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.
427 PerlIO * (*Open)(...);
429 The C<Open()> method has lots of arguments because it combines the
430 functions of perl's C<open>, C<PerlIO_open>, perl's C<sysopen>,
431 C<PerlIO_fdopen> and C<PerlIO_reopen>. The full prototype is as
434 PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab,
437 int fd, int imode, int perm,
439 int narg, SV **args);
441 Open should (perhaps indirectly) call C<PerlIO_allocate()> to allocate
442 a slot in the table and associate it with the layers information for
443 the opened file, by calling C<PerlIO_push>. The I<layers> AV is an
444 array of all the layers destined for the C<PerlIO *>, and any
445 arguments passed to them, I<n> is the index into that array of the
446 layer being called. The macro C<PerlIOArg> will return a (possibly
447 C<NULL>) SV * for the argument passed to the layer.
449 The I<mode> string is an "C<fopen()>-like" string which would match
450 the regular expression C</^[I#]?[rwa]\+?[bt]?$/>.
452 The C<'I'> prefix is used during creation of C<stdin>..C<stderr> via
453 special C<PerlIO_fdopen> calls; the C<'#'> prefix means that this is
454 C<sysopen> and that I<imode> and I<perm> should be passed to
455 C<PerlLIO_open3>; C<'r'> means B<r>ead, C<'w'> means B<w>rite and
456 C<'a'> means B<a>ppend. The C<'+'> suffix means that both reading and
457 writing/appending are permitted. The C<'b'> suffix means file should
458 be binary, and C<'t'> means it is text. (Almost all layers should do
459 the IO in binary mode, and ignore the b/t bits. The C<:crlf> layer
460 should be pushed to handle the distinction.)
462 If I<old> is not C<NULL> then this is a C<PerlIO_reopen>. Perl itself
463 does not use this (yet?) and semantics are a little vague.
465 If I<fd> not negative then it is the numeric file descriptor I<fd>,
466 which will be open in a manner compatible with the supplied mode
467 string, the call is thus equivalent to C<PerlIO_fdopen>. In this case
468 I<nargs> will be zero.
470 If I<nargs> is greater than zero then it gives the number of arguments
471 passed to C<open>, otherwise it will be 1 if for example
472 C<PerlIO_open> was called. In simple cases SvPV_nolen(*args) is the
475 Having said all that translation-only layers do not need to provide
476 C<Open()> at all, but rather leave the opening to a lower level layer
477 and wait to be "pushed". If a layer does provide C<Open()> it should
478 normally call the C<Open()> method of next layer down (if any) and
479 then push itself on top if that succeeds.
481 Returns C<NULL> on failure.
485 IV (*Binmode)(pTHX_ PerlIO *f);
487 Optional. Used when C<:raw> layer is pushed (explicitly or as a result
488 of binmode(FH)). If not present layer will be popped. If present
489 should configure layer as binary (or pop itself) and return 0.
490 If it returns -1 for error C<binmode> will fail with layer
495 SV * (*Getarg)(pTHX_ PerlIO *f,
496 CLONE_PARAMS *param, int flags);
498 Optional. If present should return an SV * representing the string
499 argument passed to the layer when it was
500 pushed. e.g. ":encoding(ascii)" would return an SvPV with value
501 "ascii". (I<param> and I<flags> arguments can be ignored in most
506 IV (*Fileno)(pTHX_ PerlIO *f);
508 Returns the Unix/Posix numeric file descriptor for the handle. Normally
509 C<PerlIOBase_fileno()> (which just asks next layer down) will suffice
512 Returns -1 on error, which is considered to include the case where the
513 layer cannot provide such a file descriptor.
517 PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
518 CLONE_PARAMS *param, int flags);
520 XXX: Needs more docs.
522 Used as part of the "clone" process when a thread is spawned (in which
523 case param will be non-NULL) and when a stream is being duplicated via
526 Similar to C<Open>, returns PerlIO* on success, C<NULL> on failure.
530 SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
532 Basic read operation.
534 Typically will call C<Fill> and manipulate pointers (possibly via the
535 API). C<PerlIOBuf_read()> may be suitable for derived classes which
536 provide "fast gets" methods.
538 Returns actual bytes read, or -1 on an error.
542 SSize_t (*Unread)(pTHX_ PerlIO *f,
543 const void *vbuf, Size_t count);
545 A superset of stdio's C<ungetc()>. Should arrange for future reads to
546 see the bytes in C<vbuf>. If there is no obviously better implementation
547 then C<PerlIOBase_unread()> provides the function by pushing a "fake"
548 "pending" layer above the calling layer.
550 Returns the number of unread chars.
554 SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count);
556 Basic write operation.
558 Returns bytes written or -1 on an error.
562 IV (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
564 Position the file pointer. Should normally call its own C<Flush>
565 method and then the C<Seek> method of next layer down.
