perliol


SYNOPSIS
           /* Defining a layer ... */
           #include <perliol.h>

DESCRIPTION
       This document describes the behavior and implementation of the PerlIO
       abstraction described in perlapio when "USE_PERLIO" is defined (and
       "USE_SFIO" is not).

   History and Background
       The PerlIO abstraction was introduced in perl5.003_02 but languished as
       just an abstraction until perl5.7.0. However during that time a number
       of perl extensions switched to using it, so the API is mostly fixed to
       maintain (source) compatibility.

       The aim of the implementation is to provide the PerlIO API in a
       flexible and platform neutral manner. It is also a trial of an "Object
       Oriented C, with vtables" approach which may be applied to Perl 6.

   Basic Structure
       PerlIO is a stack of layers.

       The low levels of the stack work with the low-level operating system
       calls (file descriptors in C) getting bytes in and out, the higher
       layers of the stack buffer, filter, and otherwise manipulate the I/O,
       and return characters (or bytes) to Perl.  Terms above and below are
       used to refer to the relative positioning of the stack layers.

       A layer contains a "vtable", the table of I/O operations (at C level a
       table of function pointers), and status flags.  The functions in the
       vtable implement operations like "open", "read", and "write".

       When I/O, for example "read", is requested, the request goes from Perl
       first down the stack using "read" functions of each layer, then at the
       bottom the input is requested from the operating system services, then
       the result is returned up the stack, finally being interpreted as Perl
       data.

       The requests do not necessarily go always all the way down to the
       operating system: that's where PerlIO buffering comes into play.

       When you do an open() and specify extra PerlIO layers to be deployed,
       the layers you specify are "pushed" on top of the already existing
       default stack.  One way to see it is that "operating system is on the
       left" and "Perl is on the right".

       What exact layers are in this default stack depends on a lot of things:
       your operating system, Perl version, Perl compile time configuration,
       and Perl runtime configuration.  See PerlIO, "PERLIO" in perlrun, and
       open for more information.

       binmode() operates similarly to open(): by default the specified layers
       are pushed on top of the existing stack.
       layers, it uses only the "unix" layer, which is a rather thin wrapper
       on top of the Unix file descriptors.

   Layers vs Disciplines
       Initial discussion of the ability to modify IO streams behaviour used
       the term "discipline" for the entities which were added. This came (I
       believe) from the use of the term in "sfio", which in turn borrowed it
       from "line disciplines" on Unix terminals. However, this document (and
       the C code) uses the term "layer".

       This is, I hope, a natural term given the implementation, and should
       avoid connotations that are inherent in earlier uses of "discipline"
       for things which are rather different.

   Data Structures
       The basic data structure is a PerlIOl:

               typedef struct _PerlIO PerlIOl;
               typedef struct _PerlIO_funcs PerlIO_funcs;
               typedef PerlIOl *PerlIO;

               struct _PerlIO
               {
                PerlIOl *      next;       /* Lower layer */
                PerlIO_funcs * tab;        /* Functions for this layer */
                IV             flags;      /* Various flags for state */
               };

       A "PerlIOl *" is a pointer to the struct, and the application level
       "PerlIO *" is a pointer to a "PerlIOl *" - i.e. a pointer to a pointer
       to the struct. This allows the application level "PerlIO *" to remain
       constant while the actual "PerlIOl *" underneath changes. (Compare
       perl's "SV *" which remains constant while its "sv_any" field changes
       as the scalar's type changes.) An IO stream is then in general
       represented as a pointer to this linked-list of "layers".

       It should be noted that because of the double indirection in a "PerlIO
       *", a "&(perlio->next)" "is" a "PerlIO *", and so to some degree at
       least one layer can use the "standard" API on the next layer down.

