to translate from UTF-8 bytes, as invalid UTF-8 or binary\ndata will result in malformed Perl strings. It is unlikely to produce invalid UTF-8 when\nused for output, though it will instead produce UTF-EBCDIC on EBCDIC systems. The\n\":encoding(UTF-8)\" layer (hyphen is significant) is preferred as it will ensure translation\nbetween valid UTF-8 bytes and valid Unicode characters.\n\n:bytes\nThis is the inverse of the \":utf8\" pseudo-layer. It turns off the flag on the layer below so\nthat data read from it is considered to be Perl's internal downgraded encoding, thus\ninterpreted as the native single-byte encoding of Latin-1 or EBCDIC. Likewise on output Perl\nwill warn if a \"wide\" character (a codepoint not in the range 0..255) is written to a such a\nstream.\n\nThis is very dangerous to push on a handle using an \":encoding\" layer, as such a layer\nassumes to be working with Perl's internal upgraded encoding, so you will likely get a\nmangled result. Instead use \":raw\" or \":pop\" to remove encoding layers.\n\n:raw\nThe \":raw\" pseudo-layer is *defined* as being identical to calling \"binmode($fh)\" - the\nstream is made suitable for passing binary data, i.e. each byte is passed as-is. The stream\nwill 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 the\nstream are also disabled. If you want UNIX line endings on a platform that normally does\nCRLF translation, but still want UTF-8 or encoding defaults, the appropriate thing to do is\nto add \":perlio\" to the PERLIO environment variable, or open the handle explicitly with that\nlayer, to replace the platform default of \":crlf\".\n\nThe implementation of \":raw\" is as a pseudo-layer which when \"pushed\" pops itself and then\nany layers which would modify the binary data stream. (Undoing \":utf8\" and \":crlf\" may be\nimplemented by clearing flags rather than popping layers but that is an implementation\ndetail.)\n\nAs a consequence of the fact that \":raw\" normally pops layers, it usually only makes sense\nto have it as the only or first element in a layer specification. When used as the first\nelement 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 UTF-8\ntranslation.\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\n:win32\nOn Win32 platforms this *experimental* layer uses the native \"handle\" IO rather than the\nunix-like numeric file descriptor layer. Known to be buggy as of Perl 5.8.2.\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\". See\nPerlIO::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 use\nwhen 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 PerlIO::mmap\nfor 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 place\nof the platform defaults for IO when opening such a handle. As such, the scalar is expected\nto act like a file, only containing or storing bytes. See PerlIO::scalar for more\ninformation.\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 use\nPerlIO 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 on\nmodern 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 both\nthe compile-time and runtime configurations of Perl. The default can be overridden by setting\nthe environment variable PERLIO to a space or colon separated list of layers, however this\ncannot 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($fh, output => 1);\n\n(Usually the layers are identical on either side of a filehandle but for example with sockets\nthere may be differences.)\n\nThere is no setlayers(), nor does getlayers() return a tied array mirroring the stack, or\nanything fancy like that. This is not accidental or unintentional. The PerlIO layer stack is a\nbit more complicated than just a stack (see for example the behaviour of \":raw\"). You are\nsupposed to use open() and binmode() to manipulate the stack.\n\nImplementation details follow, please close your eyes.\n\nThe arguments to layers are by default returned in parentheses after the name of the layer, and\ncertain layers (like \":utf8\") are not real layers but instead flags on real layers; to get all\nof these returned separately, use the optional \"details\" argument:\n\nmy @layerandargsandflags = PerlIO::getlayers($fh, details => 1);\n\nThe result will be up to be three times the number of layers: the first element will be a name,\nthe second element the arguments (unspecified arguments will be \"undef\"), the third element the\nflags, the fourth element a name again, and so forth.\n\nYou may open your eyes now.\n"
                }
            ]
        },
        "AUTHOR": {
            "content": "Nick Ing-Simmons <nick@ing-simmons.net>\n",
            "subsections": []
        },
        "SEE ALSO": {
            "content": "\"binmode\" in perlfunc, \"open\" in perlfunc, perlunicode, perliol, Encode\n",
            "subsections": []
        }
    },
    "summary": "PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space",
    "flags": [],
    "examples": [],
    "see_also": []
}