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 Basic Structure
29 PerlIO is a stack of layers.
31 The low levels of the stack work with the low-level operating system
32 calls (file descriptors in C) getting bytes in and out, the higher
33 layers of the stack buffer, filter, and otherwise manipulate the I/O,
34 and return characters (or bytes) to Perl. Terms I<above> and I<below>
35 are used to refer to the relative positioning of the stack layers.
37 A layer contains a "vtable", the table of I/O operations (at C level
38 a table of function pointers), and status flags. The functions in the
39 vtable implement operations like "open", "read", and "write".
41 When I/O, for example "read", is requested, the request goes from Perl
42 first down the stack using "read" functions of each layer, then at the
43 bottom the input is requested from the operating system services, then
44 the result is returned up the stack, finally being interpreted as Perl
47 The requests do not necessarily go always all the way down to the
48 operating system: that's where PerlIO buffering comes into play.
50 When you do an open() and specify extra PerlIO layers to be deployed,
51 the layers you specify are "pushed" on top of the already existing
52 default stack. One way to see it is that "operating system is
53 on the left" and "Perl is on the right".
55 What exact layers are in this default stack depends on a lot of
56 things: your operating system, Perl version, Perl compile time
57 configuration, and Perl runtime configuration. See L<PerlIO>,
58 L<perlrun/PERLIO>, and L<open> for more information.
60 binmode() operates similarly to open(): by default the specified
61 layers are pushed on top of the existing stack.
63 However, note that even as the specified layers are "pushed on top"
64 for open() and binmode(), this doesn't mean that the effects are
65 limited to the "top": PerlIO layers can be very 'active' and inspect
66 and affect layers also deeper in the stack. As an example there
67 is a layer called "raw" which repeatedly "pops" layers until
68 it reaches the first layer that has declared itself capable of
69 handling binary data. The "pushed" layers are processed in left-to-right
72 sysopen() operates (unsurprisingly) at a lower level in the stack than
73 open(). For example in UNIX or UNIX-like systems sysopen() operates
74 directly at the level of file descriptors: in the terms of PerlIO
75 layers, it uses only the "unix" layer, which is a rather thin wrapper
76 on top of the UNIX file descriptors.
78 =head2 Layers vs Disciplines
80 Initial discussion of the ability to modify IO streams behaviour used
81 the term "discipline" for the entities which were added. This came (I
82 believe) from the use of the term in "sfio", which in turn borrowed it
83 from "line disciplines" on Unix terminals. However, this document (and
84 the C code) uses the term "layer".
86 This is, I hope, a natural term given the implementation, and should
87 avoid connotations that are inherent in earlier uses of "discipline"
88 for things which are rather different.
90 =head2 Data Structures
92 The basic data structure is a PerlIOl:
94 typedef struct _PerlIO PerlIOl;
95 typedef struct _PerlIO_funcs PerlIO_funcs;
96 typedef PerlIOl *PerlIO;
100 PerlIOl * next; /* Lower layer */
101 PerlIO_funcs * tab; /* Functions for this layer */
102 IV flags; /* Various flags for state */
105 A C<PerlIOl *> is a pointer to the struct, and the I<application>
106 level C<PerlIO *> is a pointer to a C<PerlIOl *> - i.e. a pointer
107 to a pointer to the struct. This allows the application level C<PerlIO *>
108 to remain constant while the actual C<PerlIOl *> underneath
109 changes. (Compare perl's C<SV *> which remains constant while its
110 C<sv_any> field changes as the scalar's type changes.) An IO stream is
111 then in general represented as a pointer to this linked-list of
114 It should be noted that because of the double indirection in a C<PerlIO *>,
115 a C<< &(perlio->next) >> "is" a C<PerlIO *>, and so to some degree
116 at least one layer can use the "standard" API on the next layer down.
