3 perliol - C API for Perl's implementation of IO in Layers.
7 /* Defining a layer ... */
12 This document describes the behavior and implementation of the PerlIO
13 abstraction described in L<perlapio> when C<USE_PERLIO> is defined (and
16 =head2 History and Background
18 The PerlIO abstraction was introduced in perl5.003_02 but languished as
19 just an abstraction until perl5.7.0. However during that time a number
20 of perl extensions switched to using it, so the API is mostly fixed to
21 maintain (source) compatibility.
23 The aim of the implementation is to provide the PerlIO API in a flexible
24 and platform neutral manner. It is also a trial of an "Object Oriented
25 C, with vtables" approach which may be applied to perl6.
27 =head2 Layers vs Disciplines
29 Initial discussion of the ability to modify IO streams behaviour used
30 the term "discipline" for the entities which were added. This came (I
31 believe) from the use of the term in "sfio", which in turn borrowed it
32 from "line disciplines" on Unix terminals. However, this document (and
33 the C code) uses the term "layer".
35 This is, I hope, a natural term given the implementation, and should
36 avoid connotations that are inherent in earlier uses of "discipline"
37 for things which are rather different.
39 =head2 Data Structures
41 The basic data structure is a PerlIOl:
43 typedef struct _PerlIO PerlIOl;
44 typedef struct _PerlIO_funcs PerlIO_funcs;
45 typedef PerlIOl *PerlIO;
49 PerlIOl * next; /* Lower layer */
50 PerlIO_funcs * tab; /* Functions for this layer */
51 IV flags; /* Various flags for state */
54 A C<PerlIOl *> is a pointer to the struct, and the I<application>
55 level C<PerlIO *> is a pointer to a C<PerlIOl *> - i.e. a pointer
56 to a pointer to the struct. This allows the application level C<PerlIO *>
57 to remain constant while the actual C<PerlIOl *> underneath
58 changes. (Compare perl's C<SV *> which remains constant while its
59 C<sv_any> field changes as the scalar's type changes.) An IO stream is
60 then in general represented as a pointer to this linked-list of
63 It should be noted that because of the double indirection in a C<PerlIO *>,
64 a C<< &(perlio->next) >> "is" a C<PerlIO *>, and so to some degree
65 at least one layer can use the "standard" API on the next layer down.
67 A "layer" is composed of two parts:
73 The functions and attributes of the "layer class".
77 The per-instance data for a particular handle.
81 =head2 Functions and Attributes
83 The functions and attributes are accessed via the "tab" (for table)
84 member of C<PerlIOl>. The functions (methods of the layer "class") are
85 fixed, and are defined by the C<PerlIO_funcs> type. They are broadly the
86 same as the public C<PerlIO_xxxxx> functions:
94 IV (*Pushed)(pTHX_ PerlIO *f,const char *mode,SV *arg, PerlIO_funcs *tab);
95 IV (*Popped)(pTHX_ PerlIO *f);
96 PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab,
99 int fd, int imode, int perm,
101 int narg, SV **args);
102 IV (*Binmode)(pTHX_ PerlIO *f);
103 SV * (*Getarg)(pTHX_ PerlIO *f, CLONE_PARAMS *param, int flags)
104 IV (*Fileno)(pTHX_ PerlIO *f);
105 PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o, CLONE_PARAMS *param, int flags)
106 /* Unix-like functions - cf sfio line disciplines */
107 SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
108 SSize_t (*Unread)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
109 SSize_t (*Write)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
110 IV (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
111 Off_t (*Tell)(pTHX_ PerlIO *f);
112 IV (*Close)(pTHX_ PerlIO *f);
113 /* Stdio-like buffered IO functions */
114 IV (*Flush)(pTHX_ PerlIO *f);
115 IV (*Fill)(pTHX_ PerlIO *f);
116 IV (*Eof)(pTHX_ PerlIO *f);
117 IV (*Error)(pTHX_ PerlIO *f);
118 void (*Clearerr)(pTHX_ PerlIO *f);
119 void (*Setlinebuf)(pTHX_ PerlIO *f);
120 /* Perl's snooping functions */
121 STDCHAR * (*Get_base)(pTHX_ PerlIO *f);
122 Size_t (*Get_bufsiz)(pTHX_ PerlIO *f);
123 STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f);
124 SSize_t (*Get_cnt)(pTHX_ PerlIO *f);
125 void (*Set_ptrcnt)(pTHX_ PerlIO *f,STDCHAR *ptr,SSize_t cnt);
128 The first few members of the struct give a function table size for
129 compatibility check "name" for the layer, the size to C<malloc> for the per-instance data,
130 and some flags which are attributes of the class as whole (such as whether it is a buffering
131 layer), then follow the functions which fall into four basic groups:
137 Opening and setup functions
145 Stdio class buffering options.
149 Functions to support Perl's traditional "fast" access to the buffer.
153 A layer does not have to implement all the functions, but the whole
154 table has to be present. Unimplemented slots can be NULL (which will
155 result in an error when called) or can be filled in with stubs to
156 "inherit" behaviour from a "base class". This "inheritance" is fixed
157 for all instances of the layer, but as the layer chooses which stubs
158 to populate the table, limited "multiple inheritance" is possible.
160 =head2 Per-instance Data
162 The per-instance data are held in memory beyond the basic PerlIOl
163 struct, by making a PerlIOl the first member of the layer's struct
168 struct _PerlIO base; /* Base "class" info */
169 STDCHAR * buf; /* Start of buffer */
170 STDCHAR * end; /* End of valid part of buffer */
171 STDCHAR * ptr; /* Current position in buffer */
172 Off_t posn; /* Offset of buf into the file */
173 Size_t bufsiz; /* Real size of buffer */
174 IV oneword; /* Emergency buffer */
177 In this way (as for perl's scalars) a pointer to a PerlIOBuf can be
178 treated as a pointer to a PerlIOl.
180 =head2 Layers in action.
184 +-----------+ +----------+ +--------+
185 PerlIO ->| |--->| next |--->| NULL |
186 +-----------+ +----------+ +--------+
187 | | | buffer | | fd |
188 +-----------+ | | +--------+
192 The above attempts to show how the layer scheme works in a simple case.
193 The application's C<PerlIO *> points to an entry in the table(s)
194 representing open (allocated) handles. For example the first three slots
195 in the table correspond to C<stdin>,C<stdout> and C<stderr>. The table
196 in turn points to the current "top" layer for the handle - in this case
197 an instance of the generic buffering layer "perlio". That layer in turn
198 points to the next layer down - in this case the lowlevel "unix" layer.
200 The above is roughly equivalent to a "stdio" buffered stream, but with
201 much more flexibility:
207 If Unix level C<read>/C<write>/C<lseek> is not appropriate for (say)
208 sockets then the "unix" layer can be replaced (at open time or even
209 dynamically) with a "socket" layer.
213 Different handles can have different buffering schemes. The "top"
214 layer could be the "mmap" layer if reading disk files was quicker
215 using C<mmap> than C<read>. An "unbuffered" stream can be implemented
216 simply by not having a buffer layer.
220 Extra layers can be inserted to process the data as it flows through.
221 This was the driving need for including the scheme in perl 5.7.0+ - we
222 needed a mechanism to allow data to be translated between perl's
223 internal encoding (conceptually at least Unicode as UTF-8), and the
224 "native" format used by the system. This is provided by the
225 ":encoding(xxxx)" layer which typically sits above the buffering layer.
229 A layer can be added that does "\n" to CRLF translation. This layer
230 can be used on any platform, not just those that normally do such
235 =head2 Per-instance flag bits
237 The generic flag bits are a hybrid of C<O_XXXXX> style flags deduced
238 from the mode string passed to C<PerlIO_open()>, and state bits for
239 typical buffer layers.