       A "layer" is composed of two parts:

       1.  The functions and attributes of the "layer class".

       2.  The per-instance data for a particular handle.

   Functions and Attributes
       The functions and attributes are accessed via the "tab" (for table)
       member of "PerlIOl". The functions (methods of the layer "class") are
       fixed, and are defined by the "PerlIO_funcs" type. They are broadly the
       same as the public "PerlIO_xxxxx" functions:

         struct _PerlIO_funcs
         {
          IV           (*Binmode)(pTHX_ PerlIO *f);
          SV *         (*Getarg)(pTHX_ PerlIO *f, CLONE_PARAMS *param, int flags)
          IV           (*Fileno)(pTHX_ PerlIO *f);
          PerlIO *     (*Dup)(pTHX_ PerlIO *f, PerlIO *o, CLONE_PARAMS *param, int flags)
          /* Unix-like functions - cf sfio line disciplines */
          SSize_t      (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
          SSize_t      (*Unread)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
          SSize_t      (*Write)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
          IV           (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
          Off_t        (*Tell)(pTHX_ PerlIO *f);
          IV           (*Close)(pTHX_ PerlIO *f);
          /* Stdio-like buffered IO functions */
          IV           (*Flush)(pTHX_ PerlIO *f);
          IV           (*Fill)(pTHX_ PerlIO *f);
          IV           (*Eof)(pTHX_ PerlIO *f);
          IV           (*Error)(pTHX_ PerlIO *f);
          void         (*Clearerr)(pTHX_ PerlIO *f);
          void         (*Setlinebuf)(pTHX_ PerlIO *f);
          /* Perl's snooping functions */
          STDCHAR *    (*Get_base)(pTHX_ PerlIO *f);
          Size_t       (*Get_bufsiz)(pTHX_ PerlIO *f);
          STDCHAR *    (*Get_ptr)(pTHX_ PerlIO *f);
          SSize_t      (*Get_cnt)(pTHX_ PerlIO *f);
          void         (*Set_ptrcnt)(pTHX_ PerlIO *f,STDCHAR *ptr,SSize_t cnt);
         };

       The first few members of the struct give a function table size for
       compatibility check "name" for the layer, the  size to "malloc" for the
       per-instance data, and some flags which are attributes of the class as
       whole (such as whether it is a buffering layer), then follow the
       functions which fall into four basic groups:

       1.  Opening and setup functions

       2.  Basic IO operations

       3.  Stdio class buffering options.

       4.  Functions to support Perl's traditional "fast" access to the
           buffer.

       A layer does not have to implement all the functions, but the whole
       table has to be present. Unimplemented slots can be NULL (which will
       result in an error when called) or can be filled in with stubs to
       "inherit" behaviour from a "base class". This "inheritance" is fixed
       for all instances of the layer, but as the layer chooses which stubs to
       populate the table, limited "multiple inheritance" is possible.

   Per-instance Data
       The per-instance data are held in memory beyond the basic PerlIOl
       struct, by making a PerlIOl the first member of the layer's struct
       thus:

               typedef struct

   Layers in action.
                       table           perlio          unix
                   |           |
                   +-----------+    +----------+    +--------+
          PerlIO ->|           |--->|  next    |--->|  NULL  |
                   +-----------+    +----------+    +--------+
                   |           |    |  buffer  |    |   fd   |
                   +-----------+    |          |    +--------+
                   |           |    +----------+

       The above attempts to show how the layer scheme works in a simple case.
       The application's "PerlIO *" points to an entry in the table(s)
       representing open (allocated) handles. For example the first three
       slots in the table correspond to "stdin","stdout" and "stderr". The
       table in turn points to the current "top" layer for the handle - in
       this case an instance of the generic buffering layer "perlio". That
       layer in turn points to the next layer down - in this case the low-
       level "unix" layer.

       The above is roughly equivalent to a "stdio" buffered stream, but with
       much more flexibility:

       o   If Unix level "read"/"write"/"lseek" is not appropriate for (say)
           sockets then the "unix" layer can be replaced (at open time or even
           dynamically) with a "socket" layer.

       o   Different handles can have different buffering schemes. The "top"
           layer could be the "mmap" layer if reading disk files was quicker
           using "mmap" than "read". An "unbuffered" stream can be implemented
           simply by not having a buffer layer.

       o   Extra layers can be inserted to process the data as it flows
           through.  This was the driving need for including the scheme in
           perl 5.7.0+ - we needed a mechanism to allow data to be translated
           between perl's internal encoding (conceptually at least Unicode as
           UTF-8), and the "native" format used by the system. This is
           provided by the ":encoding(xxxx)" layer which typically sits above
           the buffering layer.

       o   A layer can be added that does "\n" to CRLF translation. This layer
           can be used on any platform, not just those that normally do such
           things.