118 A "layer" is composed of two parts:
124 The functions and attributes of the "layer class".
128 The per-instance data for a particular handle.
132 =head2 Functions and Attributes
134 The functions and attributes are accessed via the "tab" (for table)
135 member of C<PerlIOl>. The functions (methods of the layer "class") are
136 fixed, and are defined by the C<PerlIO_funcs> type. They are broadly the
137 same as the public C<PerlIO_xxxxx> functions:
145 IV (*Pushed)(pTHX_ PerlIO *f,const char *mode,SV *arg, PerlIO_funcs *tab);
146 IV (*Popped)(pTHX_ PerlIO *f);
147 PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab,
150 int fd, int imode, int perm,
152 int narg, SV **args);
153 IV (*Binmode)(pTHX_ PerlIO *f);
154 SV * (*Getarg)(pTHX_ PerlIO *f, CLONE_PARAMS *param, int flags)
155 IV (*Fileno)(pTHX_ PerlIO *f);
156 PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o, CLONE_PARAMS *param, int flags)
157 /* Unix-like functions - cf sfio line disciplines */
158 SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
159 SSize_t (*Unread)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
160 SSize_t (*Write)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
161 IV (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
162 Off_t (*Tell)(pTHX_ PerlIO *f);
163 IV (*Close)(pTHX_ PerlIO *f);
164 /* Stdio-like buffered IO functions */
165 IV (*Flush)(pTHX_ PerlIO *f);
166 IV (*Fill)(pTHX_ PerlIO *f);
167 IV (*Eof)(pTHX_ PerlIO *f);
168 IV (*Error)(pTHX_ PerlIO *f);
169 void (*Clearerr)(pTHX_ PerlIO *f);
170 void (*Setlinebuf)(pTHX_ PerlIO *f);
171 /* Perl's snooping functions */
172 STDCHAR * (*Get_base)(pTHX_ PerlIO *f);
173 Size_t (*Get_bufsiz)(pTHX_ PerlIO *f);
174 STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f);
175 SSize_t (*Get_cnt)(pTHX_ PerlIO *f);
176 void (*Set_ptrcnt)(pTHX_ PerlIO *f,STDCHAR *ptr,SSize_t cnt);
179 The first few members of the struct give a function table size for
180 compatibility check "name" for the layer, the size to C<malloc> for the per-instance data,
181 and some flags which are attributes of the class as whole (such as whether it is a buffering
182 layer), then follow the functions which fall into four basic groups:
188 Opening and setup functions
196 Stdio class buffering options.
200 Functions to support Perl's traditional "fast" access to the buffer.
204 A layer does not have to implement all the functions, but the whole
205 table has to be present. Unimplemented slots can be NULL (which will
206 result in an error when called) or can be filled in with stubs to
207 "inherit" behaviour from a "base class". This "inheritance" is fixed
208 for all instances of the layer, but as the layer chooses which stubs
209 to populate the table, limited "multiple inheritance" is possible.
211 =head2 Per-instance Data
213 The per-instance data are held in memory beyond the basic PerlIOl
214 struct, by making a PerlIOl the first member of the layer's struct
219 struct _PerlIO base; /* Base "class" info */
220 STDCHAR * buf; /* Start of buffer */
221 STDCHAR * end; /* End of valid part of buffer */
222 STDCHAR * ptr; /* Current position in buffer */
223 Off_t posn; /* Offset of buf into the file */
224 Size_t bufsiz; /* Real size of buffer */
225 IV oneword; /* Emergency buffer */
228 In this way (as for perl's scalars) a pointer to a PerlIOBuf can be
229 treated as a pointer to a PerlIOl.
231 =head2 Layers in action.
235 +-----------+ +----------+ +--------+
236 PerlIO ->| |--->| next |--->| NULL |
237 +-----------+ +----------+ +--------+
238 | | | buffer | | fd |
239 +-----------+ | | +--------+
243 The above attempts to show how the layer scheme works in a simple case.
244 The application's C<PerlIO *> points to an entry in the table(s)
245 representing open (allocated) handles. For example the first three slots
246 in the table correspond to C<stdin>,C<stdout> and C<stderr>. The table
247 in turn points to the current "top" layer for the handle - in this case
248 an instance of the generic buffering layer "perlio". That layer in turn
249 points to the next layer down - in this case the lowlevel "unix" layer.
