extra code in pp_concat, Take 2
[p5sagit/p5-mst-13.2.git] / pod / perliol.pod
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50b80e25 1=head1 NAME
2
3perliol - C API for Perl's implementation of IO in Layers.
4
5=head1 SYNOPSIS
6
7 /* Defining a layer ... */
8 #include <perliol.h>
9
50b80e25 10=head1 DESCRIPTION
11
9d799145 12This document describes the behavior and implementation of the PerlIO
13abstraction described in L<perlapio> when C<USE_PERLIO> is defined (and
14C<USE_SFIO> is not).
50b80e25 15
16=head2 History and Background
17
9d799145 18The PerlIO abstraction was introduced in perl5.003_02 but languished as
19just an abstraction until perl5.7.0. However during that time a number
d1be9408 20of perl extensions switched to using it, so the API is mostly fixed to
9d799145 21maintain (source) compatibility.
50b80e25 22
9d799145 23The aim of the implementation is to provide the PerlIO API in a flexible
24and platform neutral manner. It is also a trial of an "Object Oriented
25C, with vtables" approach which may be applied to perl6.
50b80e25 26
cc83745d 27=head2 Basic Structure
28
cc7ef057 29PerlIO is a stack of layers.
cc83745d 30
31The low levels of the stack work with the low-level operating system
32calls (file descriptors in C) getting bytes in and out, the higher
cc7ef057 33layers of the stack buffer, filter, and otherwise manipulate the I/O,
34and return characters (or bytes) to Perl. Terms I<above> and I<below>
35are used to refer to the relative positioning of the stack layers.
cc83745d 36
37A layer contains a "vtable", the table of I/O operations (at C level
38a table of function pointers), and status flags. The functions in the
39vtable implement operations like "open", "read", and "write".
40
41When I/O, for example "read", is requested, the request goes from Perl
42first down the stack using "read" functions of each layer, then at the
43bottom the input is requested from the operating system services, then
44the result is returned up the stack, finally being interpreted as Perl
45data.
46
cc7ef057 47The requests do not necessarily go always all the way down to the
48operating system: that's where PerlIO buffering comes into play.
49
cc83745d 50When you do an open() and specify extra PerlIO layers to be deployed,
51the layers you specify are "pushed" on top of the already existing
cc7ef057 52default stack. One way to see it is that "operating system is
53on the left" and "Perl is on the right".
54
55What exact layers are in this default stack depends on a lot of
56things: your operating system, Perl version, Perl compile time
57configuration, and Perl runtime configuration. See L<PerlIO>,
cc83745d 58L<perlrun/PERLIO>, and L<open> for more information.
59
60binmode() operates similarly to open(): by default the specified
61layers are pushed on top of the existing stack.
62
63However, note that even as the specified layers are "pushed on top"
64for open() and binmode(), this doesn't mean that the effects are
65limited to the "top": PerlIO layers can be very 'active' and inspect
66and affect layers also deeper in the stack. As an example there
67is a layer called "raw" which repeatedly "pops" layers until
68it reaches the first layer that has declared itself capable of
69handling binary data. The "pushed" layers are processed in left-to-right
70order.
71
72sysopen() operates (unsurprisingly) at a lower level in the stack than
73open(). For example in UNIX or UNIX-like systems sysopen() operates
74directly at the level of file descriptors: in the terms of PerlIO
75layers, it uses only the "unix" layer, which is a rather thin wrapper
76on top of the UNIX file descriptors.
77
50b80e25 78=head2 Layers vs Disciplines
79
9d799145 80Initial discussion of the ability to modify IO streams behaviour used
81the term "discipline" for the entities which were added. This came (I
82believe) from the use of the term in "sfio", which in turn borrowed it
83from "line disciplines" on Unix terminals. However, this document (and
84the C code) uses the term "layer".
85
1d11c889 86This is, I hope, a natural term given the implementation, and should
87avoid connotations that are inherent in earlier uses of "discipline"
88for things which are rather different.
50b80e25 89
90=head2 Data Structures
91
92The basic data structure is a PerlIOl:
93
94 typedef struct _PerlIO PerlIOl;
95 typedef struct _PerlIO_funcs PerlIO_funcs;
96 typedef PerlIOl *PerlIO;
97
98 struct _PerlIO
99 {
100 PerlIOl * next; /* Lower layer */
101 PerlIO_funcs * tab; /* Functions for this layer */
102 IV flags; /* Various flags for state */
103 };
104
1d11c889 105A C<PerlIOl *> is a pointer to the struct, and the I<application>
106level C<PerlIO *> is a pointer to a C<PerlIOl *> - i.e. a pointer
107to a pointer to the struct. This allows the application level C<PerlIO *>
108to remain constant while the actual C<PerlIOl *> underneath
109changes. (Compare perl's C<SV *> which remains constant while its
110C<sv_any> field changes as the scalar's type changes.) An IO stream is
111then in general represented as a pointer to this linked-list of
112"layers".
