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NAME

       perliol - C API for Perl's implementation of IO in Layers.

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.

       However, note that even as the specified layers are "pushed on top" for open() and
       binmode(), this doesn't mean that the effects are limited to the "top": PerlIO layers can
       be very 'active' and inspect and affect layers also deeper in the stack.  As an example
       there is a layer called "raw" which repeatedly "pops" layers until it reaches the first
       layer that has declared itself capable of handling binary data.  The "pushed" layers are
       processed in left-to-right order.

       sysopen() operates (unsurprisingly) at a lower level in the stack than open().  For
       example in Unix or Unix-like systems sysopen() operates directly at the level of file
       descriptors: in the terms of PerlIO 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
         {
          Size_t               fsize;
          char *               name;
          Size_t               size;
          IV           kind;
          IV           (*Pushed)(pTHX_ PerlIO *f,const char *mode,SV *arg, PerlIO_funcs *tab);
          IV           (*Popped)(pTHX_ PerlIO *f);
          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);
          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
               {
                struct _PerlIO base;       /* Base "class" info */
                STDCHAR *      buf;        /* Start of buffer */
                STDCHAR *      end;        /* End of valid part of buffer */
                STDCHAR *      ptr;        /* Current position in buffer */
                Off_t          posn;       /* Offset of buf into the file */
                Size_t         bufsiz;     /* Real size of buffer */
                IV             oneword;    /* Emergency buffer */
               } PerlIOBuf;

       In this way (as for perl's scalars) a pointer to a PerlIOBuf can be treated as a pointer
       to a PerlIOl.

   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:

       ·   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.

       ·   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.

       ·   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.

       ·   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_CANREAD
           Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick).

       PERLIO_F_ERROR
           An error has occurred (for "PerlIO_error()").

       PERLIO_F_TRUNCATE
           Truncate file suggested by open mode.

       PERLIO_F_APPEND
           All writes should be appends.

       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.

       name
                   char * name;

           The name of the layer whose open() method Perl should invoke on open().  For example
           if the layer is called APR, you will call:

             open $fh, ">:APR", ...

           and Perl knows that it has to invoke the PerlIOAPR_open() method implemented by the
           APR layer.

       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;

           ·   PERLIO_K_BUFFERED

               The layer is buffered.

           ·   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.

           ·   PERLIO_K_CANCRLF

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

           ·   PERLIO_K_FASTGETS

               Layer allows buffer snooping.

           ·   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 save the one passed to it, nor provide "Getarg()" (it could perhaps
           "Perl_warn" that the argument was un-expected).

           Returns 0 on success. On failure returns -1 and should set errno.

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

           Called when the layer is popped from the stack. A layer will normally be popped after
           "Close()" is called. But a layer can be popped without being closed if the program is
           dynamically managing layers on the stream. In such cases "Popped()" should free any
           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 nargs is greater than zero then it gives the number of arguments passed to "open",
           otherwise it will be 1 if for example "PerlIO_open" was called.  In simple cases
           SvPV_nolen(*args) is the pathname to open.

           If a layer provides "Open()" it should normally call the "Open()" method of next layer
           down (if any) and then push itself on top if that succeeds.  "PerlIOBase_open" is
           provided to do exactly that, so in most cases you don't have to write your own
           "Open()" method.  If this method is not defined, other layers may have difficulty
           pushing themselves on top of it during open.

           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.

           Typically will call "Fill" and manipulate pointers (possibly via the API).
           "PerlIOBuf_read()" may be suitable for derived classes which provide "fast gets"
           methods.

           Returns actual bytes read, or -1 on an error.

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

           A superset of stdio's "ungetc()". Should arrange for future reads 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 below.  When you
           "subclass" PerlIOBuf layer, you want to use its _read method and to supply your own
           fill method, which fills the PerlIOBuf's buffer.

           Returns 0 on success, -1 on failure.

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

           Return end-of-file indicator. "PerlIOBase_eof()" is normally sufficient.

           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
       To ask for the next layer down use PerlIONext(PerlIO *f).

       To check that a PerlIO* is valid use PerlIOValid(PerlIO *f).  (All this does is really
       just to check that the pointer is non-NULL and that the pointer behind that is non-NULL.)

       PerlIOBase(PerlIO *f) returns the "Base" pointer, or in other words, the "PerlIOl*"
       pointer.

       PerlIOSelf(PerlIO* f, type) return the PerlIOBase cast to a type.

       Perl_PerlIO_or_Base(PerlIO* f, callback, base, failure, args) either 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:

       ·   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.

       ·   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

           Clearerr    PerlIOBase_clearerr
           Close       PerlIOBase_close
           Dup         PerlIOBase_dup
           Eof         PerlIOBase_eof
           Error       PerlIOBase_error
           Fileno      PerlIOBase_fileno
           Fill        FAILURE
           Flush       SUCCESS
           Getarg      SUCCESS
           Get_base    FAILURE
           Get_bufsiz  FAILURE
           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 which transforms data in some way.

       "mmap"
           If Configure detects "mmap()" functions this layer is provided (with "perlio" as a
           "base") which does "read" operations by mmap()ing the file. Performance improvement is
           marginal on modern systems, so it is mainly there as a proof of concept. It is likely
           to be unbundled from the core at some point. The "mmap" layer is a reasonable model
           for a minimalist "derived" layer.

       "pending"
           An "internal" derivative of "perlio" which can be used to provide 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 be 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", and determined via "tell".

           Please note that this layer is implied when calling open() thus:

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

       ":via"
           Provided to allow layers to be implemented as Perl code.  For instance:

              use PerlIO::via::StripHTML;
              open( my $fh, "<:via(StripHTML)", "index.html" );

           See PerlIO::via for details.

TODO

       Things that need to be done to improve this document.

       ·   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;
             }

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

       ·   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.

       ·   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).