251 The above is roughly equivalent to a "stdio" buffered stream, but with
252 much more flexibility:
258 If Unix level C<read>/C<write>/C<lseek> is not appropriate for (say)
259 sockets then the "unix" layer can be replaced (at open time or even
260 dynamically) with a "socket" layer.
264 Different handles can have different buffering schemes. The "top"
265 layer could be the "mmap" layer if reading disk files was quicker
266 using C<mmap> than C<read>. An "unbuffered" stream can be implemented
267 simply by not having a buffer layer.
271 Extra layers can be inserted to process the data as it flows through.
272 This was the driving need for including the scheme in perl 5.7.0+ - we
273 needed a mechanism to allow data to be translated between perl's
274 internal encoding (conceptually at least Unicode as UTF-8), and the
275 "native" format used by the system. This is provided by the
276 ":encoding(xxxx)" layer which typically sits above the buffering layer.
280 A layer can be added that does "\n" to CRLF translation. This layer
281 can be used on any platform, not just those that normally do such
286 =head2 Per-instance flag bits
288 The generic flag bits are a hybrid of C<O_XXXXX> style flags deduced
289 from the mode string passed to C<PerlIO_open()>, and state bits for
290 typical buffer layers.
298 =item PERLIO_F_CANWRITE
300 Writes are permitted, i.e. opened as "w" or "r+" or "a", etc.
302 =item PERLIO_F_CANREAD
304 Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick).
308 An error has occurred (for C<PerlIO_error()>).
310 =item PERLIO_F_TRUNCATE
312 Truncate file suggested by open mode.
314 =item PERLIO_F_APPEND
316 All writes should be appends.
320 Layer is performing Win32-like "\n" mapped to CR,LF for output and CR,LF
321 mapped to "\n" for input. Normally the provided "crlf" layer is the only
322 layer that need bother about this. C<PerlIO_binmode()> will mess with this
323 flag rather than add/remove layers if the C<PERLIO_K_CANCRLF> bit is set
324 for the layers class.
328 Data written to this layer should be UTF-8 encoded; data provided
329 by this layer should be considered UTF-8 encoded. Can be set on any layer
330 by ":utf8" dummy layer. Also set on ":encoding" layer.
334 Layer is unbuffered - i.e. write to next layer down should occur for
335 each write to this layer.
339 The buffer for this layer currently holds data written to it but not sent
344 The buffer for this layer currently holds unconsumed data read from
347 =item PERLIO_F_LINEBUF
349 Layer is line buffered. Write data should be passed to next layer down
350 whenever a "\n" is seen. Any data beyond the "\n" should then be
355 File has been C<unlink()>ed, or should be deleted on C<close()>.
361 =item PERLIO_F_FASTGETS
363 This instance of this layer supports the "fast C<gets>" interface.
364 Normally set based on C<PERLIO_K_FASTGETS> for the class and by the
365 existence of the function(s) in the table. However a class that
366 normally provides that interface may need to avoid it on a
367 particular instance. The "pending" layer needs to do this when
368 it is pushed above a layer which does not support the interface.
369 (Perl's C<sv_gets()> does not expect the streams fast C<gets> behaviour
370 to change during one "get".)
374 =head2 Methods in Detail
382 Size of the function table. This is compared against the value PerlIO
383 code "knows" as a compatibility check. Future versions I<may> be able
384 to tolerate layers compiled against an old version of the headers.
390 The name of the layer whose open() method Perl should invoke on
391 open(). For example if the layer is called APR, you will call:
393 open $fh, ">:APR", ...
395 and Perl knows that it has to invoke the PerlIOAPR_open() method
396 implemented by the APR layer.