247 =item PERLIO_F_CANWRITE
249 Writes are permitted, i.e. opened as "w" or "r+" or "a", etc.
251 =item PERLIO_F_CANREAD
253 Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick).
257 An error has occurred (for C<PerlIO_error()>).
259 =item PERLIO_F_TRUNCATE
261 Truncate file suggested by open mode.
263 =item PERLIO_F_APPEND
265 All writes should be appends.
269 Layer is performing Win32-like "\n" mapped to CR,LF for output and CR,LF
270 mapped to "\n" for input. Normally the provided "crlf" layer is the only
271 layer that need bother about this. C<PerlIO_binmode()> will mess with this
272 flag rather than add/remove layers if the C<PERLIO_K_CANCRLF> bit is set
273 for the layers class.
277 Data written to this layer should be UTF-8 encoded; data provided
278 by this layer should be considered UTF-8 encoded. Can be set on any layer
279 by ":utf8" dummy layer. Also set on ":encoding" layer.
283 Layer is unbuffered - i.e. write to next layer down should occur for
284 each write to this layer.
288 The buffer for this layer currently holds data written to it but not sent
293 The buffer for this layer currently holds unconsumed data read from
296 =item PERLIO_F_LINEBUF
298 Layer is line buffered. Write data should be passed to next layer down
299 whenever a "\n" is seen. Any data beyond the "\n" should then be
304 File has been C<unlink()>ed, or should be deleted on C<close()>.
310 =item PERLIO_F_FASTGETS
312 This instance of this layer supports the "fast C<gets>" interface.
313 Normally set based on C<PERLIO_K_FASTGETS> for the class and by the
314 existence of the function(s) in the table. However a class that
315 normally provides that interface may need to avoid it on a
316 particular instance. The "pending" layer needs to do this when
317 it is pushed above a layer which does not support the interface.
318 (Perl's C<sv_gets()> does not expect the streams fast C<gets> behaviour
319 to change during one "get".)
323 =head2 Methods in Detail
331 Size of the function table. This is compared against the value PerlIO
332 code "knows" as a compatibility check. Future versions I<may> be able
333 to tolerate layers compiled against an old version of the headers.
339 The name of the layer whose open() method Perl should invoke on
340 open(). For example if the layer is called APR, you will call:
342 open $fh, ">:APR", ...
344 and Perl knows that it has to invoke the PerlIOAPR_open() method
345 implemented by the APR layer.
351 The size of the per-instance data structure, e.g.:
355 If this field is zero then C<PerlIO_pushed> does not malloc anything
356 and assumes layer's Pushed function will do any required layer stack
357 manipulation - used to avoid malloc/free overhead for dummy layers.
358 If the field is non-zero it must be at least the size of C<PerlIOl>,
359 C<PerlIO_pushed> will allocate memory for the layer's data structures
360 and link new layer onto the stream's stack. (If the layer's Pushed
361 method returns an error indication the layer is popped again.)
369 =item * PERLIO_K_BUFFERED
371 The layer is buffered.
375 The layer is acceptable to have in a binmode(FH) stack - i.e. it does not
376 (or will configure itself not to) transform bytes passing through it.
378 =item * PERLIO_K_CANCRLF
380 Layer can translate between "\n" and CRLF line ends.
382 =item * PERLIO_K_FASTGETS
384 Layer allows buffer snooping.
386 =item * PERLIO_K_MULTIARG
388 Used when the layer's open() accepts more arguments than usual. The
389 extra arguments should come not before the C<MODE> argument. When this
390 flag is used it's up to the layer to validate the args.
