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.
15 =head2 History and Background
17 The PerlIO abstraction was introduced in perl5.003_02 but languished as
18 just an abstraction until perl5.7.0. However during that time a number
19 of perl extensions switched to using it, so the API is mostly fixed to
20 maintain (source) compatibility.
22 The aim of the implementation is to provide the PerlIO API in a flexible
23 and platform neutral manner. It is also a trial of an "Object Oriented
24 C, with vtables" approach which may be applied to Perl 6.
26 =head2 Basic Structure
28 PerlIO is a stack of layers.
30 The low levels of the stack work with the low-level operating system
31 calls (file descriptors in C) getting bytes in and out, the higher
32 layers of the stack buffer, filter, and otherwise manipulate the I/O,
33 and return characters (or bytes) to Perl. Terms I<above> and I<below>
34 are used to refer to the relative positioning of the stack layers.
36 A layer contains a "vtable", the table of I/O operations (at C level
37 a table of function pointers), and status flags. The functions in the
38 vtable implement operations like "open", "read", and "write".
40 When I/O, for example "read", is requested, the request goes from Perl
41 first down the stack using "read" functions of each layer, then at the
42 bottom the input is requested from the operating system services, then
43 the result is returned up the stack, finally being interpreted as Perl
46 The requests do not necessarily go always all the way down to the
47 operating system: that's where PerlIO buffering comes into play.
49 When you do an open() and specify extra PerlIO layers to be deployed,
50 the layers you specify are "pushed" on top of the already existing
51 default stack. One way to see it is that "operating system is
52 on the left" and "Perl is on the right".
54 What exact layers are in this default stack depends on a lot of
55 things: your operating system, Perl version, Perl compile time
56 configuration, and Perl runtime configuration. See L<PerlIO>,
57 L<perlrun/PERLIO>, and L<open> for more information.
59 binmode() operates similarly to open(): by default the specified
60 layers are pushed on top of the existing stack.
62 However, note that even as the specified layers are "pushed on top"
63 for open() and binmode(), this doesn't mean that the effects are
64 limited to the "top": PerlIO layers can be very 'active' and inspect
65 and affect layers also deeper in the stack. As an example there
66 is a layer called "raw" which repeatedly "pops" layers until
67 it reaches the first layer that has declared itself capable of
68 handling binary data. The "pushed" layers are processed in left-to-right
71 sysopen() operates (unsurprisingly) at a lower level in the stack than
72 open(). For example in Unix or Unix-like systems sysopen() operates
73 directly at the level of file descriptors: in the terms of PerlIO
74 layers, it uses only the "unix" layer, which is a rather thin wrapper
75 on top of the Unix file descriptors.
77 =head2 Layers vs Disciplines
79 Initial discussion of the ability to modify IO streams behaviour used
80 the term "discipline" for the entities which were added. This came (I
81 believe) from the use of the term in "sfio", which in turn borrowed it
82 from "line disciplines" on Unix terminals. However, this document (and
83 the C code) uses the term "layer".
85 This is, I hope, a natural term given the implementation, and should
86 avoid connotations that are inherent in earlier uses of "discipline"
87 for things which are rather different.
89 =head2 Data Structures
91 The basic data structure is a PerlIOl:
93 typedef struct _PerlIO PerlIOl;
94 typedef struct _PerlIO_funcs PerlIO_funcs;
95 typedef PerlIOl *PerlIO;
99 PerlIOl * next; /* Lower layer */
100 PerlIO_funcs * tab; /* Functions for this layer */
101 U32 flags; /* Various flags for state */
104 A C<PerlIOl *> is a pointer to the struct, and the I<application>
105 level C<PerlIO *> is a pointer to a C<PerlIOl *> - i.e. a pointer
106 to a pointer to the struct. This allows the application level C<PerlIO *>
107 to remain constant while the actual C<PerlIOl *> underneath
108 changes. (Compare perl's C<SV *> which remains constant while its
109 C<sv_any> field changes as the scalar's type changes.) An IO stream is
110 then in general represented as a pointer to this linked-list of
113 It should be noted that because of the double indirection in a C<PerlIO *>,
114 a C<< &(perlio->next) >> "is" a C<PerlIO *>, and so to some degree
115 at least one layer can use the "standard" API on the next layer down.