402 The size of the per-instance data structure, e.g.:
406 If this field is zero then C<PerlIO_pushed> does not malloc anything
407 and assumes layer's Pushed function will do any required layer stack
408 manipulation - used to avoid malloc/free overhead for dummy layers.
409 If the field is non-zero it must be at least the size of C<PerlIOl>,
410 C<PerlIO_pushed> will allocate memory for the layer's data structures
411 and link new layer onto the stream's stack. (If the layer's Pushed
412 method returns an error indication the layer is popped again.)
420 =item * PERLIO_K_BUFFERED
422 The layer is buffered.
426 The layer is acceptable to have in a binmode(FH) stack - i.e. it does not
427 (or will configure itself not to) transform bytes passing through it.
429 =item * PERLIO_K_CANCRLF
431 Layer can translate between "\n" and CRLF line ends.
433 =item * PERLIO_K_FASTGETS
435 Layer allows buffer snooping.
437 =item * PERLIO_K_MULTIARG
439 Used when the layer's open() accepts more arguments than usual. The
440 extra arguments should come not before the C<MODE> argument. When this
441 flag is used it's up to the layer to validate the args.
447 IV (*Pushed)(pTHX_ PerlIO *f,const char *mode, SV *arg);
449 The only absolutely mandatory method. Called when the layer is pushed
450 onto the stack. The C<mode> argument may be NULL if this occurs
451 post-open. The C<arg> will be non-C<NULL> if an argument string was
452 passed. In most cases this should call C<PerlIOBase_pushed()> to
453 convert C<mode> into the appropriate C<PERLIO_F_XXXXX> flags in
454 addition to any actions the layer itself takes. If a layer is not
455 expecting an argument it need neither save the one passed to it, nor
456 provide C<Getarg()> (it could perhaps C<Perl_warn> that the argument
459 Returns 0 on success. On failure returns -1 and should set errno.
463 IV (*Popped)(pTHX_ PerlIO *f);
465 Called when the layer is popped from the stack. A layer will normally
466 be popped after C<Close()> is called. But a layer can be popped
467 without being closed if the program is dynamically managing layers on
468 the stream. In such cases C<Popped()> should free any resources
469 (buffers, translation tables, ...) not held directly in the layer's
470 struct. It should also C<Unread()> any unconsumed data that has been
471 read and buffered from the layer below back to that layer, so that it
472 can be re-provided to what ever is now above.
474 Returns 0 on success and failure. If C<Popped()> returns I<true> then
475 I<perlio.c> assumes that either the layer has popped itself, or the
476 layer is super special and needs to be retained for other reasons.
477 In most cases it should return I<false>.
481 PerlIO * (*Open)(...);
483 The C<Open()> method has lots of arguments because it combines the
484 functions of perl's C<open>, C<PerlIO_open>, perl's C<sysopen>,
485 C<PerlIO_fdopen> and C<PerlIO_reopen>. The full prototype is as
488 PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab,
491 int fd, int imode, int perm,
493 int narg, SV **args);
495 Open should (perhaps indirectly) call C<PerlIO_allocate()> to allocate
496 a slot in the table and associate it with the layers information for
497 the opened file, by calling C<PerlIO_push>. The I<layers> AV is an
498 array of all the layers destined for the C<PerlIO *>, and any
499 arguments passed to them, I<n> is the index into that array of the
500 layer being called. The macro C<PerlIOArg> will return a (possibly
501 C<NULL>) SV * for the argument passed to the layer.
503 The I<mode> string is an "C<fopen()>-like" string which would match
504 the regular expression C</^[I#]?[rwa]\+?[bt]?$/>.
506 The C<'I'> prefix is used during creation of C<stdin>..C<stderr> via
507 special C<PerlIO_fdopen> calls; the C<'#'> prefix means that this is
508 C<sysopen> and that I<imode> and I<perm> should be passed to
509 C<PerlLIO_open3>; C<'r'> means B<r>ead, C<'w'> means B<w>rite and
510 C<'a'> means B<a>ppend. The C<'+'> suffix means that both reading and
511 writing/appending are permitted. The C<'b'> suffix means file should
512 be binary, and C<'t'> means it is text. (Almost all layers should do
513 the IO in binary mode, and ignore the b/t bits. The C<:crlf> layer
514 should be pushed to handle the distinction.)
