{
    "mode": "man",
    "parameter": "PERLIOL",
    "section": "1",
    "url": "https://www.chedong.com/phpMan.php/man/PERLIOL/1/json",
    "generated": "2026-06-14T11:11:01Z",
    "synopsis": "/* Defining a layer ... */\n#include <perliol.h>",
    "sections": {
        "NAME": {
            "content": "perliol - C API for Perl's implementation of IO in Layers.\n",
            "subsections": []
        },
        "SYNOPSIS": {
            "content": "/* Defining a layer ... */\n#include <perliol.h>\n",
            "subsections": []
        },
        "DESCRIPTION": {
            "content": "This document describes the behavior and implementation of the PerlIO abstraction described\nin perlapio when \"USEPERLIO\" is defined.\n",
            "subsections": [
                {
                    "name": "History and Background",
                    "content": "The PerlIO abstraction was introduced in perl5.00302 but languished as just an abstraction\nuntil perl5.7.0. However during that time a number of perl extensions switched to using it,\nso the API is mostly fixed to maintain (source) compatibility.\n\nThe aim of the implementation is to provide the PerlIO API in a flexible and platform neutral\nmanner. It is also a trial of an \"Object Oriented C, with vtables\" approach which may be\napplied to Raku.\n"
                },
                {
                    "name": "Basic Structure",
                    "content": "PerlIO is a stack of layers.\n\nThe low levels of the stack work with the low-level operating system calls (file descriptors\nin C) getting bytes in and out, the higher layers of the stack buffer, filter, and otherwise\nmanipulate the I/O, and return characters (or bytes) to Perl.  Terms above and below are used\nto refer to the relative positioning of the stack layers.\n\nA layer contains a \"vtable\", the table of I/O operations (at C level a table of function\npointers), and status flags.  The functions in the vtable implement operations like \"open\",\n\"read\", and \"write\".\n\nWhen I/O, for example \"read\", is requested, the request goes from Perl first down the stack\nusing \"read\" functions of each layer, then at the bottom the input is requested from the\noperating system services, then the result is returned up the stack, finally being\ninterpreted as Perl data.\n\nThe requests do not necessarily go always all the way down to the operating system: that's\nwhere PerlIO buffering comes into play.\n\nWhen you do an open() and specify extra PerlIO layers to be deployed, the layers you specify\nare \"pushed\" on top of the already existing default stack.  One way to see it is that\n\"operating system is on the left\" and \"Perl is on the right\".\n\nWhat exact layers are in this default stack depends on a lot of things: your operating\nsystem, Perl version, Perl compile time configuration, and Perl runtime configuration.  See\nPerlIO, \"PERLIO\" in perlrun, and open for more information.\n\nbinmode() operates similarly to open(): by default the specified layers are pushed on top of\nthe existing stack.\n\nHowever, note that even as the specified layers are \"pushed on top\" for open() and binmode(),\nthis doesn't mean that the effects are limited to the \"top\": PerlIO layers can be very\n'active' and inspect and affect layers also deeper in the stack.  As an example there is a\nlayer called \"raw\" which repeatedly \"pops\" layers until it reaches the first layer that has\ndeclared itself capable of handling binary data.  The \"pushed\" layers are processed in left-\nto-right order.\n\nsysopen() operates (unsurprisingly) at a lower level in the stack than open().  For example\nin Unix or Unix-like systems sysopen() operates directly at the level of file descriptors: in\nthe terms of PerlIO layers, it uses only the \"unix\" layer, which is a rather thin wrapper on\ntop of the Unix file descriptors.\n"
                },
                {
                    "name": "Layers vs Disciplines",
                    "content": "Initial discussion of the ability to modify IO streams behaviour used the term \"discipline\"\nfor the entities which were added. This came (I believe) from the use of the term in \"sfio\",\nwhich in turn borrowed it from \"line disciplines\" on Unix terminals. However, this document\n(and the C code) uses the term \"layer\".\n\nThis is, I hope, a natural term given the implementation, and should avoid connotations that\nare inherent in earlier uses of \"discipline\" for things which are rather different.\n"