   Per-instance flag bits
       The generic flag bits are a hybrid of "O_XXXXX" style flags deduced
       from the mode string passed to "PerlIO_open()", and state bits for
       typical buffer layers.

       PERLIO_F_EOF
           End of file.

       PERLIO_F_CANWRITE
           Writes are permitted, i.e. opened as "w" or "r+" or "a", etc.

       PERLIO_F_CRLF
           Layer is performing Win32-like "\n" mapped to CR,LF for output and
           CR,LF mapped to "\n" for input. Normally the provided "crlf" layer
           is the only layer that need bother about this. "PerlIO_binmode()"
           will mess with this flag rather than add/remove layers if the
           "PERLIO_K_CANCRLF" bit is set for the layers class.

       PERLIO_F_UTF8
           Data written to this layer should be UTF-8 encoded; data provided
           by this layer should be considered UTF-8 encoded. Can be set on any
           layer by ":utf8" dummy layer. Also set on ":encoding" layer.

       PERLIO_F_UNBUF
           Layer is unbuffered - i.e. write to next layer down should occur
           for each write to this layer.

       PERLIO_F_WRBUF
           The buffer for this layer currently holds data written to it but
           not sent to next layer.

       PERLIO_F_RDBUF
           The buffer for this layer currently holds unconsumed data read from
           layer below.

       PERLIO_F_LINEBUF
           Layer is line buffered. Write data should be passed to next layer
           down whenever a "\n" is seen. Any data beyond the "\n" should then
           be processed.

       PERLIO_F_TEMP
           File has been "unlink()"ed, or should be deleted on "close()".

       PERLIO_F_OPEN
           Handle is open.

       PERLIO_F_FASTGETS
           This instance of this layer supports the "fast "gets"" interface.
           Normally set based on "PERLIO_K_FASTGETS" for the class and by the
           existence of the function(s) in the table. However a class that
           normally provides that interface may need to avoid it on a
           particular instance. The "pending" layer needs to do this when it
           is pushed above a layer which does not support the interface.
           (Perl's "sv_gets()" does not expect the streams fast "gets"
           behaviour to change during one "get".)

   Methods in Detail
       fsize
                   Size_t fsize;

           Size of the function table. This is compared against the value
           PerlIO code "knows" as a compatibility check. Future versions may
           be able to tolerate layers compiled against an old version of the
           headers.

       size
                   Size_t size;

           The size of the per-instance data structure, e.g.:

             sizeof(PerlIOAPR)

           If this field is zero then "PerlIO_pushed" does not malloc anything
           and assumes layer's Pushed function will do any required layer
           stack manipulation - used to avoid malloc/free overhead for dummy
           layers.  If the field is non-zero it must be at least the size of
           "PerlIOl", "PerlIO_pushed" will allocate memory for the layer's
           data structures and link new layer onto the stream's stack. (If the
           layer's Pushed method returns an error indication the layer is
           popped again.)

       kind
                   IV kind;

           o   PERLIO_K_BUFFERED

               The layer is buffered.

           o   PERLIO_K_RAW

               The layer is acceptable to have in a binmode(FH) stack - i.e.
               it does not (or will configure itself not to) transform bytes
               passing through it.

           o   PERLIO_K_CANCRLF

               Layer can translate between "\n" and CRLF line ends.

           o   PERLIO_K_FASTGETS

               Layer allows buffer snooping.

           o   PERLIO_K_MULTIARG

               Used when the layer's open() accepts more arguments than usual.
               The extra arguments should come not before the "MODE" argument.
               When this flag is used it's up to the layer to validate the
               args.