50b80e25 113
9d799145 114It should be noted that because of the double indirection in a C<PerlIO *>,
d4165bde 115a C<< &(perlio->next) >> "is" a C<PerlIO *>, and so to some degree
11e1c8f2 116at least one layer can use the "standard" API on the next layer down.
50b80e25 117
118A "layer" is composed of two parts:
119
120=over 4
121
210b36aa 122=item 1.
50b80e25 123
210b36aa 124The functions and attributes of the "layer class".
125
126=item 2.
127
128The per-instance data for a particular handle.
50b80e25 129
130=back
131
132=head2 Functions and Attributes
133
9d799145 134The functions and attributes are accessed via the "tab" (for table)
135member of C<PerlIOl>. The functions (methods of the layer "class") are
136fixed, and are defined by the C<PerlIO_funcs> type. They are broadly the
137same as the public C<PerlIO_xxxxx> functions:
50b80e25 138
b76cc8ba 139 struct _PerlIO_funcs
140 {
2dc2558e 141 Size_t fsize;
b76cc8ba 142 char * name;
143 Size_t size;
144 IV kind;
2dc2558e 145 IV (*Pushed)(pTHX_ PerlIO *f,const char *mode,SV *arg, PerlIO_funcs *tab);
d4165bde 146 IV (*Popped)(pTHX_ PerlIO *f);
b76cc8ba 147 PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab,
148 AV *layers, IV n,
149 const char *mode,
150 int fd, int imode, int perm,
151 PerlIO *old,
152 int narg, SV **args);
86e05cf2 153 IV (*Binmode)(pTHX_ PerlIO *f);
d4165bde 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)
b76cc8ba 157 /* Unix-like functions - cf sfio line disciplines */
d4165bde 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);
b76cc8ba 164 /* Stdio-like buffered IO functions */
d4165bde 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);
b76cc8ba 171 /* Perl's snooping functions */
d4165bde 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);
b76cc8ba 177 };
178
2dc2558e 179The first few members of the struct give a function table size for
180compatibility check "name" for the layer, the size to C<malloc> for the per-instance data,
181and some flags which are attributes of the class as whole (such as whether it is a buffering
9d799145 182layer), then follow the functions which fall into four basic groups:
50b80e25 183
184=over 4
185
aa500c9e 186=item 1.
50b80e25 187
aa500c9e 188Opening and setup functions
50b80e25 189
aa500c9e 190=item 2.
50b80e25 191
aa500c9e 192Basic IO operations
193
194=item 3.
195
196Stdio class buffering options.
197
198=item 4.
199
200Functions to support Perl's traditional "fast" access to the buffer.
50b80e25 201
202=back
203
1d11c889 204A layer does not have to implement all the functions, but the whole
205table has to be present. Unimplemented slots can be NULL (which will
206result in an error when called) or can be filled in with stubs to
207"inherit" behaviour from a "base class". This "inheritance" is fixed
208for all instances of the layer, but as the layer chooses which stubs
209to populate the table, limited "multiple inheritance" is possible.
50b80e25 210
211=head2 Per-instance Data
212
1d11c889 213The per-instance data are held in memory beyond the basic PerlIOl
214struct, by making a PerlIOl the first member of the layer's struct
215thus:
50b80e25 216
217 typedef struct
218 {
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 */
226 } PerlIOBuf;
227
1d11c889 228In this way (as for perl's scalars) a pointer to a PerlIOBuf can be
229treated as a pointer to a PerlIOl.
50b80e25 230
231=head2 Layers in action.
232
233 table perlio unix
234 | |
235 +-----------+ +----------+ +--------+
236 PerlIO ->| |--->| next |--->| NULL |
237 +-----------+ +----------+ +--------+
238 | | | buffer | | fd |
239 +-----------+ | | +--------+
240 | | +----------+
241
242
243The above attempts to show how the layer scheme works in a simple case.
9d799145 244The application's C<PerlIO *> points to an entry in the table(s)
245representing open (allocated) handles. For example the first three slots
246in the table correspond to C<stdin>,C<stdout> and C<stderr>. The table
247in turn points to the current "top" layer for the handle - in this case
248an instance of the generic buffering layer "perlio". That layer in turn
249points to the next layer down - in this case the lowlevel "unix" layer.
50b80e25 250
9d799145 251The above is roughly equivalent to a "stdio" buffered stream, but with
252much more flexibility:
50b80e25 253
254=over 4
255
256=item *
257
9d799145 258If Unix level C<read>/C<write>/C<lseek> is not appropriate for (say)
259sockets then the "unix" layer can be replaced (at open time or even
260dynamically) with a "socket" layer.
50b80e25 261
262=item *
263
1d11c889 264Different handles can have different buffering schemes. The "top"
265layer could be the "mmap" layer if reading disk files was quicker
266using C<mmap> than C<read>. An "unbuffered" stream can be implemented
267simply by not having a buffer layer.
50b80e25 268
269=item *
270
271Extra layers can be inserted to process the data as it flows through.