                },
                {
                    "name": "Data Structures",
                    "content": "The basic data structure is a PerlIOl:\n\ntypedef struct PerlIO PerlIOl;\ntypedef struct PerlIOfuncs PerlIOfuncs;\ntypedef PerlIOl *PerlIO;\n\nstruct PerlIO\n{\nPerlIOl *      next;       /* Lower layer */\nPerlIOfuncs * tab;        /* Functions for this layer */\nU32            flags;      /* Various flags for state */\n};\n\nA \"PerlIOl *\" is a pointer to the struct, and the application level \"PerlIO *\" is a pointer\nto a \"PerlIOl *\" - i.e. a pointer to a pointer to the struct. This allows the application\nlevel \"PerlIO *\" to remain constant while the actual \"PerlIOl *\" underneath changes. (Compare\nperl's \"SV *\" which remains constant while its \"svany\" field changes as the scalar's type\nchanges.) An IO stream is then in general represented as a pointer to this linked-list of\n\"layers\".\n\nIt should be noted that because of the double indirection in a \"PerlIO *\", a\n\"&(perlio->next)\" \"is\" a \"PerlIO *\", and so to some degree at least one layer can use the\n\"standard\" API on the next layer down.\n\nA \"layer\" is composed of two parts:\n\n1.  The functions and attributes of the \"layer class\".\n\n2.  The per-instance data for a particular handle.\n"
                },
                {
                    "name": "Functions and Attributes",
                    "content": "The functions and attributes are accessed via the \"tab\" (for table) member of \"PerlIOl\". The\nfunctions (methods of the layer \"class\") are fixed, and are defined by the \"PerlIOfuncs\"\ntype. They are broadly the same as the public \"PerlIOxxxxx\" functions:\n\nstruct PerlIOfuncs\n{\nSizet     fsize;\nchar *     name;\nSizet     size;\nIV         kind;\nIV         (*Pushed)(pTHX PerlIO *f,\nconst char *mode,\nSV *arg,\nPerlIOfuncs *tab);\nIV         (*Popped)(pTHX PerlIO *f);\nPerlIO *   (*Open)(pTHX PerlIOfuncs *tab,\nPerlIOlistt *layers, IV n,\nconst char *mode,\nint fd, int imode, int perm,\nPerlIO *old,\nint narg, SV args);\nIV         (*Binmode)(pTHX PerlIO *f);\nSV *       (*Getarg)(pTHX PerlIO *f, CLONEPARAMS *param, int flags)\nIV         (*Fileno)(pTHX PerlIO *f);\nPerlIO *   (*Dup)(pTHX PerlIO *f,\nPerlIO *o,\nCLONEPARAMS *param,\nint flags)\n/* Unix-like functions - cf sfio line disciplines */\nSSizet    (*Read)(pTHX PerlIO *f, void *vbuf, Sizet count);\nSSizet    (*Unread)(pTHX PerlIO *f, const void *vbuf, Sizet count);\nSSizet    (*Write)(pTHX PerlIO *f, const void *vbuf, Sizet count);\nIV         (*Seek)(pTHX PerlIO *f, Offt offset, int whence);\nOfft      (*Tell)(pTHX PerlIO *f);\nIV         (*Close)(pTHX PerlIO *f);\n/* Stdio-like buffered IO functions */\nIV         (*Flush)(pTHX PerlIO *f);\nIV         (*Fill)(pTHX PerlIO *f);\nIV         (*Eof)(pTHX PerlIO *f);\nIV         (*Error)(pTHX PerlIO *f);\nvoid       (*Clearerr)(pTHX PerlIO *f);\nvoid       (*Setlinebuf)(pTHX PerlIO *f);\n/* Perl's snooping functions */\nSTDCHAR *  (*Getbase)(pTHX PerlIO *f);\nSizet     (*Getbufsiz)(pTHX PerlIO *f);\nSTDCHAR *  (*Getptr)(pTHX PerlIO *f);\nSSizet    (*Getcnt)(pTHX PerlIO *f);\nvoid       (*Setptrcnt)(pTHX PerlIO *f,STDCHAR *ptr,SSizet cnt);\n};\n\nThe first few members of the struct give a function table size for compatibility check \"name\"\nfor the layer, the  size to \"malloc\" for the per-instance data, and some flags which are\nattributes of the class as whole (such as whether it is a buffering layer), then follow the\nfunctions which fall into four basic groups:\n\n1.  Opening and setup functions\n\n2.  Basic IO operations\n\n3.  Stdio class buffering options.\n\n4.  Functions to support Perl's traditional \"fast\" access to the buffer.\n\nA layer does not have to implement all the functions, but the whole table has to be present.\nUnimplemented slots can be NULL (which will result in an error when called) or can be filled\nin with stubs to \"inherit\" behaviour from a \"base class\". This \"inheritance\" is fixed for all\ninstances of the layer, but as the layer chooses which stubs to populate the table, limited\n\"multiple inheritance\" is possible.\n"
                },
                {
                    "name": "Per-instance Data",