396 IV (*Pushed)(pTHX_ PerlIO *f,const char *mode, SV *arg);
398 The only absolutely mandatory method. Called when the layer is pushed
399 onto the stack. The C<mode> argument may be NULL if this occurs
400 post-open. The C<arg> will be non-C<NULL> if an argument string was
401 passed. In most cases this should call C<PerlIOBase_pushed()> to
402 convert C<mode> into the appropriate C<PERLIO_F_XXXXX> flags in
403 addition to any actions the layer itself takes. If a layer is not
404 expecting an argument it need neither save the one passed to it, nor
405 provide C<Getarg()> (it could perhaps C<Perl_warn> that the argument
408 Returns 0 on success. On failure returns -1 and should set errno.
412 IV (*Popped)(pTHX_ PerlIO *f);
414 Called when the layer is popped from the stack. A layer will normally
415 be popped after C<Close()> is called. But a layer can be popped
416 without being closed if the program is dynamically managing layers on
417 the stream. In such cases C<Popped()> should free any resources
418 (buffers, translation tables, ...) not held directly in the layer's
419 struct. It should also C<Unread()> any unconsumed data that has been
420 read and buffered from the layer below back to that layer, so that it
421 can be re-provided to what ever is now above.
423 Returns 0 on success and failure. If C<Popped()> returns I<true> then
424 I<perlio.c> assumes that either the layer has popped itself, or the
425 layer is super special and needs to be retained for other reasons.
426 In most cases it should return I<false>.
430 PerlIO * (*Open)(...);
432 The C<Open()> method has lots of arguments because it combines the
433 functions of perl's C<open>, C<PerlIO_open>, perl's C<sysopen>,
434 C<PerlIO_fdopen> and C<PerlIO_reopen>. The full prototype is as
437 PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab,
440 int fd, int imode, int perm,
442 int narg, SV **args);
444 Open should (perhaps indirectly) call C<PerlIO_allocate()> to allocate
445 a slot in the table and associate it with the layers information for
446 the opened file, by calling C<PerlIO_push>. The I<layers> AV is an
447 array of all the layers destined for the C<PerlIO *>, and any
448 arguments passed to them, I<n> is the index into that array of the
449 layer being called. The macro C<PerlIOArg> will return a (possibly
450 C<NULL>) SV * for the argument passed to the layer.
452 The I<mode> string is an "C<fopen()>-like" string which would match
453 the regular expression C</^[I#]?[rwa]\+?[bt]?$/>.
455 The C<'I'> prefix is used during creation of C<stdin>..C<stderr> via
456 special C<PerlIO_fdopen> calls; the C<'#'> prefix means that this is
457 C<sysopen> and that I<imode> and I<perm> should be passed to
458 C<PerlLIO_open3>; C<'r'> means B<r>ead, C<'w'> means B<w>rite and
459 C<'a'> means B<a>ppend. The C<'+'> suffix means that both reading and
460 writing/appending are permitted. The C<'b'> suffix means file should
461 be binary, and C<'t'> means it is text. (Almost all layers should do
462 the IO in binary mode, and ignore the b/t bits. The C<:crlf> layer
463 should be pushed to handle the distinction.)
465 If I<old> is not C<NULL> then this is a C<PerlIO_reopen>. Perl itself
466 does not use this (yet?) and semantics are a little vague.
468 If I<fd> not negative then it is the numeric file descriptor I<fd>,
469 which will be open in a manner compatible with the supplied mode
470 string, the call is thus equivalent to C<PerlIO_fdopen>. In this case
471 I<nargs> will be zero.
473 If I<nargs> is greater than zero then it gives the number of arguments
474 passed to C<open>, otherwise it will be 1 if for example
475 C<PerlIO_open> was called. In simple cases SvPV_nolen(*args) is the
478 Having said all that translation-only layers do not need to provide
479 C<Open()> at all, but rather leave the opening to a lower level layer
480 and wait to be "pushed". If a layer does provide C<Open()> it should
481 normally call the C<Open()> method of next layer down (if any) and
482 then push itself on top if that succeeds.
484 If C<PerlIO_push> was performed and open has failed, it must
485 C<PerlIO_pop> itself, since if it's not, the layer won't be removed
486 and may cause bad problems.