9d799145 272This was the driving need for including the scheme in perl 5.7.0+ - we
d1be9408 273needed a mechanism to allow data to be translated between perl's
9d799145 274internal 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.
50b80e25 277
278=item *
279
1d11c889 280A layer can be added that does "\n" to CRLF translation. This layer
281can be used on any platform, not just those that normally do such
282things.
50b80e25 283
284=back
285
286=head2 Per-instance flag bits
287
1d11c889 288The generic flag bits are a hybrid of C<O_XXXXX> style flags deduced
289from the mode string passed to C<PerlIO_open()>, and state bits for
290typical buffer layers.
50b80e25 291
9d799145 292=over 4
50b80e25 293
294=item PERLIO_F_EOF
295
296End of file.
297
298=item PERLIO_F_CANWRITE
299
3039a93d 300Writes are permitted, i.e. opened as "w" or "r+" or "a", etc.
50b80e25 301
302=item PERLIO_F_CANREAD
303
3039a93d 304Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick).
50b80e25 305
306=item PERLIO_F_ERROR
307
d4165bde 308An error has occurred (for C<PerlIO_error()>).
50b80e25 309
310=item PERLIO_F_TRUNCATE
311
312Truncate file suggested by open mode.
313
314=item PERLIO_F_APPEND
315
316All writes should be appends.
317
318=item PERLIO_F_CRLF
319
11e1c8f2 320Layer is performing Win32-like "\n" mapped to CR,LF for output and CR,LF
321mapped to "\n" for input. Normally the provided "crlf" layer is the only
322layer that need bother about this. C<PerlIO_binmode()> will mess with this
9d799145 323flag rather than add/remove layers if the C<PERLIO_K_CANCRLF> bit is set
324for the layers class.
50b80e25 325
326=item PERLIO_F_UTF8
327
3039a93d 328Data written to this layer should be UTF-8 encoded; data provided
50b80e25 329by this layer should be considered UTF-8 encoded. Can be set on any layer
330by ":utf8" dummy layer. Also set on ":encoding" layer.
331
332=item PERLIO_F_UNBUF
333
334Layer is unbuffered - i.e. write to next layer down should occur for
335each write to this layer.
336
337=item PERLIO_F_WRBUF
338
339The buffer for this layer currently holds data written to it but not sent
340to next layer.
341
342=item PERLIO_F_RDBUF
343
344The buffer for this layer currently holds unconsumed data read from
345layer below.
346
347=item PERLIO_F_LINEBUF
348
9d799145 349Layer is line buffered. Write data should be passed to next layer down
350whenever a "\n" is seen. Any data beyond the "\n" should then be
351processed.
50b80e25 352
353=item PERLIO_F_TEMP
354
9d799145 355File has been C<unlink()>ed, or should be deleted on C<close()>.
50b80e25 356
357=item PERLIO_F_OPEN
358
359Handle is open.
360
361=item PERLIO_F_FASTGETS
362
9d799145 363This instance of this layer supports the "fast C<gets>" interface.
364Normally set based on C<PERLIO_K_FASTGETS> for the class and by the
d1be9408 365existence of the function(s) in the table. However a class that
50b80e25 366normally provides that interface may need to avoid it on a
367particular instance. The "pending" layer needs to do this when
d1be9408 368it is pushed above a layer which does not support the interface.
9d799145 369(Perl's C<sv_gets()> does not expect the streams fast C<gets> behaviour
50b80e25 370to change during one "get".)
371
372=back
373
374=head2 Methods in Detail
375
376=over 4
377
e2d9456f 378=item fsize
2dc2558e 379
380 Size_t fsize;
381
a489db4d 382Size of the function table. This is compared against the value PerlIO
383code "knows" as a compatibility check. Future versions I<may> be able
384to tolerate layers compiled against an old version of the headers.
2dc2558e 385
5cb3728c 386=item name
387
388 char * name;
d4165bde 389
390The name of the layer whose open() method Perl should invoke on
391open(). For example if the layer is called APR, you will call:
392
393 open $fh, ">:APR", ...
394
395and Perl knows that it has to invoke the PerlIOAPR_open() method
396implemented by the APR layer.
397
5cb3728c 398=item size
399
400 Size_t size;
d4165bde 401
402The size of the per-instance data structure, e.g.:
403
404 sizeof(PerlIOAPR)
405
a489db4d 406If this field is zero then C<PerlIO_pushed> does not malloc anything
407and assumes layer's Pushed function will do any required layer stack
408manipulation - used to avoid malloc/free overhead for dummy layers.
2dc2558e 409If the field is non-zero it must be at least the size of C<PerlIOl>,
410C<PerlIO_pushed> will allocate memory for the layer's data structures
411and link new layer onto the stream's stack. (If the layer's Pushed
412method returns an error indication the layer is popped again.)
413
5cb3728c 414=item kind
415
416 IV kind;
d4165bde 417
d4165bde 418=over 4
419
420=item * PERLIO_K_BUFFERED
421
86e05cf2 422The layer is buffered.
423
424=item * PERLIO_K_RAW
425
426The 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.