       Pushed
                   IV      (*Pushed)(pTHX_ PerlIO *f,const char *mode, SV *arg);

           The only absolutely mandatory method. Called when the layer is
           pushed onto the stack.  The "mode" argument may be NULL if this
           occurs post-open. The "arg" will be non-"NULL" if an argument
           string was passed. In most cases this should call
           "PerlIOBase_pushed()" to convert "mode" into the appropriate
           "PERLIO_F_XXXXX" flags in addition to any actions the layer itself
           takes.  If a layer is not expecting an argument it need neither
           resources (buffers, translation tables, ...) not held directly in
           the layer's struct.  It should also "Unread()" any unconsumed data
           that has been read and buffered from the layer below back to that
           layer, so that it can be re-provided to what ever is now above.

           Returns 0 on success and failure.  If "Popped()" returns true then
           perlio.c assumes that either the layer has popped itself, or the
           layer is super special and needs to be retained for other reasons.
           In most cases it should return false.

       Open
                   PerlIO *        (*Open)(...);

           The "Open()" method has lots of arguments because it combines the
           functions of perl's "open", "PerlIO_open", perl's "sysopen",
           "PerlIO_fdopen" and "PerlIO_reopen".  The full prototype is as
           follows:

            PerlIO *       (*Open)(pTHX_ PerlIO_funcs *tab,
                                   PerlIO_list_t *layers, IV n,
                                   const char *mode,
                                   int fd, int imode, int perm,
                                   PerlIO *old,
                                   int narg, SV **args);

           Open should (perhaps indirectly) call "PerlIO_allocate()" to
           allocate a slot in the table and associate it with the layers
           information for the opened file, by calling "PerlIO_push".  The
           layers is an array of all the layers destined for the "PerlIO *",
           and any arguments passed to them, n is the index into that array of
           the layer being called. The macro "PerlIOArg" will return a
           (possibly "NULL") SV * for the argument passed to the layer.

           The mode string is an ""fopen()"-like" string which would match the
           regular expression "/^[I#]?[rwa]\+?[bt]?$/".

           The 'I' prefix is used during creation of "stdin".."stderr" via
           special "PerlIO_fdopen" calls; the '#' prefix means that this is
           "sysopen" and that imode and perm should be passed to
           "PerlLIO_open3"; 'r' means read, 'w' means write and 'a' means
           append. The '+' suffix means that both reading and
           writing/appending are permitted.  The 'b' suffix means file should
           be binary, and 't' means it is text. (Almost all layers should do
           the IO in binary mode, and ignore the b/t bits. The ":crlf" layer
           should be pushed to handle the distinction.)

           If old is not "NULL" then this is a "PerlIO_reopen". Perl itself
           does not use this (yet?) and semantics are a little vague.

           If fd not negative then it is the numeric file descriptor fd, which
           will be open in a manner compatible with the supplied mode string,
           the call is thus equivalent to "PerlIO_fdopen". In this case nargs
           will be zero.

           If "PerlIO_push" was performed and open has failed, it must
           "PerlIO_pop" itself, since if it's not, the layer won't be removed
           and may cause bad problems.

           Returns "NULL" on failure.

       Binmode
                   IV        (*Binmode)(pTHX_ PerlIO *f);

           Optional. Used when ":raw" layer is pushed (explicitly or as a
           result of binmode(FH)). If not present layer will be popped. If
           present should configure layer as binary (or pop itself) and return
           0.  If it returns -1 for error "binmode" will fail with layer still
           on the stack.

       Getarg
                   SV *      (*Getarg)(pTHX_ PerlIO *f,
                                       CLONE_PARAMS *param, int flags);

           Optional. If present should return an SV * representing the string
           argument passed to the layer when it was pushed. e.g.
           ":encoding(ascii)" would return an SvPV with value "ascii". (param
           and flags arguments can be ignored in most cases)

           "Dup" uses "Getarg" to retrieve the argument originally passed to
           "Pushed", so you must implement this function if your layer has an
           extra argument to "Pushed" and will ever be "Dup"ed.