516 If I<old> is not C<NULL> then this is a C<PerlIO_reopen>. Perl itself
517 does not use this (yet?) and semantics are a little vague.
519 If I<fd> not negative then it is the numeric file descriptor I<fd>,
520 which will be open in a manner compatible with the supplied mode
521 string, the call is thus equivalent to C<PerlIO_fdopen>. In this case
522 I<nargs> will be zero.
524 If I<nargs> is greater than zero then it gives the number of arguments
525 passed to C<open>, otherwise it will be 1 if for example
526 C<PerlIO_open> was called. In simple cases SvPV_nolen(*args) is the
529 Having said all that translation-only layers do not need to provide
530 C<Open()> at all, but rather leave the opening to a lower level layer
531 and wait to be "pushed". If a layer does provide C<Open()> it should
532 normally call the C<Open()> method of next layer down (if any) and
533 then push itself on top if that succeeds.
535 If C<PerlIO_push> was performed and open has failed, it must
536 C<PerlIO_pop> itself, since if it's not, the layer won't be removed
537 and may cause bad problems.
539 Returns C<NULL> on failure.
543 IV (*Binmode)(pTHX_ PerlIO *f);
545 Optional. Used when C<:raw> layer is pushed (explicitly or as a result
546 of binmode(FH)). If not present layer will be popped. If present
547 should configure layer as binary (or pop itself) and return 0.
548 If it returns -1 for error C<binmode> will fail with layer
553 SV * (*Getarg)(pTHX_ PerlIO *f,
554 CLONE_PARAMS *param, int flags);
556 Optional. If present should return an SV * representing the string
557 argument passed to the layer when it was
558 pushed. e.g. ":encoding(ascii)" would return an SvPV with value
559 "ascii". (I<param> and I<flags> arguments can be ignored in most
564 IV (*Fileno)(pTHX_ PerlIO *f);
566 Returns the Unix/Posix numeric file descriptor for the handle. Normally
567 C<PerlIOBase_fileno()> (which just asks next layer down) will suffice
570 Returns -1 on error, which is considered to include the case where the
571 layer cannot provide such a file descriptor.
575 PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
576 CLONE_PARAMS *param, int flags);
578 XXX: Needs more docs.
580 Used as part of the "clone" process when a thread is spawned (in which
581 case param will be non-NULL) and when a stream is being duplicated via
584 Similar to C<Open>, returns PerlIO* on success, C<NULL> on failure.
588 SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
590 Basic read operation.
592 Typically will call C<Fill> and manipulate pointers (possibly via the
593 API). C<PerlIOBuf_read()> may be suitable for derived classes which
594 provide "fast gets" methods.
596 Returns actual bytes read, or -1 on an error.
600 SSize_t (*Unread)(pTHX_ PerlIO *f,
601 const void *vbuf, Size_t count);
603 A superset of stdio's C<ungetc()>. Should arrange for future reads to
604 see the bytes in C<vbuf>. If there is no obviously better implementation
605 then C<PerlIOBase_unread()> provides the function by pushing a "fake"
606 "pending" layer above the calling layer.
608 Returns the number of unread chars.
612 SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count);
614 Basic write operation.
616 Returns bytes written or -1 on an error.
620 IV (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
622 Position the file pointer. Should normally call its own C<Flush>
623 method and then the C<Seek> method of next layer down.
625 Returns 0 on success, -1 on failure.
629 Off_t (*Tell)(pTHX_ PerlIO *f);
631 Return the file pointer. May be based on layers cached concept of
632 position to avoid overhead.