428
d4165bde 429=item * PERLIO_K_CANCRLF
430
86e05cf2 431Layer can translate between "\n" and CRLF line ends.
432
d4165bde 433=item * PERLIO_K_FASTGETS
434
86e05cf2 435Layer allows buffer snooping.
436
d4165bde 437=item * PERLIO_K_MULTIARG
438
439Used when the layer's open() accepts more arguments than usual. The
440extra arguments should come not before the C<MODE> argument. When this
441flag is used it's up to the layer to validate the args.
442
d4165bde 443=back
444
5cb3728c 445=item Pushed
446
447 IV (*Pushed)(pTHX_ PerlIO *f,const char *mode, SV *arg);
50b80e25 448
1d11c889 449The only absolutely mandatory method. Called when the layer is pushed
450onto the stack. The C<mode> argument may be NULL if this occurs
451post-open. The C<arg> will be non-C<NULL> if an argument string was
452passed. In most cases this should call C<PerlIOBase_pushed()> to
453convert C<mode> into the appropriate C<PERLIO_F_XXXXX> flags in
454addition to any actions the layer itself takes. If a layer is not
455expecting an argument it need neither save the one passed to it, nor
456provide C<Getarg()> (it could perhaps C<Perl_warn> that the argument
457was un-expected).
50b80e25 458
d4165bde 459Returns 0 on success. On failure returns -1 and should set errno.
460
5cb3728c 461=item Popped
462
463 IV (*Popped)(pTHX_ PerlIO *f);
50b80e25 464
1d11c889 465Called when the layer is popped from the stack. A layer will normally
466be popped after C<Close()> is called. But a layer can be popped
467without being closed if the program is dynamically managing layers on
468the stream. In such cases C<Popped()> should free any resources
469(buffers, translation tables, ...) not held directly in the layer's
470struct. It should also C<Unread()> any unconsumed data that has been
471read and buffered from the layer below back to that layer, so that it
472can be re-provided to what ever is now above.
b76cc8ba 473
3077d0b1 474Returns 0 on success and failure. If C<Popped()> returns I<true> then
475I<perlio.c> assumes that either the layer has popped itself, or the
476layer is super special and needs to be retained for other reasons.
477In most cases it should return I<false>.
d4165bde 478
5cb3728c 479=item Open
480
481 PerlIO * (*Open)(...);
b76cc8ba 482
1d11c889 483The C<Open()> method has lots of arguments because it combines the
484functions of perl's C<open>, C<PerlIO_open>, perl's C<sysopen>,
485C<PerlIO_fdopen> and C<PerlIO_reopen>. The full prototype is as
486follows:
b76cc8ba 487
488 PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab,
489 AV *layers, IV n,
490 const char *mode,
491 int fd, int imode, int perm,
492 PerlIO *old,
493 int narg, SV **args);
494
1d11c889 495Open should (perhaps indirectly) call C<PerlIO_allocate()> to allocate
496a slot in the table and associate it with the layers information for
497the opened file, by calling C<PerlIO_push>. The I<layers> AV is an
498array of all the layers destined for the C<PerlIO *>, and any
499arguments passed to them, I<n> is the index into that array of the
500layer being called. The macro C<PerlIOArg> will return a (possibly
501C<NULL>) SV * for the argument passed to the layer.
502
503The I<mode> string is an "C<fopen()>-like" string which would match
504the regular expression C</^[I#]?[rwa]\+?[bt]?$/>.
505
506The C<'I'> prefix is used during creation of C<stdin>..C<stderr> via
507special C<PerlIO_fdopen> calls; the C<'#'> prefix means that this is
508C<sysopen> and that I<imode> and I<perm> should be passed to
509C<PerlLIO_open3>; C<'r'> means B<r>ead, C<'w'> means B<w>rite and
510C<'a'> means B<a>ppend. The C<'+'> suffix means that both reading and
a489db4d 511writing/appending are permitted. The C<'b'> suffix means file should
512be binary, and C<'t'> means it is text. (Almost all layers should do
513the IO in binary mode, and ignore the b/t bits. The C<:crlf> layer
514should be pushed to handle the distinction.)
1d11c889 515
516If I<old> is not C<NULL> then this is a C<PerlIO_reopen>. Perl itself
517does not use this (yet?) and semantics are a little vague.
518
519If I<fd> not negative then it is the numeric file descriptor I<fd>,
520which will be open in a manner compatible with the supplied mode
521string, the call is thus equivalent to C<PerlIO_fdopen>. In this case
522I<nargs> will be zero.
523
524If I<nargs> is greater than zero then it gives the number of arguments
525passed to C<open>, otherwise it will be 1 if for example
526C<PerlIO_open> was called. In simple cases SvPV_nolen(*args) is the
527pathname to open.
528
529Having said all that translation-only layers do not need to provide
530C<Open()> at all, but rather leave the opening to a lower level layer
531and wait to be "pushed". If a layer does provide C<Open()> it should
532normally call the C<Open()> method of next layer down (if any) and
533then push itself on top if that succeeds.
b76cc8ba 534
3077d0b1 535If C<PerlIO_push> was performed and open has failed, it must
536C<PerlIO_pop> itself, since if it's not, the layer won't be removed
537and may cause bad problems.