117 A "layer" is composed of two parts:
123 The functions and attributes of the "layer class".
127 The per-instance data for a particular handle.
131 =head2 Functions and Attributes
133 The functions and attributes are accessed via the "tab" (for table)
134 member of C<PerlIOl>. The functions (methods of the layer "class") are
135 fixed, and are defined by the C<PerlIO_funcs> type. They are broadly the
136 same as the public C<PerlIO_xxxxx> functions:
144 IV (*Pushed)(pTHX_ PerlIO *f,
148 IV (*Popped)(pTHX_ PerlIO *f);
149 PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab,
150 PerlIO_list_t *layers, IV n,
152 int fd, int imode, int perm,
154 int narg, SV **args);
155 IV (*Binmode)(pTHX_ PerlIO *f);
156 SV * (*Getarg)(pTHX_ PerlIO *f, CLONE_PARAMS *param, int flags)
157 IV (*Fileno)(pTHX_ PerlIO *f);
158 PerlIO * (*Dup)(pTHX_ PerlIO *f,
162 /* Unix-like functions - cf sfio line disciplines */
163 SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
164 SSize_t (*Unread)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
165 SSize_t (*Write)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
166 IV (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
167 Off_t (*Tell)(pTHX_ PerlIO *f);
168 IV (*Close)(pTHX_ PerlIO *f);
169 /* Stdio-like buffered IO functions */
170 IV (*Flush)(pTHX_ PerlIO *f);
171 IV (*Fill)(pTHX_ PerlIO *f);
172 IV (*Eof)(pTHX_ PerlIO *f);
173 IV (*Error)(pTHX_ PerlIO *f);
174 void (*Clearerr)(pTHX_ PerlIO *f);
175 void (*Setlinebuf)(pTHX_ PerlIO *f);
176 /* Perl's snooping functions */
177 STDCHAR * (*Get_base)(pTHX_ PerlIO *f);
178 Size_t (*Get_bufsiz)(pTHX_ PerlIO *f);
179 STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f);
180 SSize_t (*Get_cnt)(pTHX_ PerlIO *f);
181 void (*Set_ptrcnt)(pTHX_ PerlIO *f,STDCHAR *ptr,SSize_t cnt);
184 The first few members of the struct give a function table size for
185 compatibility check "name" for the layer, the size to C<malloc> for the per-instance data,
186 and some flags which are attributes of the class as whole (such as whether it is a buffering
187 layer), then follow the functions which fall into four basic groups:
193 Opening and setup functions
201 Stdio class buffering options.
205 Functions to support Perl's traditional "fast" access to the buffer.
209 A layer does not have to implement all the functions, but the whole
210 table has to be present. Unimplemented slots can be NULL (which will
211 result in an error when called) or can be filled in with stubs to
212 "inherit" behaviour from a "base class". This "inheritance" is fixed
213 for all instances of the layer, but as the layer chooses which stubs
214 to populate the table, limited "multiple inheritance" is possible.
216 =head2 Per-instance Data
218 The per-instance data are held in memory beyond the basic PerlIOl
219 struct, by making a PerlIOl the first member of the layer's struct
224 struct _PerlIO base; /* Base "class" info */
225 STDCHAR * buf; /* Start of buffer */
226 STDCHAR * end; /* End of valid part of buffer */
227 STDCHAR * ptr; /* Current position in buffer */
228 Off_t posn; /* Offset of buf into the file */
229 Size_t bufsiz; /* Real size of buffer */
230 IV oneword; /* Emergency buffer */
233 In this way (as for perl's scalars) a pointer to a PerlIOBuf can be
234 treated as a pointer to a PerlIOl.
236 =head2 Layers in action.