488 Returns C<NULL> on failure.
492 IV (*Binmode)(pTHX_ PerlIO *f);
494 Optional. Used when C<:raw> layer is pushed (explicitly or as a result
495 of binmode(FH)). If not present layer will be popped. If present
496 should configure layer as binary (or pop itself) and return 0.
497 If it returns -1 for error C<binmode> will fail with layer
502 SV * (*Getarg)(pTHX_ PerlIO *f,
503 CLONE_PARAMS *param, int flags);
505 Optional. If present should return an SV * representing the string
506 argument passed to the layer when it was
507 pushed. e.g. ":encoding(ascii)" would return an SvPV with value
508 "ascii". (I<param> and I<flags> arguments can be ignored in most
513 IV (*Fileno)(pTHX_ PerlIO *f);
515 Returns the Unix/Posix numeric file descriptor for the handle. Normally
516 C<PerlIOBase_fileno()> (which just asks next layer down) will suffice
519 Returns -1 on error, which is considered to include the case where the
520 layer cannot provide such a file descriptor.
524 PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
525 CLONE_PARAMS *param, int flags);
527 XXX: Needs more docs.
529 Used as part of the "clone" process when a thread is spawned (in which
530 case param will be non-NULL) and when a stream is being duplicated via
533 Similar to C<Open>, returns PerlIO* on success, C<NULL> on failure.
537 SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
539 Basic read operation.
541 Typically will call C<Fill> and manipulate pointers (possibly via the
542 API). C<PerlIOBuf_read()> may be suitable for derived classes which
543 provide "fast gets" methods.
545 Returns actual bytes read, or -1 on an error.
549 SSize_t (*Unread)(pTHX_ PerlIO *f,
550 const void *vbuf, Size_t count);
552 A superset of stdio's C<ungetc()>. Should arrange for future reads to
553 see the bytes in C<vbuf>. If there is no obviously better implementation
554 then C<PerlIOBase_unread()> provides the function by pushing a "fake"
555 "pending" layer above the calling layer.
557 Returns the number of unread chars.
561 SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count);
563 Basic write operation.
565 Returns bytes written or -1 on an error.
569 IV (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
571 Position the file pointer. Should normally call its own C<Flush>
572 method and then the C<Seek> method of next layer down.
574 Returns 0 on success, -1 on failure.
578 Off_t (*Tell)(pTHX_ PerlIO *f);
580 Return the file pointer. May be based on layers cached concept of
581 position to avoid overhead.
583 Returns -1 on failure to get the file pointer.
587 IV (*Close)(pTHX_ PerlIO *f);
589 Close the stream. Should normally call C<PerlIOBase_close()> to flush
590 itself and close layers below, and then deallocate any data structures
591 (buffers, translation tables, ...) not held directly in the data
594 Returns 0 on success, -1 on failure.
598 IV (*Flush)(pTHX_ PerlIO *f);
600 Should make stream's state consistent with layers below. That is, any
601 buffered write data should be written, and file position of lower layers
602 adjusted for data read from below but not actually consumed.
603 (Should perhaps C<Unread()> such data to the lower layer.)
605 Returns 0 on success, -1 on failure.
609 IV (*Fill)(pTHX_ PerlIO *f);
611 The buffer for this layer should be filled (for read) from layer
612 below. When you "subclass" PerlIOBuf layer, you want to use its
613 I<_read> method and to supply your own fill method, which fills the
616 Returns 0 on success, -1 on failure.
620 IV (*Eof)(pTHX_ PerlIO *f);
622 Return end-of-file indicator. C<PerlIOBase_eof()> is normally sufficient.
624 Returns 0 on end-of-file, 1 if not end-of-file, -1 on error.
628 IV (*Error)(pTHX_ PerlIO *f);
630 Return error indicator. C<PerlIOBase_error()> is normally sufficient.
632 Returns 1 if there is an error (usually when C<PERLIO_F_ERROR> is set,
637 void (*Clearerr)(pTHX_ PerlIO *f);
639 Clear end-of-file and error indicators. Should call C<PerlIOBase_clearerr()>
640 to set the C<PERLIO_F_XXXXX> flags, which may suffice.