                    "content": "The per-instance data are held in memory beyond the basic PerlIOl struct, by making a PerlIOl\nthe first member of the layer's struct thus:\n\ntypedef struct\n{\nstruct PerlIO base;       /* Base \"class\" info */\nSTDCHAR *      buf;        /* Start of buffer */\nSTDCHAR *      end;        /* End of valid part of buffer */\nSTDCHAR *      ptr;        /* Current position in buffer */\nOfft          posn;       /* Offset of buf into the file */\nSizet         bufsiz;     /* Real size of buffer */\nIV             oneword;    /* Emergency buffer */\n} PerlIOBuf;\n\nIn this way (as for perl's scalars) a pointer to a PerlIOBuf can be treated as a pointer to a\nPerlIOl.\n"
                },
                {
                    "name": "Layers in action.",
                    "content": "table           perlio          unix\n|           |\n+-----------+    +----------+    +--------+\nPerlIO ->|           |--->|  next    |--->|  NULL  |\n+-----------+    +----------+    +--------+\n|           |    |  buffer  |    |   fd   |\n+-----------+    |          |    +--------+\n|           |    +----------+\n\nThe above attempts to show how the layer scheme works in a simple case.  The application's\n\"PerlIO *\" points to an entry in the table(s) representing open (allocated) handles. For\nexample the first three slots in the table correspond to \"stdin\",\"stdout\" and \"stderr\". The\ntable in turn points to the current \"top\" layer for the handle - in this case an instance of\nthe generic buffering layer \"perlio\". That layer in turn points to the next layer down - in\nthis case the low-level \"unix\" layer.\n\nThe above is roughly equivalent to a \"stdio\" buffered stream, but with much more flexibility:\n\n•   If Unix level \"read\"/\"write\"/\"lseek\" is not appropriate for (say) sockets then the \"unix\"\nlayer can be replaced (at open time or even dynamically) with a \"socket\" layer.\n\n•   Different handles can have different buffering schemes. The \"top\" layer could be the\n\"mmap\" layer if reading disk files was quicker using \"mmap\" than \"read\". An \"unbuffered\"\nstream can be implemented simply by not having a buffer layer.\n\n•   Extra layers can be inserted to process the data as it flows through.  This was the\ndriving need for including the scheme in perl 5.7.0+ - we needed a mechanism to allow\ndata to be translated between perl's internal encoding (conceptually at least Unicode as\nUTF-8), and the \"native\" format used by the system. This is provided by the\n\":encoding(xxxx)\" layer which typically sits above the buffering layer.\n\n•   A layer can be added that does \"\\n\" to CRLF translation. This layer can be used on any\nplatform, not just those that normally do such things.\n"
                },
                {
                    "name": "Per-instance flag bits",
                    "content": "The generic flag bits are a hybrid of \"OXXXXX\" style flags deduced from the mode string\npassed to \"PerlIOopen()\", and state bits for typical buffer layers.\n\nPERLIOFEOF\nEnd of file.\n\nPERLIOFCANWRITE\nWrites are permitted, i.e. opened as \"w\" or \"r+\" or \"a\", etc.\n\nPERLIOFCANREAD\nReads are permitted i.e. opened \"r\" or \"w+\" (or even \"a+\" - ick).\n\nPERLIOFERROR\nAn error has occurred (for \"PerlIOerror()\").\n\nPERLIOFTRUNCATE\nTruncate file suggested by open mode.\n\nPERLIOFAPPEND\nAll writes should be appends.\n\nPERLIOFCRLF\nLayer is performing Win32-like \"\\n\" mapped to CR,LF for output and CR,LF mapped to \"\\n\"\nfor input. Normally the provided \"crlf\" layer is the only layer that need bother about\nthis. \"PerlIObinmode()\" will mess with this flag rather than add/remove layers if the\n\"PERLIOKCANCRLF\" bit is set for the layers class.\n\nPERLIOFUTF8\nData written to this layer should be UTF-8 encoded; data provided by this layer should be\nconsidered UTF-8 encoded. Can be set on any layer by \":utf8\" dummy layer. Also set on\n\":encoding\" layer.\n\nPERLIOFUNBUF\nLayer is unbuffered - i.e. write to next layer down should occur for each write to this\nlayer.\n\nPERLIOFWRBUF\nThe buffer for this layer currently holds data written to it but not sent to next layer.\n\nPERLIOFRDBUF\nThe buffer for this layer currently holds unconsumed data read from layer below.\n\nPERLIOFLINEBUF\nLayer is line buffered. Write data should be passed to next layer down whenever a \"\\n\" is\nseen. Any data beyond the \"\\n\" should then be processed.\n\nPERLIOFTEMP\nFile has been \"unlink()\"ed, or should be deleted on \"close()\".\n\nPERLIOFOPEN\nHandle is open.\n\nPERLIOFFASTGETS\nThis instance of this layer supports the \"fast \"gets\"\" interface.  Normally set based on\n\"PERLIOKFASTGETS\" for the class and by the existence of the function(s) in the table.\nHowever a class that normally provides that interface may need to avoid it on a\nparticular instance. The \"pending\" layer needs to do this when it is pushed above a layer\nwhich does not support the interface.  (Perl's \"svgets()\" does not expect the streams\nfast \"gets\" behaviour to change during one \"get\".)\n"