       Fileno
                   IV        (*Fileno)(pTHX_ PerlIO *f);

           Returns the Unix/Posix numeric file descriptor for the handle.
           Normally "PerlIOBase_fileno()" (which just asks next layer down)
           will suffice for this.

           Returns -1 on error, which is considered to include the case where
           the layer cannot provide such a file descriptor.

       Dup
                   PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
                                   CLONE_PARAMS *param, int flags);

           XXX: Needs more docs.

           Used as part of the "clone" process when a thread is spawned (in
           which case param will be non-NULL) and when a stream is being
           duplicated via '&' in the "open".

           Similar to "Open", returns PerlIO* on success, "NULL" on failure.

       Read
                   SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);

           Basic read operation.
           to see the bytes in "vbuf". If there is no obviously better
           implementation then "PerlIOBase_unread()" provides the function by
           pushing a "fake" "pending" layer above the calling layer.

           Returns the number of unread chars.

       Write
                   SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count);

           Basic write operation.

           Returns bytes written or -1 on an error.

       Seek
                   IV      (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);

           Position the file pointer. Should normally call its own "Flush"
           method and then the "Seek" method of next layer down.

           Returns 0 on success, -1 on failure.

       Tell
                   Off_t   (*Tell)(pTHX_ PerlIO *f);

           Return the file pointer. May be based on layers cached concept of
           position to avoid overhead.

           Returns -1 on failure to get the file pointer.

       Close
                   IV      (*Close)(pTHX_ PerlIO *f);

           Close the stream. Should normally call "PerlIOBase_close()" to
           flush itself and close layers below, and then deallocate any data
           structures (buffers, translation tables, ...) not  held directly in
           the data structure.

           Returns 0 on success, -1 on failure.

       Flush
                   IV      (*Flush)(pTHX_ PerlIO *f);

           Should make stream's state consistent with layers below. That is,
           any buffered write data should be written, and file position of
           lower layers adjusted for data read from below but not actually
           consumed.  (Should perhaps "Unread()" such data to the lower
           layer.)

           Returns 0 on success, -1 on failure.

       Fill
                   IV      (*Fill)(pTHX_ PerlIO *f);

           The buffer for this layer should be filled (for read) from layer
           Returns 0 on end-of-file, 1 if not end-of-file, -1 on error.

       Error
                   IV      (*Error)(pTHX_ PerlIO *f);

           Return error indicator. "PerlIOBase_error()" is normally
           sufficient.

           Returns 1 if there is an error (usually when "PERLIO_F_ERROR" is
           set, 0 otherwise.

       Clearerr
                   void    (*Clearerr)(pTHX_ PerlIO *f);

           Clear end-of-file and error indicators. Should call
           "PerlIOBase_clearerr()" to set the "PERLIO_F_XXXXX" flags, which
           may suffice.

       Setlinebuf
                   void    (*Setlinebuf)(pTHX_ PerlIO *f);

           Mark the stream as line buffered. "PerlIOBase_setlinebuf()" sets
           the PERLIO_F_LINEBUF flag and is normally sufficient.

       Get_base
                   STDCHAR *       (*Get_base)(pTHX_ PerlIO *f);

           Allocate (if not already done so) the read buffer for this layer
           and return pointer to it. Return NULL on failure.

       Get_bufsiz
                   Size_t  (*Get_bufsiz)(pTHX_ PerlIO *f);

           Return the number of bytes that last "Fill()" put in the buffer.

       Get_ptr
                   STDCHAR *       (*Get_ptr)(pTHX_ PerlIO *f);

           Return the current read pointer relative to this layer's buffer.

       Get_cnt
                   SSize_t (*Get_cnt)(pTHX_ PerlIO *f);

           Return the number of bytes left to be read in the current buffer.

       Set_ptrcnt
                   void    (*Set_ptrcnt)(pTHX_ PerlIO *f,
                                         STDCHAR *ptr, SSize_t cnt);

           Adjust the read pointer and count of bytes to match "ptr" and/or
           "cnt".  The application (or layer above) must ensure they are
           consistent.  (Checking is allowed by the paranoid.)