644 void (*Setlinebuf)(pTHX_ PerlIO *f);
646 Mark the stream as line buffered. C<PerlIOBase_setlinebuf()> sets the
647 PERLIO_F_LINEBUF flag and is normally sufficient.
651 STDCHAR * (*Get_base)(pTHX_ PerlIO *f);
653 Allocate (if not already done so) the read buffer for this layer and
654 return pointer to it. Return NULL on failure.
658 Size_t (*Get_bufsiz)(pTHX_ PerlIO *f);
660 Return the number of bytes that last C<Fill()> put in the buffer.
664 STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f);
666 Return the current read pointer relative to this layer's buffer.
670 SSize_t (*Get_cnt)(pTHX_ PerlIO *f);
672 Return the number of bytes left to be read in the current buffer.
676 void (*Set_ptrcnt)(pTHX_ PerlIO *f,
677 STDCHAR *ptr, SSize_t cnt);
679 Adjust the read pointer and count of bytes to match C<ptr> and/or C<cnt>.
680 The application (or layer above) must ensure they are consistent.
681 (Checking is allowed by the paranoid.)
685 =head2 Implementing PerlIO Layers
687 If you are creating a PerlIO layer, you may want to be lazy, in other
688 words, implement only the methods that interest you. The other methods
689 you can either replace with the "blank" methods
694 (which do nothing, and return zero and -1, respectively) or for
695 certain methods you may assume a default behaviour by using a NULL
696 method. The Open method looks for help in the 'parent' layer.
697 The following table summarizes the behaviour:
699 method behaviour with NULL
701 Clearerr PerlIOBase_clearerr
702 Close PerlIOBase_close
705 Error PerlIOBase_error
706 Fileno PerlIOBase_fileno
721 Setlinebuf PerlIOBase_setlinebuf
723 Unread PerlIOBase_unread
726 FAILURE Set errno (to EINVAL in UNIXish, to LIB$_INVARG in VMS) and
727 return -1 (for numeric return values) or NULL (for pointers)
728 INHERITED Inherited from the layer below
729 SUCCESS Return 0 (for numeric return values) or a pointer
733 The file C<perlio.c> provides the following layers:
739 A basic non-buffered layer which calls Unix/POSIX C<read()>, C<write()>,
740 C<lseek()>, C<close()>. No buffering. Even on platforms that distinguish
741 between O_TEXT and O_BINARY this layer is always O_BINARY.
745 A very complete generic buffering layer which provides the whole of
746 PerlIO API. It is also intended to be used as a "base class" for other
747 layers. (For example its C<Read()> method is implemented in terms of
748 the C<Get_cnt()>/C<Get_ptr()>/C<Set_ptrcnt()> methods).
750 "perlio" over "unix" provides a complete replacement for stdio as seen
751 via PerlIO API. This is the default for USE_PERLIO when system's stdio
752 does not permit perl's "fast gets" access, and which do not
753 distinguish between C<O_TEXT> and C<O_BINARY>.
757 A layer which provides the PerlIO API via the layer scheme, but
758 implements it by calling system's stdio. This is (currently) the default
759 if system's stdio provides sufficient access to allow perl's "fast gets"
760 access and which do not distinguish between C<O_TEXT> and C<O_BINARY>.
764 A layer derived using "perlio" as a base class. It provides Win32-like
765 "\n" to CR,LF translation. Can either be applied above "perlio" or serve
766 as the buffer layer itself. "crlf" over "unix" is the default if system
767 distinguishes between C<O_TEXT> and C<O_BINARY> opens. (At some point
768 "unix" will be replaced by a "native" Win32 IO layer on that platform,
769 as Win32's read/write layer has various drawbacks.) The "crlf" layer is
770 a reasonable model for a layer which transforms data in some way.