567 Returns 0 on success, -1 on failure.
571 Off_t (*Tell)(pTHX_ PerlIO *f);
573 Return the file pointer. May be based on layers cached concept of
574 position to avoid overhead.
576 Returns -1 on failure to get the file pointer.
580 IV (*Close)(pTHX_ PerlIO *f);
582 Close the stream. Should normally call C<PerlIOBase_close()> to flush
583 itself and close layers below, and then deallocate any data structures
584 (buffers, translation tables, ...) not held directly in the data
587 Returns 0 on success, -1 on failure.
591 IV (*Flush)(pTHX_ PerlIO *f);
593 Should make stream's state consistent with layers below. That is, any
594 buffered write data should be written, and file position of lower layers
595 adjusted for data read from below but not actually consumed.
596 (Should perhaps C<Unread()> such data to the lower layer.)
598 Returns 0 on success, -1 on failure.
602 IV (*Fill)(pTHX_ PerlIO *f);
604 The buffer for this layer should be filled (for read) from layer
605 below. When you "subclass" PerlIOBuf layer, you want to use its
606 I<_read> method and to supply your own fill method, which fills the
609 Returns 0 on success, -1 on failure.
613 IV (*Eof)(pTHX_ PerlIO *f);
615 Return end-of-file indicator. C<PerlIOBase_eof()> is normally sufficient.
617 Returns 0 on end-of-file, 1 if not end-of-file, -1 on error.
621 IV (*Error)(pTHX_ PerlIO *f);
623 Return error indicator. C<PerlIOBase_error()> is normally sufficient.
625 Returns 1 if there is an error (usually when C<PERLIO_F_ERROR> is set,
630 void (*Clearerr)(pTHX_ PerlIO *f);
632 Clear end-of-file and error indicators. Should call C<PerlIOBase_clearerr()>
633 to set the C<PERLIO_F_XXXXX> flags, which may suffice.
637 void (*Setlinebuf)(pTHX_ PerlIO *f);
639 Mark the stream as line buffered. C<PerlIOBase_setlinebuf()> sets the
640 PERLIO_F_LINEBUF flag and is normally sufficient.
644 STDCHAR * (*Get_base)(pTHX_ PerlIO *f);
646 Allocate (if not already done so) the read buffer for this layer and
647 return pointer to it. Return NULL on failure.
651 Size_t (*Get_bufsiz)(pTHX_ PerlIO *f);
653 Return the number of bytes that last C<Fill()> put in the buffer.
657 STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f);
659 Return the current read pointer relative to this layer's buffer.
663 SSize_t (*Get_cnt)(pTHX_ PerlIO *f);
665 Return the number of bytes left to be read in the current buffer.
669 void (*Set_ptrcnt)(pTHX_ PerlIO *f,
670 STDCHAR *ptr, SSize_t cnt);
672 Adjust the read pointer and count of bytes to match C<ptr> and/or C<cnt>.
673 The application (or layer above) must ensure they are consistent.
674 (Checking is allowed by the paranoid.)
678 =head2 Implementing PerlIO Layers
680 If you are creating a PerlIO layer, you may want to be lazy, in other
681 words, implement only the methods that interest you. The other methods
682 you can either replace with the "blank" methods
687 (which do nothing, and return zero and -1, respectively) or for
688 certain methods you may assume a default behaviour by using a NULL
689 method. The Open method looks for help in the 'parent' layer.
690 The following table summarizes the behaviour:
692 method behaviour with NULL
694 Clearerr PerlIOBase_clearerr
695 Close PerlIOBase_close
698 Error PerlIOBase_error
699 Fileno PerlIOBase_fileno
714 Setlinebuf PerlIOBase_setlinebuf
716 Unread PerlIOBase_unread
719 FAILURE Set errno (to EINVAL in UNIXish, to LIB$_INVARG in VMS) and
720 return -1 (for numeric return values) or NULL (for pointers)
721 INHERITED Inherited from the layer below
722 SUCCESS Return 0 (for numeric return values) or a pointer
726 The file C<perlio.c> provides the following layers:
732 A basic non-buffered layer which calls Unix/POSIX C<read()>, C<write()>,
733 C<lseek()>, C<close()>. No buffering. Even on platforms that distinguish
734 between O_TEXT and O_BINARY this layer is always O_BINARY.
738 A very complete generic buffering layer which provides the whole of
739 PerlIO API. It is also intended to be used as a "base class" for other
740 layers. (For example its C<Read()> method is implemented in terms of
741 the C<Get_cnt()>/C<Get_ptr()>/C<Set_ptrcnt()> methods).
743 "perlio" over "unix" provides a complete replacement for stdio as seen
744 via PerlIO API. This is the default for USE_PERLIO when system's stdio
745 does not permit perl's "fast gets" access, and which do not
746 distinguish between C<O_TEXT> and C<O_BINARY>.