                },
                {
                    "name": "Methods in Detail",
                    "content": "fsize\nSizet fsize;\n\nSize of the function table. This is compared against the value PerlIO code \"knows\" as a\ncompatibility check. Future versions may be able to tolerate layers compiled against an\nold version of the headers.\n\nname\nchar * name;\n\nThe name of the layer whose open() method Perl should invoke on open().  For example if\nthe layer is called APR, you will call:\n\nopen $fh, \">:APR\", ...\n\nand Perl knows that it has to invoke the PerlIOAPRopen() method implemented by the APR\nlayer.\n\nsize\nSizet size;\n\nThe size of the per-instance data structure, e.g.:\n\nsizeof(PerlIOAPR)\n\nIf this field is zero then \"PerlIOpushed\" does not malloc anything and assumes layer's\nPushed function will do any required layer stack manipulation - used to avoid malloc/free\noverhead for dummy layers.  If the field is non-zero it must be at least the size of\n\"PerlIOl\", \"PerlIOpushed\" will allocate memory for the layer's data structures and link\nnew layer onto the stream's stack. (If the layer's Pushed method returns an error\nindication the layer is popped again.)\n\nkind\nIV kind;\n\n•   PERLIOKBUFFERED\n\nThe layer is buffered.\n\n•   PERLIOKRAW\n\nThe layer is acceptable to have in a binmode(FH) stack - i.e. it does not (or will\nconfigure itself not to) transform bytes passing through it.\n\n•   PERLIOKCANCRLF\n\nLayer can translate between \"\\n\" and CRLF line ends.\n\n•   PERLIOKFASTGETS\n\nLayer allows buffer snooping.\n\n•   PERLIOKMULTIARG\n\nUsed when the layer's open() accepts more arguments than usual. The extra arguments\nshould come not before the \"MODE\" argument. When this flag is used it's up to the\nlayer to validate the args.\n\nPushed\nIV     (*Pushed)(pTHX PerlIO *f,const char *mode, SV *arg);\n\nThe only absolutely mandatory method. Called when the layer is pushed onto the stack.\nThe \"mode\" argument may be NULL if this occurs post-open. The \"arg\" will be non-\"NULL\" if\nan argument string was passed. In most cases this should call \"PerlIOBasepushed()\" to\nconvert \"mode\" into the appropriate \"PERLIOFXXXXX\" flags in addition to any actions the\nlayer itself takes.  If a layer is not expecting an argument it need neither save the one\npassed to it, nor provide \"Getarg()\" (it could perhaps \"Perlwarn\" that the argument was\nun-expected).\n\nReturns 0 on success. On failure returns -1 and should set errno.\n\nPopped\nIV      (*Popped)(pTHX PerlIO *f);\n\nCalled when the layer is popped from the stack. A layer will normally be popped after\n\"Close()\" is called. But a layer can be popped without being closed if the program is\ndynamically managing layers on the stream. In such cases \"Popped()\" should free any\nresources (buffers, translation tables, ...) not held directly in the layer's struct.  It\nshould also \"Unread()\" any unconsumed data that has been read and buffered from the layer\nbelow back to that layer, so that it can be re-provided to what ever is now above.\n\nReturns 0 on success and failure.  If \"Popped()\" returns true then perlio.c assumes that\neither the layer has popped itself, or the layer is super special and needs to be\nretained for other reasons.  In most cases it should return false.\n\nOpen\nPerlIO *        (*Open)(...);\n\nThe \"Open()\" method has lots of arguments because it combines the functions of perl's\n\"open\", \"PerlIOopen\", perl's \"sysopen\", \"PerlIOfdopen\" and \"PerlIOreopen\".  The full\nprototype is as follows:\n\nPerlIO *       (*Open)(pTHX PerlIOfuncs *tab,\nPerlIOlistt *layers, IV n,\nconst char *mode,\nint fd, int imode, int perm,\nPerlIO *old,\nint narg, SV args);\n\nOpen should (perhaps indirectly) call \"PerlIOallocate()\" to allocate a slot in the table\nand associate it with the layers information for the opened file, by calling\n\"PerlIOpush\".  The layers is an array of all the layers destined for the \"PerlIO *\", and\nany arguments passed to them, n is the index into that array of the layer being called.\nThe macro \"PerlIOArg\" will return a (possibly \"NULL\") SV * for the argument passed to the\nlayer.