634 Returns -1 on failure to get the file pointer.
638 IV (*Close)(pTHX_ PerlIO *f);
640 Close the stream. Should normally call C<PerlIOBase_close()> to flush
641 itself and close layers below, and then deallocate any data structures
642 (buffers, translation tables, ...) not held directly in the data
645 Returns 0 on success, -1 on failure.
649 IV (*Flush)(pTHX_ PerlIO *f);
651 Should make stream's state consistent with layers below. That is, any
652 buffered write data should be written, and file position of lower layers
653 adjusted for data read from below but not actually consumed.
654 (Should perhaps C<Unread()> such data to the lower layer.)
656 Returns 0 on success, -1 on failure.
660 IV (*Fill)(pTHX_ PerlIO *f);
662 The buffer for this layer should be filled (for read) from layer
663 below. When you "subclass" PerlIOBuf layer, you want to use its
664 I<_read> method and to supply your own fill method, which fills the
667 Returns 0 on success, -1 on failure.
671 IV (*Eof)(pTHX_ PerlIO *f);
673 Return end-of-file indicator. C<PerlIOBase_eof()> is normally sufficient.
675 Returns 0 on end-of-file, 1 if not end-of-file, -1 on error.
679 IV (*Error)(pTHX_ PerlIO *f);
681 Return error indicator. C<PerlIOBase_error()> is normally sufficient.
683 Returns 1 if there is an error (usually when C<PERLIO_F_ERROR> is set,
688 void (*Clearerr)(pTHX_ PerlIO *f);
690 Clear end-of-file and error indicators. Should call C<PerlIOBase_clearerr()>
691 to set the C<PERLIO_F_XXXXX> flags, which may suffice.
695 void (*Setlinebuf)(pTHX_ PerlIO *f);
697 Mark the stream as line buffered. C<PerlIOBase_setlinebuf()> sets the
698 PERLIO_F_LINEBUF flag and is normally sufficient.
702 STDCHAR * (*Get_base)(pTHX_ PerlIO *f);
704 Allocate (if not already done so) the read buffer for this layer and
705 return pointer to it. Return NULL on failure.
709 Size_t (*Get_bufsiz)(pTHX_ PerlIO *f);
711 Return the number of bytes that last C<Fill()> put in the buffer.
715 STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f);
717 Return the current read pointer relative to this layer's buffer.
721 SSize_t (*Get_cnt)(pTHX_ PerlIO *f);
723 Return the number of bytes left to be read in the current buffer.
727 void (*Set_ptrcnt)(pTHX_ PerlIO *f,
728 STDCHAR *ptr, SSize_t cnt);
730 Adjust the read pointer and count of bytes to match C<ptr> and/or C<cnt>.
731 The application (or layer above) must ensure they are consistent.
732 (Checking is allowed by the paranoid.)
738 To ask for the next layer down use PerlIONext(PerlIO *f).
740 To check that a PerlIO* is valid use PerlIOValid(PerlIO *f). (All
741 this does is really just to check that the pointer is non-NULL and
742 that the pointer behind that is non-NULL.)
744 PerlIOBase(PerlIO *f) returns the "Base" pointer, or in other words,
745 the C<PerlIOl*> pointer.
747 PerlIOSelf(PerlIO* f, type) return the PerlIOBase cast to a type.
749 Perl_PerlIO_or_Base(PerlIO* f, callback, base, failure, args) either
750 calls the I<callback> from the functions of the layer I<f> (just by
751 the name of the IO function, like "Read") with the I<args>, or if
752 there is no such callback, calls the I<base> version of the callback
753 with the same args, or if the f is invalid, set errno to EBADF and
756 Perl_PerlIO_or_fail(PerlIO* f, callback, failure, args) either calls
757 the I<callback> of the functions of the layer I<f> with the I<args>,
758 or if there is no such callback, set errno to EINVAL. Or if the f is
759 invalid, set errno to EBADF and return I<failure>.