240 +-----------+ +----------+ +--------+
241 PerlIO ->| |--->| next |--->| NULL |
242 +-----------+ +----------+ +--------+
243 | | | buffer | | fd |
244 +-----------+ | | +--------+
248 The above attempts to show how the layer scheme works in a simple case.
249 The application's C<PerlIO *> points to an entry in the table(s)
250 representing open (allocated) handles. For example the first three slots
251 in the table correspond to C<stdin>,C<stdout> and C<stderr>. The table
252 in turn points to the current "top" layer for the handle - in this case
253 an instance of the generic buffering layer "perlio". That layer in turn
254 points to the next layer down - in this case the low-level "unix" layer.
256 The above is roughly equivalent to a "stdio" buffered stream, but with
257 much more flexibility:
263 If Unix level C<read>/C<write>/C<lseek> is not appropriate for (say)
264 sockets then the "unix" layer can be replaced (at open time or even
265 dynamically) with a "socket" layer.
269 Different handles can have different buffering schemes. The "top"
270 layer could be the "mmap" layer if reading disk files was quicker
271 using C<mmap> than C<read>. An "unbuffered" stream can be implemented
272 simply by not having a buffer layer.
276 Extra layers can be inserted to process the data as it flows through.
277 This was the driving need for including the scheme in perl 5.7.0+ - we
278 needed a mechanism to allow data to be translated between perl's
279 internal encoding (conceptually at least Unicode as UTF-8), and the
280 "native" format used by the system. This is provided by the
281 ":encoding(xxxx)" layer which typically sits above the buffering layer.
285 A layer can be added that does "\n" to CRLF translation. This layer
286 can be used on any platform, not just those that normally do such
291 =head2 Per-instance flag bits
293 The generic flag bits are a hybrid of C<O_XXXXX> style flags deduced
294 from the mode string passed to C<PerlIO_open()>, and state bits for
295 typical buffer layers.
303 =item PERLIO_F_CANWRITE
305 Writes are permitted, i.e. opened as "w" or "r+" or "a", etc.
307 =item PERLIO_F_CANREAD
309 Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick).
313 An error has occurred (for C<PerlIO_error()>).
315 =item PERLIO_F_TRUNCATE
317 Truncate file suggested by open mode.
319 =item PERLIO_F_APPEND
321 All writes should be appends.
325 Layer is performing Win32-like "\n" mapped to CR,LF for output and CR,LF
326 mapped to "\n" for input. Normally the provided "crlf" layer is the only
327 layer that need bother about this. C<PerlIO_binmode()> will mess with this
328 flag rather than add/remove layers if the C<PERLIO_K_CANCRLF> bit is set
329 for the layers class.
333 Data written to this layer should be UTF-8 encoded; data provided
334 by this layer should be considered UTF-8 encoded. Can be set on any layer
335 by ":utf8" dummy layer. Also set on ":encoding" layer.
339 Layer is unbuffered - i.e. write to next layer down should occur for
340 each write to this layer.
344 The buffer for this layer currently holds data written to it but not sent
349 The buffer for this layer currently holds unconsumed data read from
352 =item PERLIO_F_LINEBUF
354 Layer is line buffered. Write data should be passed to next layer down
355 whenever a "\n" is seen. Any data beyond the "\n" should then be
360 File has been C<unlink()>ed, or should be deleted on C<close()>.
366 =item PERLIO_F_FASTGETS
368 This instance of this layer supports the "fast C<gets>" interface.
369 Normally set based on C<PERLIO_K_FASTGETS> for the class and by the
370 existence of the function(s) in the table. However a class that
371 normally provides that interface may need to avoid it on a
372 particular instance. The "pending" layer needs to do this when
373 it is pushed above a layer which does not support the interface.
374 (Perl's C<sv_gets()> does not expect the streams fast C<gets> behaviour
375 to change during one "get".)
379 =head2 Methods in Detail
387 Size of the function table. This is compared against the value PerlIO
388 code "knows" as a compatibility check. Future versions I<may> be able
389 to tolerate layers compiled against an old version of the headers.
395 The name of the layer whose open() method Perl should invoke on
396 open(). For example if the layer is called APR, you will call:
398 open $fh, ">:APR", ...