\n\nWhere a layer opens or takes ownership of a file descriptor, that layer is responsible\nfor getting the file descriptor's close-on-exec flag into the correct state.  The flag\nshould be clear for a file descriptor numbered less than or equal to \"PLmaxsysfd\", and\nset for any file descriptor numbered higher.  For thread safety, when a layer opens a new\nfile descriptor it should if possible open it with the close-on-exec flag initially set.\n\nThe mode string is an \"\"fopen()\"-like\" string which would match the regular expression\n\"/^[I#]?[rwa]\\+?[bt]?$/\".\n\nThe 'I' prefix is used during creation of \"stdin\"..\"stderr\" via special \"PerlIOfdopen\"\ncalls; the '#' prefix means that this is \"sysopen\" and that imode and perm should be\npassed to \"PerlLIOopen3\"; 'r' means read, 'w' means write and 'a' means append. The '+'\nsuffix means that both reading and writing/appending are permitted.  The 'b' suffix means\nfile should be binary, and 't' means it is text. (Almost all layers should do the IO in\nbinary mode, and ignore the b/t bits. The \":crlf\" layer should be pushed to handle the\ndistinction.)\n\nIf old is not \"NULL\" then this is a \"PerlIOreopen\". Perl itself does not use this (yet?)\nand semantics are a little vague.\n\nIf fd not negative then it is the numeric file descriptor fd, which will be open in a\nmanner compatible with the supplied mode string, the call is thus equivalent to\n\"PerlIOfdopen\". In this case nargs will be zero.  The file descriptor may have the\nclose-on-exec flag either set or clear; it is the responsibility of the layer that takes\nownership of it to get the flag into the correct state.\n\nIf nargs is greater than zero then it gives the number of arguments passed to \"open\",\notherwise it will be 1 if for example \"PerlIOopen\" was called.  In simple cases\nSvPVnolen(*args) is the pathname to open.\n\nIf a layer provides \"Open()\" it should normally call the \"Open()\" method of next layer\ndown (if any) and then push itself on top if that succeeds.  \"PerlIOBaseopen\" is\nprovided to do exactly that, so in most cases you don't have to write your own \"Open()\"\nmethod.  If this method is not defined, other layers may have difficulty pushing\nthemselves on top of it during open.\n\nIf \"PerlIOpush\" was performed and open has failed, it must \"PerlIOpop\" itself, since if\nit's not, the layer won't be removed and may cause bad problems.\n\nReturns \"NULL\" on failure.\n\nBinmode\nIV        (*Binmode)(pTHX PerlIO *f);\n\nOptional. Used when \":raw\" layer is pushed (explicitly or as a result of binmode(FH)). If\nnot present layer will be popped. If present should configure layer as binary (or pop\nitself) and return 0.  If it returns -1 for error \"binmode\" will fail with layer still on\nthe stack.\n\nGetarg\nSV *      (*Getarg)(pTHX PerlIO *f,\nCLONEPARAMS *param, int flags);\n\nOptional. If present should return an SV * representing the string argument passed to the\nlayer when it was pushed. e.g. \":encoding(ascii)\" would return an SvPV with value\n\"ascii\". (param and flags arguments can be ignored in most cases)\n\n\"Dup\" uses \"Getarg\" to retrieve the argument originally passed to \"Pushed\", so you must\nimplement this function if your layer has an extra argument to \"Pushed\" and will ever be\n\"Dup\"ed.\n\nFileno\nIV        (*Fileno)(pTHX PerlIO *f);\n\nReturns the Unix/Posix numeric file descriptor for the handle. Normally\n\"PerlIOBasefileno()\" (which just asks next layer down) will suffice for this.\n\nReturns -1 on error, which is considered to include the case where the layer cannot\nprovide such a file descriptor.\n\nDup\nPerlIO * (*Dup)(pTHX PerlIO *f, PerlIO *o,\nCLONEPARAMS *param, int flags);\n\nXXX: Needs more docs.\n\nUsed as part of the \"clone\" process when a thread is spawned (in which case param will be\nnon-NULL) and when a stream is being duplicated via '&' in the \"open\".\n\nSimilar to \"Open\", returns PerlIO* on success, \"NULL\" on failure.\n\nRead\nSSizet (*Read)(pTHX PerlIO *f, void *vbuf, Sizet count);\n\nBasic read operation.\n\nTypically will call \"Fill\" and manipulate pointers (possibly via the API).\n\"PerlIOBufread()\" may be suitable for derived classes which provide \"fast gets\" methods.\n\nReturns actual bytes read, or -1 on an error.\n\nUnread\nSSizet (*Unread)(pTHX PerlIO *f,\nconst void *vbuf, Sizet count);\n\nA superset of stdio's \"ungetc()\". Should arrange for future reads to see the bytes in\n\"vbuf\". If there is no obviously better implementation then \"PerlIOBaseunread()\"\nprovides the function by pushing a \"fake\" \"pending\" layer above the calling layer.