774 If Configure detects C<mmap()> functions this layer is provided (with
775 "perlio" as a "base") which does "read" operations by mmap()ing the
776 file. Performance improvement is marginal on modern systems, so it is
777 mainly there as a proof of concept. It is likely to be unbundled from
778 the core at some point. The "mmap" layer is a reasonable model for a
779 minimalist "derived" layer.
783 An "internal" derivative of "perlio" which can be used to provide
784 Unread() function for layers which have no buffer or cannot be
785 bothered. (Basically this layer's C<Fill()> pops itself off the stack
786 and so resumes reading from layer below.)
790 A dummy layer which never exists on the layer stack. Instead when
791 "pushed" it actually pops the stack removing itself, it then calls
792 Binmode function table entry on all the layers in the stack - normally
793 this (via PerlIOBase_binmode) removes any layers which do not have
794 C<PERLIO_K_RAW> bit set. Layers can modify that behaviour by defining
795 their own Binmode entry.
799 Another dummy layer. When pushed it pops itself and sets the
800 C<PERLIO_F_UTF8> flag on the layer which was (and now is once more)
801 the top of the stack.
805 In addition F<perlio.c> also provides a number of C<PerlIOBase_xxxx()>
806 functions which are intended to be used in the table slots of classes
807 which do not need to do anything special for a particular method.
809 =head2 Extension Layers
811 Layers can made available by extension modules. When an unknown layer
812 is encountered the PerlIO code will perform the equivalent of :
816 Where I<layer> is the unknown layer. F<PerlIO.pm> will then attempt to:
818 require PerlIO::layer;
820 If after that process the layer is still not defined then the C<open>
823 The following extension layers are bundled with perl:
831 makes this layer available, although F<PerlIO.pm> "knows" where to
832 find it. It is an example of a layer which takes an argument as it is
835 open( $fh, "<:encoding(iso-8859-7)", $pathname );
839 Provides support for reading data from and writing data to a scalar.
841 open( $fh, "+<:scalar", \$scalar );
843 When a handle is so opened, then reads get bytes from the string value
844 of I<$scalar>, and writes change the value. In both cases the position
845 in I<$scalar> starts as zero but can be altered via C<seek>, and
846 determined via C<tell>.
848 Please note that this layer is implied when calling open() thus:
850 open( $fh, "+<", \$scalar );
854 Provided to allow layers to be implemented as Perl code. For instance:
856 use PerlIO::via::StripHTML;
857 open( my $fh, "<:via(StripHTML)", "index.html" );
859 See L<PerlIO::via> for details.
865 Things that need to be done to improve this document.
871 Explain how to make a valid fh without going through open()(i.e. apply
872 a layer). For example if the file is not opened through perl, but we
873 want to get back a fh, like it was opened by Perl.
875 How PerlIO_apply_layera fits in, where its docs, was it made public?
877 Currently the example could be something like this:
879 PerlIO *foo_to_PerlIO(pTHX_ char *mode, ...)
881 char *mode; /* "w", "r", etc */
882 const char *layers = ":APR"; /* the layer name */
883 PerlIO *f = PerlIO_allocate(aTHX);
888 PerlIO_apply_layers(aTHX_ f, mode, layers);
891 PerlIOAPR *st = PerlIOSelf(f, PerlIOAPR);
892 /* fill in the st struct, as in _open() */
894 PerlIOBase(f)->flags |= PERLIO_F_OPEN;
903 fix/add the documentation in places marked as XXX.
907 The handling of errors by the layer is not specified. e.g. when $!
908 should be set explicitly, when the error handling should be just
909 delegated to the top layer.
911 Probably give some hints on using SETERRNO() or pointers to where they
916 I think it would help to give some concrete examples to make it easier
917 to understand the API. Of course I agree that the API has to be
918 concise, but since there is no second document that is more of a
919 guide, I think that it'd make it easier to start with the doc which is
920 an API, but has examples in it in places where things are unclear, to
921 a person who is not a PerlIO guru (yet).