400 and Perl knows that it has to invoke the PerlIOAPR_open() method
401 implemented by the APR layer.
407 The size of the per-instance data structure, e.g.:
411 If this field is zero then C<PerlIO_pushed> does not malloc anything
412 and assumes layer's Pushed function will do any required layer stack
413 manipulation - used to avoid malloc/free overhead for dummy layers.
414 If the field is non-zero it must be at least the size of C<PerlIOl>,
415 C<PerlIO_pushed> will allocate memory for the layer's data structures
416 and link new layer onto the stream's stack. (If the layer's Pushed
417 method returns an error indication the layer is popped again.)
425 =item * PERLIO_K_BUFFERED
427 The layer is buffered.
431 The layer is acceptable to have in a binmode(FH) stack - i.e. it does not
432 (or will configure itself not to) transform bytes passing through it.
434 =item * PERLIO_K_CANCRLF
436 Layer can translate between "\n" and CRLF line ends.
438 =item * PERLIO_K_FASTGETS
440 Layer allows buffer snooping.
442 =item * PERLIO_K_MULTIARG
444 Used when the layer's open() accepts more arguments than usual. The
445 extra arguments should come not before the C<MODE> argument. When this
446 flag is used it's up to the layer to validate the args.
452 IV (*Pushed)(pTHX_ PerlIO *f,const char *mode, SV *arg);
454 The only absolutely mandatory method. Called when the layer is pushed
455 onto the stack. The C<mode> argument may be NULL if this occurs
456 post-open. The C<arg> will be non-C<NULL> if an argument string was
457 passed. In most cases this should call C<PerlIOBase_pushed()> to
458 convert C<mode> into the appropriate C<PERLIO_F_XXXXX> flags in
459 addition to any actions the layer itself takes. If a layer is not
460 expecting an argument it need neither save the one passed to it, nor
461 provide C<Getarg()> (it could perhaps C<Perl_warn> that the argument
464 Returns 0 on success. On failure returns -1 and should set errno.
468 IV (*Popped)(pTHX_ PerlIO *f);
470 Called when the layer is popped from the stack. A layer will normally
471 be popped after C<Close()> is called. But a layer can be popped
472 without being closed if the program is dynamically managing layers on
473 the stream. In such cases C<Popped()> should free any resources
474 (buffers, translation tables, ...) not held directly in the layer's
475 struct. It should also C<Unread()> any unconsumed data that has been
476 read and buffered from the layer below back to that layer, so that it
477 can be re-provided to what ever is now above.
479 Returns 0 on success and failure. If C<Popped()> returns I<true> then
480 I<perlio.c> assumes that either the layer has popped itself, or the
481 layer is super special and needs to be retained for other reasons.
482 In most cases it should return I<false>.
486 PerlIO * (*Open)(...);
488 The C<Open()> method has lots of arguments because it combines the
489 functions of perl's C<open>, C<PerlIO_open>, perl's C<sysopen>,
490 C<PerlIO_fdopen> and C<PerlIO_reopen>. The full prototype is as
493 PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab,
494 PerlIO_list_t *layers, IV n,
496 int fd, int imode, int perm,
498 int narg, SV **args);
500 Open should (perhaps indirectly) call C<PerlIO_allocate()> to allocate
501 a slot in the table and associate it with the layers information for
502 the opened file, by calling C<PerlIO_push>. The I<layers> is an
503 array of all the layers destined for the C<PerlIO *>, and any
504 arguments passed to them, I<n> is the index into that array of the
505 layer being called. The macro C<PerlIOArg> will return a (possibly
506 C<NULL>) SV * for the argument passed to the layer.
508 The I<mode> string is an "C<fopen()>-like" string which would match
509 the regular expression C</^[I#]?[rwa]\+?[bt]?$/>.