538
d4165bde 539Returns C<NULL> on failure.
540
86e05cf2 541=item Binmode
542
543 IV (*Binmode)(pTHX_ PerlIO *f);
544
545Optional. Used when C<:raw> layer is pushed (explicitly or as a result
546of binmode(FH)). If not present layer will be popped. If present
547should configure layer as binary (or pop itself) and return 0.
548If it returns -1 for error C<binmode> will fail with layer
549still on the stack.
550
5cb3728c 551=item Getarg
552
553 SV * (*Getarg)(pTHX_ PerlIO *f,
554 CLONE_PARAMS *param, int flags);
b76cc8ba 555
d4165bde 556Optional. If present should return an SV * representing the string
557argument passed to the layer when it was
558pushed. e.g. ":encoding(ascii)" would return an SvPV with value
559"ascii". (I<param> and I<flags> arguments can be ignored in most
560cases)
b76cc8ba 561
15355bd6 562C<Dup> uses C<Getarg> to retrieve the argument originally passed to
563C<Pushed>, so you must implement this function if your layer has an
564extra argument to C<Pushed> and will ever be C<Dup>ed.
8741baa6 565
5cb3728c 566=item Fileno
567
568 IV (*Fileno)(pTHX_ PerlIO *f);
b76cc8ba 569
d1be9408 570Returns the Unix/Posix numeric file descriptor for the handle. Normally
b76cc8ba 571C<PerlIOBase_fileno()> (which just asks next layer down) will suffice
572for this.
50b80e25 573
a489db4d 574Returns -1 on error, which is considered to include the case where the
575layer cannot provide such a file descriptor.
d4165bde 576
5cb3728c 577=item Dup
578
579 PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
580 CLONE_PARAMS *param, int flags);
d4165bde 581
2dc2558e 582XXX: Needs more docs.
583
a489db4d 584Used as part of the "clone" process when a thread is spawned (in which
585case param will be non-NULL) and when a stream is being duplicated via
586'&' in the C<open>.
d4165bde 587
588Similar to C<Open>, returns PerlIO* on success, C<NULL> on failure.
589
5cb3728c 590=item Read
591
592 SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
d4165bde 593
594Basic read operation.
50b80e25 595
d4165bde 596Typically will call C<Fill> and manipulate pointers (possibly via the
597API). C<PerlIOBuf_read()> may be suitable for derived classes which
598provide "fast gets" methods.
50b80e25 599
d4165bde 600Returns actual bytes read, or -1 on an error.
601
5cb3728c 602=item Unread
603
604 SSize_t (*Unread)(pTHX_ PerlIO *f,
605 const void *vbuf, Size_t count);
50b80e25 606
9d799145 607A superset of stdio's C<ungetc()>. Should arrange for future reads to
608see the bytes in C<vbuf>. If there is no obviously better implementation
609then C<PerlIOBase_unread()> provides the function by pushing a "fake"
610"pending" layer above the calling layer.
50b80e25 611
d4165bde 612Returns the number of unread chars.
613
5cb3728c 614=item Write
615
616 SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count);
50b80e25 617
d4165bde 618Basic write operation.
50b80e25 619
d4165bde 620Returns bytes written or -1 on an error.
621
5cb3728c 622=item Seek
623
624 IV (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
50b80e25 625
1d11c889 626Position the file pointer. Should normally call its own C<Flush>
627method and then the C<Seek> method of next layer down.
50b80e25 628
d4165bde 629Returns 0 on success, -1 on failure.
630
5cb3728c 631=item Tell
632
633 Off_t (*Tell)(pTHX_ PerlIO *f);
50b80e25 634
9d799145 635Return the file pointer. May be based on layers cached concept of
636position to avoid overhead.
50b80e25 637
d4165bde 638Returns -1 on failure to get the file pointer.
639
5cb3728c 640=item Close
641
642 IV (*Close)(pTHX_ PerlIO *f);
50b80e25 643
9d799145 644Close the stream. Should normally call C<PerlIOBase_close()> to flush
645itself and close layers below, and then deallocate any data structures
646(buffers, translation tables, ...) not held directly in the data
647structure.
50b80e25 648
d4165bde 649Returns 0 on success, -1 on failure.
650
5cb3728c 651=item Flush
652
653 IV (*Flush)(pTHX_ PerlIO *f);
50b80e25 654
9d799145 655Should make stream's state consistent with layers below. That is, any
656buffered write data should be written, and file position of lower layers
d1be9408 657adjusted for data read from below but not actually consumed.
b76cc8ba 658(Should perhaps C<Unread()> such data to the lower layer.)
50b80e25 659
d4165bde 660Returns 0 on success, -1 on failure.