\n\nReturns the number of unread chars.\n\nWrite\nSSizet (*Write)(PerlIO *f, const void *vbuf, Sizet count);\n\nBasic write operation.\n\nReturns bytes written or -1 on an error.\n\nSeek\nIV      (*Seek)(pTHX PerlIO *f, Offt offset, int whence);\n\nPosition the file pointer. Should normally call its own \"Flush\" method and then the\n\"Seek\" method of next layer down.\n\nReturns 0 on success, -1 on failure.\n\nTell\nOfft   (*Tell)(pTHX PerlIO *f);\n\nReturn the file pointer. May be based on layers cached concept of position to avoid\noverhead.\n\nReturns -1 on failure to get the file pointer.\n\nClose\nIV      (*Close)(pTHX PerlIO *f);\n\nClose the stream. Should normally call \"PerlIOBaseclose()\" to flush itself and close\nlayers below, and then deallocate any data structures (buffers, translation tables, ...)\nnot  held directly in the data structure.\n\nReturns 0 on success, -1 on failure.\n\nFlush\nIV      (*Flush)(pTHX PerlIO *f);\n\nShould make stream's state consistent with layers below. That is, any buffered write data\nshould be written, and file position of lower layers adjusted for data read from below\nbut not actually consumed.  (Should perhaps \"Unread()\" such data to the lower layer.)\n\nReturns 0 on success, -1 on failure.\n\nFill\nIV      (*Fill)(pTHX PerlIO *f);\n\nThe buffer for this layer should be filled (for read) from layer below.  When you\n\"subclass\" PerlIOBuf layer, you want to use its read method and to supply your own fill\nmethod, which fills the PerlIOBuf's buffer.\n\nReturns 0 on success, -1 on failure.\n\nEof\nIV      (*Eof)(pTHX PerlIO *f);\n\nReturn end-of-file indicator. \"PerlIOBaseeof()\" is normally sufficient.\n\nReturns 0 on end-of-file, 1 if not end-of-file, -1 on error.\n\nError\nIV      (*Error)(pTHX PerlIO *f);\n\nReturn error indicator. \"PerlIOBaseerror()\" is normally sufficient.\n\nReturns 1 if there is an error (usually when \"PERLIOFERROR\" is set), 0 otherwise.\n\nClearerr\nvoid    (*Clearerr)(pTHX PerlIO *f);\n\nClear end-of-file and error indicators. Should call \"PerlIOBaseclearerr()\" to set the\n\"PERLIOFXXXXX\" flags, which may suffice.\n\nSetlinebuf\nvoid    (*Setlinebuf)(pTHX PerlIO *f);\n\nMark the stream as line buffered. \"PerlIOBasesetlinebuf()\" sets the PERLIOFLINEBUF\nflag and is normally sufficient.\n\nGetbase\nSTDCHAR *       (*Getbase)(pTHX PerlIO *f);\n\nAllocate (if not already done so) the read buffer for this layer and return pointer to\nit. Return NULL on failure.\n\nGetbufsiz\nSizet  (*Getbufsiz)(pTHX PerlIO *f);\n\nReturn the number of bytes that last \"Fill()\" put in the buffer.\n\nGetptr\nSTDCHAR *       (*Getptr)(pTHX PerlIO *f);\n\nReturn the current read pointer relative to this layer's buffer.\n\nGetcnt\nSSizet (*Getcnt)(pTHX PerlIO *f);\n\nReturn the number of bytes left to be read in the current buffer.\n\nSetptrcnt\nvoid    (*Setptrcnt)(pTHX PerlIO *f,\nSTDCHAR *ptr, SSizet cnt);\n\nAdjust the read pointer and count of bytes to match \"ptr\" and/or \"cnt\".  The application\n(or layer above) must ensure they are consistent.  (Checking is allowed by the paranoid.)\n"
                },
                {
                    "name": "Utilities",
                    "content": "To ask for the next layer down use PerlIONext(PerlIO *f).\n\nTo check that a PerlIO* is valid use PerlIOValid(PerlIO *f).  (All this does is really just\nto check that the pointer is non-NULL and that the pointer behind that is non-NULL.)\n\nPerlIOBase(PerlIO *f) returns the \"Base\" pointer, or in other words, the \"PerlIOl*\" pointer.\n\nPerlIOSelf(PerlIO* f, type) return the PerlIOBase cast to a type.\n\nPerlPerlIOorBase(PerlIO* f, callback, base, failure, args) either calls the callback from\nthe functions of the layer f (just by the name of the IO function, like \"Read\") with the\nargs, or if there is no such callback, calls the base version of the callback with the same\nargs, or if the f is invalid, set errno to EBADF and return failure.\n\nPerlPerlIOorfail(PerlIO* f, callback, failure, args) either calls the callback of the\nfunctions of the layer f with the args, or if there is no such callback, set errno to EINVAL.\nOr if the f is invalid, set errno to EBADF and return failure.\n\nPerlPerlIOorBasevoid(PerlIO* f, callback, base, args) either calls the callback of the\nfunctions of the layer f with the args, or if there is no such callback, calls the base\nversion of the callback with the same args, or if the f is invalid, set errno to EBADF.\n\nPerlPerlIOorfailvoid(PerlIO* f, callback, args) either calls the callback of the\nfunctions of the layer f with the args, or if there is no such callback, set errno to EINVAL.\nOr if the f is invalid, set errno to EBADF.\n"