761 Perl_PerlIO_or_Base_void(PerlIO* f, callback, base, args) either calls
762 the I<callback> of the functions of the layer I<f> with the I<args>,
763 or if there is no such callback, calls the I<base> version of the
764 callback with the same args, or if the f is invalid, set errno to
767 Perl_PerlIO_or_fail_void(PerlIO* f, callback, args) either calls the
768 I<callback> of the functions of the layer I<f> with the I<args>, or if
769 there is no such callback, set errno to EINVAL. Or if the f is
770 invalid, set errno to EBADF.
772 =head2 Implementing PerlIO Layers
774 If you find the implementation document unclear or not sufficient,
775 look at the existing PerlIO layer implementations, which include:
779 =item * C implementations
781 The F<perlio.c> and F<perliol.h> in the Perl core implement the
782 "unix", "perlio", "stdio", "crlf", "utf8", "byte", "raw", "pending"
783 layers, and also the "mmap" and "win32" layers if applicable.
784 (The "win32" is currently unfinished and unused, to see what is used
785 instead in Win32, see L<PerlIO/"Querying the layers of filehandles"> .)
787 PerlIO::encoding, PerlIO::scalar, PerlIO::via in the Perl core.
789 PerlIO::gzip and APR::PerlIO (mod_perl 2.0) on CPAN.
791 =item * Perl implementations
793 PerlIO::via::QuotedPrint in the Perl core and PerlIO::via::* on CPAN.
797 If you are creating a PerlIO layer, you may want to be lazy, in other
798 words, implement only the methods that interest you. The other methods
799 you can either replace with the "blank" methods
804 (which do nothing, and return zero and -1, respectively) or for
805 certain methods you may assume a default behaviour by using a NULL
806 method. The Open method looks for help in the 'parent' layer.
807 The following table summarizes the behaviour:
809 method behaviour with NULL
811 Clearerr PerlIOBase_clearerr
812 Close PerlIOBase_close
815 Error PerlIOBase_error
816 Fileno PerlIOBase_fileno
831 Setlinebuf PerlIOBase_setlinebuf
833 Unread PerlIOBase_unread
836 FAILURE Set errno (to EINVAL in UNIXish, to LIB$_INVARG in VMS) and
837 return -1 (for numeric return values) or NULL (for pointers)
838 INHERITED Inherited from the layer below
839 SUCCESS Return 0 (for numeric return values) or a pointer
843 The file C<perlio.c> provides the following layers:
849 A basic non-buffered layer which calls Unix/POSIX C<read()>, C<write()>,
850 C<lseek()>, C<close()>. No buffering. Even on platforms that distinguish
851 between O_TEXT and O_BINARY this layer is always O_BINARY.
855 A very complete generic buffering layer which provides the whole of
856 PerlIO API. It is also intended to be used as a "base class" for other
857 layers. (For example its C<Read()> method is implemented in terms of
858 the C<Get_cnt()>/C<Get_ptr()>/C<Set_ptrcnt()> methods).
860 "perlio" over "unix" provides a complete replacement for stdio as seen
861 via PerlIO API. This is the default for USE_PERLIO when system's stdio
862 does not permit perl's "fast gets" access, and which do not
863 distinguish between C<O_TEXT> and C<O_BINARY>.
867 A layer which provides the PerlIO API via the layer scheme, but
868 implements it by calling system's stdio. This is (currently) the default
869 if system's stdio provides sufficient access to allow perl's "fast gets"
870 access and which do not distinguish between C<O_TEXT> and C<O_BINARY>.
874 A layer derived using "perlio" as a base class. It provides Win32-like
875 "\n" to CR,LF translation. Can either be applied above "perlio" or serve
876 as the buffer layer itself. "crlf" over "unix" is the default if system
877 distinguishes between C<O_TEXT> and C<O_BINARY> opens. (At some point
878 "unix" will be replaced by a "native" Win32 IO layer on that platform,
879 as Win32's read/write layer has various drawbacks.) The "crlf" layer is
880 a reasonable model for a layer which transforms data in some way.