   Utilities
       calls the callback from the functions of the layer f (just by the name
       of the IO function, like "Read") with the args, or if there is no such
       callback, calls the base version of the callback with the same args, or
       if the f is invalid, set errno to EBADF and return failure.

       Perl_PerlIO_or_fail(PerlIO* f, callback, failure, args) either calls
       the callback of the functions of the layer f with the args, or if there
       is no such callback, set errno to EINVAL.  Or if the f is invalid, set
       errno to EBADF and return failure.

       Perl_PerlIO_or_Base_void(PerlIO* f, callback, base, args) either calls
       the callback of the functions of the layer f with the args, or if there
       is no such callback, calls the base version of the callback with the
       same args, or if the f is invalid, set errno to EBADF.

       Perl_PerlIO_or_fail_void(PerlIO* f, callback, args) either calls the
       callback of the functions of the layer f with the args, or if there is
       no such callback, set errno to EINVAL.  Or if the f is invalid, set
       errno to EBADF.

   Implementing PerlIO Layers
       If you find the implementation document unclear or not sufficient, look
       at the existing PerlIO layer implementations, which include:

       o   C implementations

           The perlio.c and perliol.h in the Perl core implement the "unix",
           "perlio", "stdio", "crlf", "utf8", "byte", "raw", "pending" layers,
           and also the "mmap" and "win32" layers if applicable.  (The "win32"
           is currently unfinished and unused, to see what is used instead in
           Win32, see "Querying the layers of filehandles" in PerlIO .)

           PerlIO::encoding, PerlIO::scalar, PerlIO::via in the Perl core.

           PerlIO::gzip and APR::PerlIO (mod_perl 2.0) on CPAN.

       o   Perl implementations

           PerlIO::via::QuotedPrint in the Perl core and PerlIO::via::* on
           CPAN.

       If you are creating a PerlIO layer, you may want to be lazy, in other
       words, implement only the methods that interest you.  The other methods
       you can either replace with the "blank" methods

           PerlIOBase_noop_ok
           PerlIOBase_noop_fail

       (which do nothing, and return zero and -1, respectively) or for certain
       methods you may assume a default behaviour by using a NULL method.  The
       Open method looks for help in the 'parent' layer.  The following table
       summarizes the behaviour:

           method      behaviour with NULL
           Get_cnt     FAILURE
           Get_ptr     FAILURE
           Open        INHERITED
           Popped      SUCCESS
           Pushed      SUCCESS
           Read        PerlIOBase_read
           Seek        FAILURE
           Set_cnt     FAILURE
           Set_ptrcnt  FAILURE
           Setlinebuf  PerlIOBase_setlinebuf
           Tell        FAILURE
           Unread      PerlIOBase_unread
           Write       FAILURE

        FAILURE        Set errno (to EINVAL in Unixish, to LIB$_INVARG in VMS) and
                       return -1 (for numeric return values) or NULL (for pointers)
        INHERITED      Inherited from the layer below
        SUCCESS        Return 0 (for numeric return values) or a pointer

   Core Layers
       The file "perlio.c" provides the following layers:

       "unix"
           A basic non-buffered layer which calls Unix/POSIX "read()",
           "write()", "lseek()", "close()". No buffering. Even on platforms
           that distinguish between O_TEXT and O_BINARY this layer is always
           O_BINARY.

       "perlio"
           A very complete generic buffering layer which provides the whole of
           PerlIO API. It is also intended to be used as a "base class" for
           other layers. (For example its "Read()" method is implemented in
           terms of the "Get_cnt()"/"Get_ptr()"/"Set_ptrcnt()" methods).

           "perlio" over "unix" provides a complete replacement for stdio as
           seen via PerlIO API. This is the default for USE_PERLIO when
           system's stdio does not permit perl's "fast gets" access, and which
           do not distinguish between "O_TEXT" and "O_BINARY".

       "stdio"
           A layer which provides the PerlIO API via the layer scheme, but
           implements it by calling system's stdio. This is (currently) the
           default if system's stdio provides sufficient access to allow
           perl's "fast gets" access and which do not distinguish between
           "O_TEXT" and "O_BINARY".