511 The C<'I'> prefix is used during creation of C<stdin>..C<stderr> via
512 special C<PerlIO_fdopen> calls; the C<'#'> prefix means that this is
513 C<sysopen> and that I<imode> and I<perm> should be passed to
514 C<PerlLIO_open3>; C<'r'> means B<r>ead, C<'w'> means B<w>rite and
515 C<'a'> means B<a>ppend. The C<'+'> suffix means that both reading and
516 writing/appending are permitted. The C<'b'> suffix means file should
517 be binary, and C<'t'> means it is text. (Almost all layers should do
518 the IO in binary mode, and ignore the b/t bits. The C<:crlf> layer
519 should be pushed to handle the distinction.)
521 If I<old> is not C<NULL> then this is a C<PerlIO_reopen>. Perl itself
522 does not use this (yet?) and semantics are a little vague.
524 If I<fd> not negative then it is the numeric file descriptor I<fd>,
525 which will be open in a manner compatible with the supplied mode
526 string, the call is thus equivalent to C<PerlIO_fdopen>. In this case
527 I<nargs> will be zero.
529 If I<nargs> is greater than zero then it gives the number of arguments
530 passed to C<open>, otherwise it will be 1 if for example
531 C<PerlIO_open> was called. In simple cases SvPV_nolen(*args) is the
534 If a layer provides C<Open()> it should normally call the C<Open()>
535 method of next layer down (if any) and then push itself on top if that
536 succeeds. C<PerlIOBase_open> is provided to do exactly that, so in
537 most cases you don't have to write your own C<Open()> method. If this
538 method is not defined, other layers may have difficulty pushing
539 themselves on top of it during open.
541 If C<PerlIO_push> was performed and open has failed, it must
542 C<PerlIO_pop> itself, since if it's not, the layer won't be removed
543 and may cause bad problems.
545 Returns C<NULL> on failure.
549 IV (*Binmode)(pTHX_ PerlIO *f);
551 Optional. Used when C<:raw> layer is pushed (explicitly or as a result
552 of binmode(FH)). If not present layer will be popped. If present
553 should configure layer as binary (or pop itself) and return 0.
554 If it returns -1 for error C<binmode> will fail with layer
559 SV * (*Getarg)(pTHX_ PerlIO *f,
560 CLONE_PARAMS *param, int flags);
562 Optional. If present should return an SV * representing the string
563 argument passed to the layer when it was
564 pushed. e.g. ":encoding(ascii)" would return an SvPV with value
565 "ascii". (I<param> and I<flags> arguments can be ignored in most
568 C<Dup> uses C<Getarg> to retrieve the argument originally passed to
569 C<Pushed>, so you must implement this function if your layer has an
570 extra argument to C<Pushed> and will ever be C<Dup>ed.
574 IV (*Fileno)(pTHX_ PerlIO *f);
576 Returns the Unix/Posix numeric file descriptor for the handle. Normally
577 C<PerlIOBase_fileno()> (which just asks next layer down) will suffice
580 Returns -1 on error, which is considered to include the case where the
581 layer cannot provide such a file descriptor.
585 PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
586 CLONE_PARAMS *param, int flags);
588 XXX: Needs more docs.
590 Used as part of the "clone" process when a thread is spawned (in which
591 case param will be non-NULL) and when a stream is being duplicated via
594 Similar to C<Open>, returns PerlIO* on success, C<NULL> on failure.
598 SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
600 Basic read operation.
602 Typically will call C<Fill> and manipulate pointers (possibly via the
603 API). C<PerlIOBuf_read()> may be suitable for derived classes which
604 provide "fast gets" methods.
606 Returns actual bytes read, or -1 on an error.
610 SSize_t (*Unread)(pTHX_ PerlIO *f,
611 const void *vbuf, Size_t count);
613 A superset of stdio's C<ungetc()>. Should arrange for future reads to
614 see the bytes in C<vbuf>. If there is no obviously better implementation
615 then C<PerlIOBase_unread()> provides the function by pushing a "fake"
616 "pending" layer above the calling layer.
618 Returns the number of unread chars.
622 SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count);
624 Basic write operation.
626 Returns bytes written or -1 on an error.
630 IV (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
632 Position the file pointer. Should normally call its own C<Flush>
633 method and then the C<Seek> method of next layer down.