661
5cb3728c 662=item Fill
663
664 IV (*Fill)(pTHX_ PerlIO *f);
d4165bde 665
666The buffer for this layer should be filled (for read) from layer
667below. When you "subclass" PerlIOBuf layer, you want to use its
668I<_read> method and to supply your own fill method, which fills the
669PerlIOBuf's buffer.
50b80e25 670
d4165bde 671Returns 0 on success, -1 on failure.
50b80e25 672
5cb3728c 673=item Eof
674
675 IV (*Eof)(pTHX_ PerlIO *f);
50b80e25 676
9d799145 677Return end-of-file indicator. C<PerlIOBase_eof()> is normally sufficient.
50b80e25 678
d4165bde 679Returns 0 on end-of-file, 1 if not end-of-file, -1 on error.
680
5cb3728c 681=item Error
682
683 IV (*Error)(pTHX_ PerlIO *f);
50b80e25 684
9d799145 685Return error indicator. C<PerlIOBase_error()> is normally sufficient.
50b80e25 686
d4165bde 687Returns 1 if there is an error (usually when C<PERLIO_F_ERROR> is set,
6880 otherwise.
689
5cb3728c 690=item Clearerr
691
692 void (*Clearerr)(pTHX_ PerlIO *f);
50b80e25 693
9d799145 694Clear end-of-file and error indicators. Should call C<PerlIOBase_clearerr()>
695to set the C<PERLIO_F_XXXXX> flags, which may suffice.
50b80e25 696
5cb3728c 697=item Setlinebuf
698
699 void (*Setlinebuf)(pTHX_ PerlIO *f);
50b80e25 700
b76cc8ba 701Mark the stream as line buffered. C<PerlIOBase_setlinebuf()> sets the
702PERLIO_F_LINEBUF flag and is normally sufficient.
50b80e25 703
5cb3728c 704=item Get_base
705
706 STDCHAR * (*Get_base)(pTHX_ PerlIO *f);
50b80e25 707
708Allocate (if not already done so) the read buffer for this layer and
d4165bde 709return pointer to it. Return NULL on failure.
50b80e25 710
5cb3728c 711=item Get_bufsiz
712
713 Size_t (*Get_bufsiz)(pTHX_ PerlIO *f);
50b80e25 714
9d799145 715Return the number of bytes that last C<Fill()> put in the buffer.
50b80e25 716
5cb3728c 717=item Get_ptr
718
719 STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f);
50b80e25 720
3039a93d 721Return the current read pointer relative to this layer's buffer.
50b80e25 722
5cb3728c 723=item Get_cnt
724
725 SSize_t (*Get_cnt)(pTHX_ PerlIO *f);
50b80e25 726
727Return the number of bytes left to be read in the current buffer.
728
5cb3728c 729=item Set_ptrcnt
730
731 void (*Set_ptrcnt)(pTHX_ PerlIO *f,
732 STDCHAR *ptr, SSize_t cnt);
50b80e25 733
734Adjust the read pointer and count of bytes to match C<ptr> and/or C<cnt>.
735The application (or layer above) must ensure they are consistent.
736(Checking is allowed by the paranoid.)
737
738=back
739
b0a2be78 740=head2 Utilities
741
742To ask for the next layer down use PerlIONext(PerlIO *f).
743
744To check that a PerlIO* is valid use PerlIOValid(PerlIO *f). (All
745this does is really just to check that the pointer is non-NULL and
746that the pointer behind that is non-NULL.)
747
748PerlIOBase(PerlIO *f) returns the "Base" pointer, or in other words,
749the C<PerlIOl*> pointer.
750
751PerlIOSelf(PerlIO* f, type) return the PerlIOBase cast to a type.
752
753Perl_PerlIO_or_Base(PerlIO* f, callback, base, failure, args) either
754calls the I<callback> from the functions of the layer I<f> (just by
755the name of the IO function, like "Read") with the I<args>, or if
756there is no such callback, calls the I<base> version of the callback
757with the same args, or if the f is invalid, set errno to EBADF and
758return I<failure>.
759
760Perl_PerlIO_or_fail(PerlIO* f, callback, failure, args) either calls
761the I<callback> of the functions of the layer I<f> with the I<args>,
762or if there is no such callback, set errno to EINVAL. Or if the f is
763invalid, set errno to EBADF and return I<failure>.
764
765Perl_PerlIO_or_Base_void(PerlIO* f, callback, base, args) either calls
766the I<callback> of the functions of the layer I<f> with the I<args>,
767or if there is no such callback, calls the I<base> version of the
768callback with the same args, or if the f is invalid, set errno to
769EBADF.
770
771Perl_PerlIO_or_fail_void(PerlIO* f, callback, args) either calls the
772I<callback> of the functions of the layer I<f> with the I<args>, or if
773there is no such callback, set errno to EINVAL. Or if the f is
774invalid, set errno to EBADF.