750 A layer which provides the PerlIO API via the layer scheme, but
751 implements it by calling system's stdio. This is (currently) the default
752 if system's stdio provides sufficient access to allow perl's "fast gets"
753 access and which do not distinguish between C<O_TEXT> and C<O_BINARY>.
757 A layer derived using "perlio" as a base class. It provides Win32-like
758 "\n" to CR,LF translation. Can either be applied above "perlio" or serve
759 as the buffer layer itself. "crlf" over "unix" is the default if system
760 distinguishes between C<O_TEXT> and C<O_BINARY> opens. (At some point
761 "unix" will be replaced by a "native" Win32 IO layer on that platform,
762 as Win32's read/write layer has various drawbacks.) The "crlf" layer is
763 a reasonable model for a layer which transforms data in some way.
767 If Configure detects C<mmap()> functions this layer is provided (with
768 "perlio" as a "base") which does "read" operations by mmap()ing the
769 file. Performance improvement is marginal on modern systems, so it is
770 mainly there as a proof of concept. It is likely to be unbundled from
771 the core at some point. The "mmap" layer is a reasonable model for a
772 minimalist "derived" layer.
776 An "internal" derivative of "perlio" which can be used to provide
777 Unread() function for layers which have no buffer or cannot be
778 bothered. (Basically this layer's C<Fill()> pops itself off the stack
779 and so resumes reading from layer below.)
783 A dummy layer which never exists on the layer stack. Instead when
784 "pushed" it actually pops the stack removing itself, it then calls
785 Binmode function table entry on all the layers in the stack - normally
786 this (via PerlIOBase_binmode) removes any layers which do not have
787 C<PERLIO_K_RAW> bit set. Layers can modify that behaviour by defining
788 their own Binmode entry.
792 Another dummy layer. When pushed it pops itself and sets the
793 C<PERLIO_F_UTF8> flag on the layer which was (and now is once more)
794 the top of the stack.
798 In addition F<perlio.c> also provides a number of C<PerlIOBase_xxxx()>
799 functions which are intended to be used in the table slots of classes
800 which do not need to do anything special for a particular method.
802 =head2 Extension Layers
804 Layers can made available by extension modules. When an unknown layer
805 is encountered the PerlIO code will perform the equivalent of :
809 Where I<layer> is the unknown layer. F<PerlIO.pm> will then attempt to:
811 require PerlIO::layer;
813 If after that process the layer is still not defined then the C<open>
816 The following extension layers are bundled with perl:
824 makes this layer available, although F<PerlIO.pm> "knows" where to
825 find it. It is an example of a layer which takes an argument as it is
828 open( $fh, "<:encoding(iso-8859-7)", $pathname );
832 Provides support for reading data from and writing data to a scalar.
834 open( $fh, "+<:scalar", \$scalar );
836 When a handle is so opened, then reads get bytes from the string value
837 of I<$scalar>, and writes change the value. In both cases the position
838 in I<$scalar> starts as zero but can be altered via C<seek>, and
839 determined via C<tell>.
841 Please note that this layer is implied when calling open() thus:
843 open( $fh, "+<", \$scalar );
847 Provided to allow layers to be implemented as Perl code. For instance:
849 use PerlIO::via::StripHTML;
850 open( my $fh, "<:via(StripHTML)", "index.html" );
852 See L<PerlIO::via> for details.
858 Things that need to be done to improve this document.
864 Explain how to make a valid fh without going through open()(i.e. apply
865 a layer). For example if the file is not opened through perl, but we
866 want to get back a fh, like it was opened by Perl.
868 How PerlIO_apply_layera fits in, where its docs, was it made public?
870 Currently the example could be something like this:
872 PerlIO *foo_to_PerlIO(pTHX_ char *mode, ...)
874 char *mode; /* "w", "r", etc */
875 const char *layers = ":APR"; /* the layer name */
876 PerlIO *f = PerlIO_allocate(aTHX);
881 PerlIO_apply_layers(aTHX_ f, mode, layers);
884 PerlIOAPR *st = PerlIOSelf(f, PerlIOAPR);
885 /* fill in the st struct, as in _open() */
887 PerlIOBase(f)->flags |= PERLIO_F_OPEN;
896 fix/add the documentation in places marked as XXX.
900 The handling of errors by the layer is not specified. e.g. when $!
901 should be set explicitly, when the error handling should be just
902 delegated to the top layer.
904 Probably give some hints on using SETERRNO() or pointers to where they
909 I think it would help to give some concrete examples to make it easier
910 to understand the API. Of course I agree that the API has to be
911 concise, but since there is no second document that is more of a
912 guide, I think that it'd make it easier to start with the doc which is
913 an API, but has examples in it in places where things are unclear, to
914 a person who is not a PerlIO guru (yet).