635 Returns 0 on success, -1 on failure.
639 Off_t (*Tell)(pTHX_ PerlIO *f);
641 Return the file pointer. May be based on layers cached concept of
642 position to avoid overhead.
644 Returns -1 on failure to get the file pointer.
648 IV (*Close)(pTHX_ PerlIO *f);
650 Close the stream. Should normally call C<PerlIOBase_close()> to flush
651 itself and close layers below, and then deallocate any data structures
652 (buffers, translation tables, ...) not held directly in the data
655 Returns 0 on success, -1 on failure.
659 IV (*Flush)(pTHX_ PerlIO *f);
661 Should make stream's state consistent with layers below. That is, any
662 buffered write data should be written, and file position of lower layers
663 adjusted for data read from below but not actually consumed.
664 (Should perhaps C<Unread()> such data to the lower layer.)
666 Returns 0 on success, -1 on failure.
670 IV (*Fill)(pTHX_ PerlIO *f);
672 The buffer for this layer should be filled (for read) from layer
673 below. When you "subclass" PerlIOBuf layer, you want to use its
674 I<_read> method and to supply your own fill method, which fills the
677 Returns 0 on success, -1 on failure.
681 IV (*Eof)(pTHX_ PerlIO *f);
683 Return end-of-file indicator. C<PerlIOBase_eof()> is normally sufficient.
685 Returns 0 on end-of-file, 1 if not end-of-file, -1 on error.
689 IV (*Error)(pTHX_ PerlIO *f);
691 Return error indicator. C<PerlIOBase_error()> is normally sufficient.
693 Returns 1 if there is an error (usually when C<PERLIO_F_ERROR> is set),
698 void (*Clearerr)(pTHX_ PerlIO *f);
700 Clear end-of-file and error indicators. Should call C<PerlIOBase_clearerr()>
701 to set the C<PERLIO_F_XXXXX> flags, which may suffice.
705 void (*Setlinebuf)(pTHX_ PerlIO *f);
707 Mark the stream as line buffered. C<PerlIOBase_setlinebuf()> sets the
708 PERLIO_F_LINEBUF flag and is normally sufficient.
712 STDCHAR * (*Get_base)(pTHX_ PerlIO *f);
714 Allocate (if not already done so) the read buffer for this layer and
715 return pointer to it. Return NULL on failure.
719 Size_t (*Get_bufsiz)(pTHX_ PerlIO *f);
721 Return the number of bytes that last C<Fill()> put in the buffer.
725 STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f);
727 Return the current read pointer relative to this layer's buffer.
731 SSize_t (*Get_cnt)(pTHX_ PerlIO *f);
733 Return the number of bytes left to be read in the current buffer.
737 void (*Set_ptrcnt)(pTHX_ PerlIO *f,
738 STDCHAR *ptr, SSize_t cnt);
740 Adjust the read pointer and count of bytes to match C<ptr> and/or C<cnt>.
741 The application (or layer above) must ensure they are consistent.
742 (Checking is allowed by the paranoid.)
748 To ask for the next layer down use PerlIONext(PerlIO *f).
750 To check that a PerlIO* is valid use PerlIOValid(PerlIO *f). (All
751 this does is really just to check that the pointer is non-NULL and
752 that the pointer behind that is non-NULL.)
754 PerlIOBase(PerlIO *f) returns the "Base" pointer, or in other words,
755 the C<PerlIOl*> pointer.
757 PerlIOSelf(PerlIO* f, type) return the PerlIOBase cast to a type.
759 Perl_PerlIO_or_Base(PerlIO* f, callback, base, failure, args) either
760 calls the I<callback> from the functions of the layer I<f> (just by
761 the name of the IO function, like "Read") with the I<args>, or if
762 there is no such callback, calls the I<base> version of the callback
763 with the same args, or if the f is invalid, set errno to EBADF and
766 Perl_PerlIO_or_fail(PerlIO* f, callback, failure, args) either calls
767 the I<callback> of the functions of the layer I<f> with the I<args>,
768 or if there is no such callback, set errno to EINVAL. Or if the f is
769 invalid, set errno to EBADF and return I<failure>.