775
210e727c 776=head2 Implementing PerlIO Layers
777
2535a4f7 778If you find the implementation document unclear or not sufficient,
b0a2be78 779look at the existing PerlIO layer implementations, which include:
2535a4f7 780
781=over
782
783=item * C implementations
784
eae154c7 785The F<perlio.c> and F<perliol.h> in the Perl core implement the
786"unix", "perlio", "stdio", "crlf", "utf8", "byte", "raw", "pending"
787layers, and also the "mmap" and "win32" layers if applicable.
788(The "win32" is currently unfinished and unused, to see what is used
789instead in Win32, see L<PerlIO/"Querying the layers of filehandles"> .)
790
2535a4f7 791PerlIO::encoding, PerlIO::scalar, PerlIO::via in the Perl core.
792
793PerlIO::gzip and APR::PerlIO (mod_perl 2.0) on CPAN.
794
795=item * Perl implementations
796
797PerlIO::via::QuotedPrint in the Perl core and PerlIO::via::* on CPAN.
798
799=back
800
210e727c 801If you are creating a PerlIO layer, you may want to be lazy, in other
802words, implement only the methods that interest you. The other methods
803you can either replace with the "blank" methods
804
805 PerlIOBase_noop_ok
806 PerlIOBase_noop_fail
807
808(which do nothing, and return zero and -1, respectively) or for
809certain methods you may assume a default behaviour by using a NULL
61bdadae 810method. The Open method looks for help in the 'parent' layer.
811The following table summarizes the behaviour:
210e727c 812
813 method behaviour with NULL
814
815 Clearerr PerlIOBase_clearerr
816 Close PerlIOBase_close
61bdadae 817 Dup PerlIOBase_dup
210e727c 818 Eof PerlIOBase_eof
819 Error PerlIOBase_error
820 Fileno PerlIOBase_fileno
821 Fill FAILURE
822 Flush SUCCESS
61bdadae 823 Getarg SUCCESS
210e727c 824 Get_base FAILURE
825 Get_bufsiz FAILURE
826 Get_cnt FAILURE
827 Get_ptr FAILURE
61bdadae 828 Open INHERITED
829 Popped SUCCESS
830 Pushed SUCCESS
210e727c 831 Read PerlIOBase_read
832 Seek FAILURE
833 Set_cnt FAILURE
834 Set_ptrcnt FAILURE
835 Setlinebuf PerlIOBase_setlinebuf
836 Tell FAILURE
837 Unread PerlIOBase_unread
838 Write FAILURE
50b80e25 839
61bdadae 840 FAILURE Set errno (to EINVAL in UNIXish, to LIB$_INVARG in VMS) and
841 return -1 (for numeric return values) or NULL (for pointers)
842 INHERITED Inherited from the layer below
843 SUCCESS Return 0 (for numeric return values) or a pointer
844
50b80e25 845=head2 Core Layers
846
847The file C<perlio.c> provides the following layers:
848
849=over 4
850
851=item "unix"
852
9d799145 853A basic non-buffered layer which calls Unix/POSIX C<read()>, C<write()>,
854C<lseek()>, C<close()>. No buffering. Even on platforms that distinguish
855between O_TEXT and O_BINARY this layer is always O_BINARY.
50b80e25 856
857=item "perlio"
858
9d799145 859A very complete generic buffering layer which provides the whole of
860PerlIO API. It is also intended to be used as a "base class" for other
1d11c889 861layers. (For example its C<Read()> method is implemented in terms of
862the C<Get_cnt()>/C<Get_ptr()>/C<Set_ptrcnt()> methods).
50b80e25 863
9d799145 864"perlio" over "unix" provides a complete replacement for stdio as seen
865via PerlIO API. This is the default for USE_PERLIO when system's stdio
1d11c889 866does not permit perl's "fast gets" access, and which do not
867distinguish between C<O_TEXT> and C<O_BINARY>.
50b80e25 868
869=item "stdio"
870
9d799145 871A layer which provides the PerlIO API via the layer scheme, but
872implements it by calling system's stdio. This is (currently) the default
873if system's stdio provides sufficient access to allow perl's "fast gets"
874access and which do not distinguish between C<O_TEXT> and C<O_BINARY>.
50b80e25 875
876=item "crlf"
877
9d799145 878A layer derived using "perlio" as a base class. It provides Win32-like
879"\n" to CR,LF translation. Can either be applied above "perlio" or serve
880as the buffer layer itself. "crlf" over "unix" is the default if system
881distinguishes between C<O_TEXT> and C<O_BINARY> opens. (At some point
882"unix" will be replaced by a "native" Win32 IO layer on that platform,
883as Win32's read/write layer has various drawbacks.) The "crlf" layer is
884a reasonable model for a layer which transforms data in some way.
50b80e25 885
886=item "mmap"
887
9d799145 888If Configure detects C<mmap()> functions this layer is provided (with
889"perlio" as a "base") which does "read" operations by mmap()ing the
890file. Performance improvement is marginal on modern systems, so it is
891mainly there as a proof of concept. It is likely to be unbundled from
892the core at some point. The "mmap" layer is a reasonable model for a
893minimalist "derived" layer.