       "crlf"
           A layer derived using "perlio" as a base class. It provides
           Win32-like "\n" to CR,LF translation. Can either be applied above
           "perlio" or serve as the buffer layer itself. "crlf" over "unix" is
           the default if system distinguishes between "O_TEXT" and "O_BINARY"
           opens. (At some point "unix" will be replaced by a "native" Win32
           IO layer on that platform, as Win32's read/write layer has various
           drawbacks.) The "crlf" layer is a reasonable model for a layer
           Unread() function for layers which have no buffer or cannot be
           bothered.  (Basically this layer's "Fill()" pops itself off the
           stack and so resumes reading from layer below.)

       "raw"
           A dummy layer which never exists on the layer stack. Instead when
           "pushed" it actually pops the stack removing itself, it then calls
           Binmode function table entry on all the layers in the stack -
           normally this (via PerlIOBase_binmode) removes any layers which do
           not have "PERLIO_K_RAW" bit set. Layers can modify that behaviour
           by defining their own Binmode entry.

       "utf8"
           Another dummy layer. When pushed it pops itself and sets the
           "PERLIO_F_UTF8" flag on the layer which was (and now is once more)
           the top of the stack.

       In addition perlio.c also provides a number of "PerlIOBase_xxxx()"
       functions which are intended to be used in the table slots of classes
       which do not need to do anything special for a particular method.

   Extension Layers
       Layers can made available by extension modules. When an unknown layer
       is encountered the PerlIO code will perform the equivalent of :

          use PerlIO 'layer';

       Where layer is the unknown layer. PerlIO.pm will then attempt to:

          require PerlIO::layer;

       If after that process the layer is still not defined then the "open"
       will fail.

       The following extension layers are bundled with perl:

       ":encoding"
              use Encoding;

           makes this layer available, although PerlIO.pm "knows" where to
           find it.  It is an example of a layer which takes an argument as it
           is called thus:

              open( $fh, "<:encoding(iso-8859-7)", $pathname );

       ":scalar"
           Provides support for reading data from and writing data to a
           scalar.

              open( $fh, "+<:scalar", \$scalar );

           When a handle is so opened, then reads get bytes from the string
           value of $scalar, and writes change the value. In both cases the
           position in $scalar starts as zero but can be altered via "seek",

           See PerlIO::via for details.

TODO
       Things that need to be done to improve this document.

       o   Explain how to make a valid fh without going through open()(i.e.
           apply a layer). For example if the file is not opened through perl,
           but we want to get back a fh, like it was opened by Perl.

           How PerlIO_apply_layera fits in, where its docs, was it made
           public?

           Currently the example could be something like this:

             PerlIO *foo_to_PerlIO(pTHX_ char *mode, ...)
             {
                 char *mode; /* "w", "r", etc */
                 const char *layers = ":APR"; /* the layer name */
                 PerlIO *f = PerlIO_allocate(aTHX);
                 if (!f) {
                     return NULL;
                 }

                 PerlIO_apply_layers(aTHX_ f, mode, layers);

                 if (f) {
                     PerlIOAPR *st = PerlIOSelf(f, PerlIOAPR);
                     /* fill in the st struct, as in _open() */
                     st->file = file;
                     PerlIOBase(f)->flags |= PERLIO_F_OPEN;

                     return f;
                 }
                 return NULL;
             }

       o   fix/add the documentation in places marked as XXX.

       o   The handling of errors by the layer is not specified. e.g. when $!
           should be set explicitly, when the error handling should be just
           delegated to the top layer.

           Probably give some hints on using SETERRNO() or pointers to where
           they can be found.

       o   I think it would help to give some concrete examples to make it
           easier to understand the API. Of course I agree that the API has to
           be concise, but since there is no second document that is more of a
           guide, I think that it'd make it easier to start with the doc which
           is an API, but has examples in it in places where things are
           unclear, to a person who is not a PerlIO guru (yet).
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