771 Perl_PerlIO_or_Base_void(PerlIO* f, callback, base, args) either calls
772 the I<callback> of the functions of the layer I<f> with the I<args>,
773 or if there is no such callback, calls the I<base> version of the
774 callback with the same args, or if the f is invalid, set errno to
777 Perl_PerlIO_or_fail_void(PerlIO* f, callback, args) either calls the
778 I<callback> of the functions of the layer I<f> with the I<args>, or if
779 there is no such callback, set errno to EINVAL. Or if the f is
780 invalid, set errno to EBADF.
782 =head2 Implementing PerlIO Layers
784 If you find the implementation document unclear or not sufficient,
785 look at the existing PerlIO layer implementations, which include:
789 =item * C implementations
791 The F<perlio.c> and F<perliol.h> in the Perl core implement the
792 "unix", "perlio", "stdio", "crlf", "utf8", "byte", "raw", "pending"
793 layers, and also the "mmap" and "win32" layers if applicable.
794 (The "win32" is currently unfinished and unused, to see what is used
795 instead in Win32, see L<PerlIO/"Querying the layers of filehandles"> .)
797 PerlIO::encoding, PerlIO::scalar, PerlIO::via in the Perl core.
799 PerlIO::gzip and APR::PerlIO (mod_perl 2.0) on CPAN.
801 =item * Perl implementations
803 PerlIO::via::QuotedPrint in the Perl core and PerlIO::via::* on CPAN.
807 If you are creating a PerlIO layer, you may want to be lazy, in other
808 words, implement only the methods that interest you. The other methods
809 you can either replace with the "blank" methods
814 (which do nothing, and return zero and -1, respectively) or for
815 certain methods you may assume a default behaviour by using a NULL
816 method. The Open method looks for help in the 'parent' layer.
817 The following table summarizes the behaviour:
819 method behaviour with NULL
821 Clearerr PerlIOBase_clearerr
822 Close PerlIOBase_close
825 Error PerlIOBase_error
826 Fileno PerlIOBase_fileno
841 Setlinebuf PerlIOBase_setlinebuf
843 Unread PerlIOBase_unread
846 FAILURE Set errno (to EINVAL in Unixish, to LIB$_INVARG in VMS)
847 and return -1 (for numeric return values) or NULL (for
849 INHERITED Inherited from the layer below
850 SUCCESS Return 0 (for numeric return values) or a pointer
854 The file C<perlio.c> provides the following layers:
860 A basic non-buffered layer which calls Unix/POSIX C<read()>, C<write()>,
861 C<lseek()>, C<close()>. No buffering. Even on platforms that distinguish
862 between O_TEXT and O_BINARY this layer is always O_BINARY.
866 A very complete generic buffering layer which provides the whole of
867 PerlIO API. It is also intended to be used as a "base class" for other
868 layers. (For example its C<Read()> method is implemented in terms of
869 the C<Get_cnt()>/C<Get_ptr()>/C<Set_ptrcnt()> methods).
871 "perlio" over "unix" provides a complete replacement for stdio as seen
872 via PerlIO API. This is the default for USE_PERLIO when system's stdio
873 does not permit perl's "fast gets" access, and which do not
874 distinguish between C<O_TEXT> and C<O_BINARY>.
878 A layer which provides the PerlIO API via the layer scheme, but
879 implements it by calling system's stdio. This is (currently) the default
880 if system's stdio provides sufficient access to allow perl's "fast gets"
881 access and which do not distinguish between C<O_TEXT> and C<O_BINARY>.
885 A layer derived using "perlio" as a base class. It provides Win32-like
886 "\n" to CR,LF translation. Can either be applied above "perlio" or serve
887 as the buffer layer itself. "crlf" over "unix" is the default if system
888 distinguishes between C<O_TEXT> and C<O_BINARY> opens. (At some point
889 "unix" will be replaced by a "native" Win32 IO layer on that platform,
890 as Win32's read/write layer has various drawbacks.) The "crlf" layer is
891 a reasonable model for a layer which transforms data in some way.