884 If Configure detects C<mmap()> functions this layer is provided (with
885 "perlio" as a "base") which does "read" operations by mmap()ing the
886 file. Performance improvement is marginal on modern systems, so it is
887 mainly there as a proof of concept. It is likely to be unbundled from
888 the core at some point. The "mmap" layer is a reasonable model for a
889 minimalist "derived" layer.
893 An "internal" derivative of "perlio" which can be used to provide
894 Unread() function for layers which have no buffer or cannot be
895 bothered. (Basically this layer's C<Fill()> pops itself off the stack
896 and so resumes reading from layer below.)
900 A dummy layer which never exists on the layer stack. Instead when
901 "pushed" it actually pops the stack removing itself, it then calls
902 Binmode function table entry on all the layers in the stack - normally
903 this (via PerlIOBase_binmode) removes any layers which do not have
904 C<PERLIO_K_RAW> bit set. Layers can modify that behaviour by defining
905 their own Binmode entry.
909 Another dummy layer. When pushed it pops itself and sets the
910 C<PERLIO_F_UTF8> flag on the layer which was (and now is once more)
911 the top of the stack.
915 In addition F<perlio.c> also provides a number of C<PerlIOBase_xxxx()>
916 functions which are intended to be used in the table slots of classes
917 which do not need to do anything special for a particular method.
919 =head2 Extension Layers
921 Layers can made available by extension modules. When an unknown layer
922 is encountered the PerlIO code will perform the equivalent of :
926 Where I<layer> is the unknown layer. F<PerlIO.pm> will then attempt to:
928 require PerlIO::layer;
930 If after that process the layer is still not defined then the C<open>
933 The following extension layers are bundled with perl:
941 makes this layer available, although F<PerlIO.pm> "knows" where to
942 find it. It is an example of a layer which takes an argument as it is
945 open( $fh, "<:encoding(iso-8859-7)", $pathname );
949 Provides support for reading data from and writing data to a scalar.
951 open( $fh, "+<:scalar", \$scalar );
953 When a handle is so opened, then reads get bytes from the string value
954 of I<$scalar>, and writes change the value. In both cases the position
955 in I<$scalar> starts as zero but can be altered via C<seek>, and
956 determined via C<tell>.
958 Please note that this layer is implied when calling open() thus:
960 open( $fh, "+<", \$scalar );
964 Provided to allow layers to be implemented as Perl code. For instance:
966 use PerlIO::via::StripHTML;
967 open( my $fh, "<:via(StripHTML)", "index.html" );
969 See L<PerlIO::via> for details.
975 Things that need to be done to improve this document.
981 Explain how to make a valid fh without going through open()(i.e. apply
982 a layer). For example if the file is not opened through perl, but we
983 want to get back a fh, like it was opened by Perl.
985 How PerlIO_apply_layera fits in, where its docs, was it made public?
987 Currently the example could be something like this:
989 PerlIO *foo_to_PerlIO(pTHX_ char *mode, ...)
991 char *mode; /* "w", "r", etc */
992 const char *layers = ":APR"; /* the layer name */
993 PerlIO *f = PerlIO_allocate(aTHX);
998 PerlIO_apply_layers(aTHX_ f, mode, layers);
1001 PerlIOAPR *st = PerlIOSelf(f, PerlIOAPR);
1002 /* fill in the st struct, as in _open() */
1004 PerlIOBase(f)->flags |= PERLIO_F_OPEN;
1013 fix/add the documentation in places marked as XXX.
1017 The handling of errors by the layer is not specified. e.g. when $!
1018 should be set explicitly, when the error handling should be just
1019 delegated to the top layer.
1021 Probably give some hints on using SETERRNO() or pointers to where they
1026 I think it would help to give some concrete examples to make it easier
1027 to understand the API. Of course I agree that the API has to be
1028 concise, but since there is no second document that is more of a
1029 guide, I think that it'd make it easier to start with the doc which is
1030 an API, but has examples in it in places where things are unclear, to
1031 a person who is not a PerlIO guru (yet).