


PERLIOL(1)       Perl Programmers Reference Guide      PERLIOL(1)


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



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       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 */



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               };

       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);



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          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:












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               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:

       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



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           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_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.



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       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.:




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             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



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           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



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           "/^[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.

           Having said all that translation-only layers do not
           need to provide "Open()" at all, but rather leave the
           opening to a lower level layer and wait to be
           "pushed".  If a layer does provide "Open()" it should
           normally call the "Open()" method of next layer down
           (if any) and then push itself on top if that succeeds.

           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




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                   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);



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           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.



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       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



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           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



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       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

           Clearerr    PerlIOBase_clearerr
           Close       PerlIOBase_close
           Dup         PerlIOBase_dup
           Eof         PerlIOBase_eof
           Error       PerlIOBase_error
           Fileno      PerlIOBase_fileno



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           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".




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

          use PerlIO 'layer';



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       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.

       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



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           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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