MirOS Manual: perliol(1)


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 perl6.

     Basic Structure

     PerlIO is a stack of layers.

     The low levels of the stack work with the low-level operat-
     ing 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.

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     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 descrip-
     tors: 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;

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             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 applica-
     tion level "PerlIO *" is a pointer to a "PerlIOl *" - i.e. a
     pointer to a pointer to the struct. This allows the applica-
     tion 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:

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       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,
                             AV *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.

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     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
     lowlevel "unix" layer.

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     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.

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     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".)

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     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 mal-
         loc 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.

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         * 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 uncon-
         sumed 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".

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         The full prototype is as follows:

          PerlIO *       (*Open)(pTHX_ PerlIO_funcs *tab,
                                 AV *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 AV 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 '+' suf-
         fix 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 distinc-
         tion.)

         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 descrip-
         tor 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

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         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
                 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 descrip-
         tor.

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

         XXX: Needs more docs.

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         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 (pos-
         sibly 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.

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         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 nor-
         mally 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.

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     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).

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     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 suf-
     ficient, 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 unfin-
         ished 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.

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     * 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

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     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 pro-
         vides 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 pro-
         vides 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 dis-
         tinguishes 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 improve-
         ment is marginal on modern systems, so it is mainly
         there as a proof of concept. It is likely to be unbun-
         dled from the core at some point. The "mmap" layer is a
         reasonable model for a minimalist "derived" layer.

     "pending"

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         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 read-
         ing from layer below.)

     "raw"
         A dummy layer which never exists on the layer stack.
         Instead when "pushed" it actually pops the stack remov-
         ing 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 any-
     thing special for a particular method.

     Extension Layers

     Layers can made available by extension modules. When an unk-
     nown 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 );

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     ":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);

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               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).

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