50b80e25 894
895=item "pending"
896
9d799145 897An "internal" derivative of "perlio" which can be used to provide
1d11c889 898Unread() function for layers which have no buffer or cannot be
899bothered. (Basically this layer's C<Fill()> pops itself off the stack
900and so resumes reading from layer below.)
50b80e25 901
902=item "raw"
903
9d799145 904A dummy layer which never exists on the layer stack. Instead when
86e05cf2 905"pushed" it actually pops the stack removing itself, it then calls
906Binmode function table entry on all the layers in the stack - normally
907this (via PerlIOBase_binmode) removes any layers which do not have
908C<PERLIO_K_RAW> bit set. Layers can modify that behaviour by defining
909their own Binmode entry.
50b80e25 910
911=item "utf8"
912
9d799145 913Another dummy layer. When pushed it pops itself and sets the
1d11c889 914C<PERLIO_F_UTF8> flag on the layer which was (and now is once more)
915the top of the stack.
50b80e25 916
917=back
918
9d799145 919In addition F<perlio.c> also provides a number of C<PerlIOBase_xxxx()>
920functions which are intended to be used in the table slots of classes
921which do not need to do anything special for a particular method.
50b80e25 922
923=head2 Extension Layers
924
1d11c889 925Layers can made available by extension modules. When an unknown layer
926is encountered the PerlIO code will perform the equivalent of :
b76cc8ba 927
928 use PerlIO 'layer';
929
1d11c889 930Where I<layer> is the unknown layer. F<PerlIO.pm> will then attempt to:
b76cc8ba 931
932 require PerlIO::layer;
933
1d11c889 934If after that process the layer is still not defined then the C<open>
935will fail.
b76cc8ba 936
937The following extension layers are bundled with perl:
50b80e25 938
939=over 4
940
b76cc8ba 941=item ":encoding"
50b80e25 942
943 use Encoding;
944
1d11c889 945makes this layer available, although F<PerlIO.pm> "knows" where to
946find it. It is an example of a layer which takes an argument as it is
947called thus:
50b80e25 948
b31b80f9 949 open( $fh, "<:encoding(iso-8859-7)", $pathname );
50b80e25 950
385e1f9f 951=item ":scalar"
b76cc8ba 952
b31b80f9 953Provides support for reading data from and writing data to a scalar.
b76cc8ba 954
385e1f9f 955 open( $fh, "+<:scalar", \$scalar );
50b80e25 956
1d11c889 957When a handle is so opened, then reads get bytes from the string value
958of I<$scalar>, and writes change the value. In both cases the position
959in I<$scalar> starts as zero but can be altered via C<seek>, and
960determined via C<tell>.
b76cc8ba 961
385e1f9f 962Please note that this layer is implied when calling open() thus:
963
964 open( $fh, "+<", \$scalar );
965
966=item ":via"
b76cc8ba 967
4f7853f4 968Provided to allow layers to be implemented as Perl code. For instance:
969
e934609f 970 use PerlIO::via::StripHTML;
385e1f9f 971 open( my $fh, "<:via(StripHTML)", "index.html" );
4f7853f4 972
e934609f 973See L<PerlIO::via> for details.
b76cc8ba 974
975=back
50b80e25 976
d4165bde 977=head1 TODO
978
979Things that need to be done to improve this document.
980
981=over
982
983=item *
984
985Explain how to make a valid fh without going through open()(i.e. apply
986a layer). For example if the file is not opened through perl, but we
987want to get back a fh, like it was opened by Perl.
988
989How PerlIO_apply_layera fits in, where its docs, was it made public?
990
991Currently the example could be something like this:
992
993 PerlIO *foo_to_PerlIO(pTHX_ char *mode, ...)
994 {
995 char *mode; /* "w", "r", etc */
996 const char *layers = ":APR"; /* the layer name */
997 PerlIO *f = PerlIO_allocate(aTHX);
998 if (!f) {
999 return NULL;
1000 }
1001
1002 PerlIO_apply_layers(aTHX_ f, mode, layers);
1003
1004 if (f) {
1005 PerlIOAPR *st = PerlIOSelf(f, PerlIOAPR);
1006 /* fill in the st struct, as in _open() */
1007 st->file = file;
1008 PerlIOBase(f)->flags |= PERLIO_F_OPEN;
1009
1010 return f;
1011 }
1012 return NULL;
1013 }
1014
1015=item *
1016
1017fix/add the documentation in places marked as XXX.
1018
1019=item *
1020
1021The handling of errors by the layer is not specified. e.g. when $!
1022should be set explicitly, when the error handling should be just
1023delegated to the top layer.
1024
1025Probably give some hints on using SETERRNO() or pointers to where they
1026can be found.
1027
1028=item *
1029
1030I think it would help to give some concrete examples to make it easier
1031to understand the API. Of course I agree that the API has to be
1032concise, but since there is no second document that is more of a
1033guide, I think that it'd make it easier to start with the doc which is
1034an API, but has examples in it in places where things are unclear, to
1035a person who is not a PerlIO guru (yet).
1036
1037=back
1038
50b80e25 1039=cut