                },
                {
                    "name": "Implementing PerlIO Layers",
                    "content": "If you find the implementation document unclear or not sufficient, look at the existing\nPerlIO layer implementations, which include:\n\n•   C implementations\n\nThe perlio.c and perliol.h in the Perl core implement the \"unix\", \"perlio\", \"stdio\",\n\"crlf\", \"utf8\", \"byte\", \"raw\", \"pending\" layers, and also the \"mmap\" and \"win32\" layers\nif applicable.  (The \"win32\" is currently unfinished and unused, to see what is used\ninstead in Win32, see \"Querying the layers of filehandles\" in PerlIO .)\n\nPerlIO::encoding, PerlIO::scalar, PerlIO::via in the Perl core.\n\nPerlIO::gzip and APR::PerlIO (modperl 2.0) on CPAN.\n\n•   Perl implementations\n\nPerlIO::via::QuotedPrint in the Perl core and PerlIO::via::* on CPAN.\n\nIf you are creating a PerlIO layer, you may want to be lazy, in other words, implement only\nthe methods that interest you.  The other methods you can either replace with the \"blank\"\nmethods\n\nPerlIOBasenoopok\nPerlIOBasenoopfail\n\n(which do nothing, and return zero and -1, respectively) or for certain methods you may\nassume a default behaviour by using a NULL method.  The Open method looks for help in the\n'parent' layer.  The following table summarizes the behaviour:\n\nmethod      behaviour with NULL\n\nClearerr    PerlIOBaseclearerr\nClose       PerlIOBaseclose\nDup         PerlIOBasedup\nEof         PerlIOBaseeof\nError       PerlIOBaseerror\nFileno      PerlIOBasefileno\nFill        FAILURE\nFlush       SUCCESS\nGetarg      SUCCESS\nGetbase    FAILURE\nGetbufsiz  FAILURE\nGetcnt     FAILURE\nGetptr     FAILURE\nOpen        INHERITED\nPopped      SUCCESS\nPushed      SUCCESS\nRead        PerlIOBaseread\nSeek        FAILURE\nSetcnt     FAILURE\nSetptrcnt  FAILURE\nSetlinebuf  PerlIOBasesetlinebuf\nTell        FAILURE\nUnread      PerlIOBaseunread\nWrite       FAILURE\n\nFAILURE        Set errno (to EINVAL in Unixish, to LIB$INVARG in VMS)\nand return -1 (for numeric return values) or NULL (for\npointers)\nINHERITED      Inherited from the layer below\nSUCCESS        Return 0 (for numeric return values) or a pointer\n"
                },
                {
                    "name": "Core Layers",
                    "content": "The file \"perlio.c\" provides the following layers:\n\n\"unix\"\nA basic non-buffered layer which calls Unix/POSIX \"read()\", \"write()\", \"lseek()\",\n\"close()\". No buffering. Even on platforms that distinguish between OTEXT and OBINARY\nthis layer is always OBINARY.\n\n\"perlio\"\nA very complete generic buffering layer which provides the whole of PerlIO API. It is\nalso intended to be used as a \"base class\" for other layers. (For example its \"Read()\"\nmethod is implemented in terms of the \"Getcnt()\"/\"Getptr()\"/\"Setptrcnt()\" methods).\n\n\"perlio\" over \"unix\" provides a complete replacement for stdio as seen via PerlIO API.\nThis is the default for USEPERLIO when system's stdio does not permit perl's \"fast gets\"\naccess, and which do not distinguish between \"OTEXT\" and \"OBINARY\".\n\n\"stdio\"\nA layer which provides the PerlIO API via the layer scheme, but implements it by calling\nsystem's stdio. This is (currently) the default if system's stdio provides sufficient\naccess to allow perl's \"fast gets\" access and which do not distinguish between \"OTEXT\"\nand \"OBINARY\".\n\n\"crlf\"\nA layer derived using \"perlio\" as a base class. It provides Win32-like \"\\n\" to CR,LF\ntranslation. Can either be applied above \"perlio\" or serve as the buffer layer itself.\n\"crlf\" over \"unix\" is the default if system distinguishes between \"OTEXT\" and \"OBINARY\"\nopens. (At some point \"unix\" will be replaced by a \"native\" Win32 IO layer on that\nplatform, as Win32's read/write layer has various drawbacks.) The \"crlf\" layer is a\nreasonable model for a layer which transforms data in some way.\n\n\"mmap\"\nIf Configure detects \"mmap()\" functions this layer is provided (with \"perlio\" as a\n\"base\") which does \"read\" operations by mmap()ing the file. Performance improvement is\nmarginal on modern systems, so it is mainly there as a proof of concept. It is likely to\nbe unbundled from the core at some point. The \"mmap\" layer is a reasonable model for a\nminimalist \"derived\" layer.