895 If Configure detects C<mmap()> functions this layer is provided (with
896 "perlio" as a "base") which does "read" operations by mmap()ing the
897 file. Performance improvement is marginal on modern systems, so it is
898 mainly there as a proof of concept. It is likely to be unbundled from
899 the core at some point. The "mmap" layer is a reasonable model for a
900 minimalist "derived" layer.
904 An "internal" derivative of "perlio" which can be used to provide
905 Unread() function for layers which have no buffer or cannot be
906 bothered. (Basically this layer's C<Fill()> pops itself off the stack
907 and so resumes reading from layer below.)
911 A dummy layer which never exists on the layer stack. Instead when
912 "pushed" it actually pops the stack removing itself, it then calls
913 Binmode function table entry on all the layers in the stack - normally
914 this (via PerlIOBase_binmode) removes any layers which do not have
915 C<PERLIO_K_RAW> bit set. Layers can modify that behaviour by defining
916 their own Binmode entry.
920 Another dummy layer. When pushed it pops itself and sets the
921 C<PERLIO_F_UTF8> flag on the layer which was (and now is once more)
922 the top of the stack.
926 In addition F<perlio.c> also provides a number of C<PerlIOBase_xxxx()>
927 functions which are intended to be used in the table slots of classes
928 which do not need to do anything special for a particular method.
930 =head2 Extension Layers
932 Layers can be made available by extension modules. When an unknown layer
933 is encountered the PerlIO code will perform the equivalent of :
937 Where I<layer> is the unknown layer. F<PerlIO.pm> will then attempt to:
939 require PerlIO::layer;
941 If after that process the layer is still not defined then the C<open>
944 The following extension layers are bundled with perl:
952 makes this layer available, although F<PerlIO.pm> "knows" where to
953 find it. It is an example of a layer which takes an argument as it is
956 open( $fh, "<:encoding(iso-8859-7)", $pathname );
960 Provides support for reading data from and writing data to a scalar.
962 open( $fh, "+<:scalar", \$scalar );
964 When a handle is so opened, then reads get bytes from the string value
965 of I<$scalar>, and writes change the value. In both cases the position
966 in I<$scalar> starts as zero but can be altered via C<seek>, and
967 determined via C<tell>.
969 Please note that this layer is implied when calling open() thus:
971 open( $fh, "+<", \$scalar );
975 Provided to allow layers to be implemented as Perl code. For instance:
977 use PerlIO::via::StripHTML;
978 open( my $fh, "<:via(StripHTML)", "index.html" );
980 See L<PerlIO::via> for details.
986 Things that need to be done to improve this document.
992 Explain how to make a valid fh without going through open()(i.e. apply
993 a layer). For example if the file is not opened through perl, but we
994 want to get back a fh, like it was opened by Perl.
996 How PerlIO_apply_layera fits in, where its docs, was it made public?
998 Currently the example could be something like this:
1000 PerlIO *foo_to_PerlIO(pTHX_ char *mode, ...)
1002 char *mode; /* "w", "r", etc */
1003 const char *layers = ":APR"; /* the layer name */
1004 PerlIO *f = PerlIO_allocate(aTHX);
1009 PerlIO_apply_layers(aTHX_ f, mode, layers);
1012 PerlIOAPR *st = PerlIOSelf(f, PerlIOAPR);
1013 /* fill in the st struct, as in _open() */
1015 PerlIOBase(f)->flags |= PERLIO_F_OPEN;
1024 fix/add the documentation in places marked as XXX.
1028 The handling of errors by the layer is not specified. e.g. when $!
1029 should be set explicitly, when the error handling should be just
1030 delegated to the top layer.
1032 Probably give some hints on using SETERRNO() or pointers to where they
1037 I think it would help to give some concrete examples to make it easier
1038 to understand the API. Of course I agree that the API has to be
1039 concise, but since there is no second document that is more of a
1040 guide, I think that it'd make it easier to start with the doc which is
1041 an API, but has examples in it in places where things are unclear, to
1042 a person who is not a PerlIO guru (yet).