\n\n\"pending\"\nAn \"internal\" derivative of \"perlio\" which can be used to provide Unread() function for\nlayers which have no buffer or cannot be bothered.  (Basically this layer's \"Fill()\" pops\nitself off the stack and so resumes reading from layer below.)\n\n\"raw\"\nA dummy layer which never exists on the layer stack. Instead when \"pushed\" it actually\npops the stack removing itself, it then calls Binmode function table entry on all the\nlayers in the stack - normally this (via PerlIOBasebinmode) removes any layers which do\nnot have \"PERLIOKRAW\" bit set. Layers can modify that behaviour by defining their own\nBinmode entry.\n\n\"utf8\"\nAnother dummy layer. When pushed it pops itself and sets the \"PERLIOFUTF8\" flag on the\nlayer which was (and now is once more) the top of the stack.\n\nIn addition perlio.c also provides a number of \"PerlIOBasexxxx()\" functions which are\nintended to be used in the table slots of classes which do not need to do anything special\nfor a particular method.\n"
                },
                {
                    "name": "Extension Layers",
                    "content": "Layers can be made available by extension modules. When an unknown layer is encountered the\nPerlIO code will perform the equivalent of :\n\nuse PerlIO 'layer';\n\nWhere layer is the unknown layer. PerlIO.pm will then attempt to:\n\nrequire PerlIO::layer;\n\nIf after that process the layer is still not defined then the \"open\" will fail.\n\nThe following extension layers are bundled with perl:\n\n\":encoding\"\nuse Encoding;\n\nmakes this layer available, although PerlIO.pm \"knows\" where to find it.  It is an\nexample of a layer which takes an argument as it is called thus:\n\nopen( $fh, \"<:encoding(iso-8859-7)\", $pathname );\n\n\":scalar\"\nProvides support for reading data from and writing data to a scalar.\n\nopen( $fh, \"+<:scalar\", \\$scalar );\n\nWhen a handle is so opened, then reads get bytes from the string value of $scalar, and\nwrites change the value. In both cases the position in $scalar starts as zero but can be\naltered via \"seek\", and determined via \"tell\".\n\nPlease note that this layer is implied when calling open() thus:\n\nopen( $fh, \"+<\", \\$scalar );\n\n\":via\"\nProvided to allow layers to be implemented as Perl code.  For instance:\n\nuse PerlIO::via::StripHTML;\nopen( my $fh, \"<:via(StripHTML)\", \"index.html\" );\n\nSee PerlIO::via for details.\n"
                }
            ]
        },
        "TODO": {
            "content": "Things that need to be done to improve this document.\n\n•   Explain how to make a valid fh without going through open()(i.e. apply a layer). For\nexample if the file is not opened through perl, but we want to get back a fh, like it was\nopened by Perl.\n\nHow PerlIOapplylayera fits in, where its docs, was it made public?\n\nCurrently the example could be something like this:\n\nPerlIO *footoPerlIO(pTHX char *mode, ...)\n{\nchar *mode; /* \"w\", \"r\", etc */\nconst char *layers = \":APR\"; /* the layer name */\nPerlIO *f = PerlIOallocate(aTHX);\nif (!f) {\nreturn NULL;\n}\n\nPerlIOapplylayers(aTHX f, mode, layers);\n\nif (f) {\nPerlIOAPR *st = PerlIOSelf(f, PerlIOAPR);\n/* fill in the st struct, as in open() */\nst->file = file;\nPerlIOBase(f)->flags |= PERLIOFOPEN;\n\nreturn f;\n}\nreturn NULL;\n}\n\n•   fix/add the documentation in places marked as XXX.\n\n•   The handling of errors by the layer is not specified. e.g. when $!  should be set\nexplicitly, when the error handling should be just delegated to the top layer.\n\nProbably give some hints on using SETERRNO() or pointers to where they can be found.\n\n•   I think it would help to give some concrete examples to make it easier to understand the\nAPI. Of course I agree that the API has to be concise, but since there is no second\ndocument that is more of a guide, I think that it'd make it easier to start with the doc\nwhich is an API, but has examples in it in places where things are unclear, to a person\nwho is not a PerlIO guru (yet).\n\n\n\nperl v5.34.0                                 2025-07-25                                   PERLIOL(1)",
            "subsections": []
        }
    },
    "summary": "perliol - C API for Perl's implementation of IO in Layers.",
    "flags": [],
    "examples": [],
    "see_also": []
}