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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
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10=head1 DESCRIPTION
11
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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).
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15
16=head2 History and Background
17
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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
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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
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27=head2 Basic Structure
28
cc7ef057 29PerlIO is a stack of layers.
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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
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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.
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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
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47The requests do not necessarily go always all the way down to the
48operating system: that's where PerlIO buffering comes into play.
49
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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
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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>,
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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
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78=head2 Layers vs Disciplines
79
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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
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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.
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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
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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.
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117
118A "layer" is composed of two parts:
119
120=over 4
121
210b36aa 122=item 1.
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124The functions and attributes of the "layer class".
125
126=item 2.
127
128The per-instance data for a particular handle.
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129
130=back
131
132=head2 Functions and Attributes
133
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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
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139 struct _PerlIO_funcs
140 {
2dc2558e 141 Size_t fsize;
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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);
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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);
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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 */
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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 */
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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 */
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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);
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177 };
178
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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:
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183
184=over 4
185
aa500c9e 186=item 1.
50b80e25 187
aa500c9e 188Opening and setup functions
50b80e25 189
aa500c9e 190=item 2.
50b80e25 191
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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.
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201
202=back
203
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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.
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210
211=head2 Per-instance Data
212
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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:
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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
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228In this way (as for perl's scalars) a pointer to a PerlIOBuf can be
229treated as a pointer to a PerlIOl.
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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.
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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
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251The above is roughly equivalent to a "stdio" buffered stream, but with
252much more flexibility:
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253
254=over 4
255
256=item *
257
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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.
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261
262=item *
263
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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.
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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
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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.
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277
278=item *
279
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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.
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283
284=back
285
286=head2 Per-instance flag bits
287
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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
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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.
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301
302=item PERLIO_F_CANREAD
303
3039a93d 304Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick).
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305
306=item PERLIO_F_ERROR
307
d4165bde 308An error has occurred (for C<PerlIO_error()>).
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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
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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
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323flag rather than add/remove layers if the C<PERLIO_K_CANCRLF> bit is set
324for the layers class.
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325
326=item PERLIO_F_UTF8
327
3039a93d 328Data written to this layer should be UTF-8 encoded; data provided
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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
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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.
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352
353=item PERLIO_F_TEMP
354
9d799145 355File has been C<unlink()>ed, or should be deleted on C<close()>.
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356
357=item PERLIO_F_OPEN
358
359Handle is open.
360
361=item PERLIO_F_FASTGETS
362
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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
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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
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370to change during one "get".)
371
372=back
373
374=head2 Methods in Detail
375
376=over 4
377
e2d9456f 378=item fsize
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379
380 Size_t fsize;
381
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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
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386=item name
387
388 char * name;
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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
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398=item size
399
400 Size_t size;
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401
402The size of the per-instance data structure, e.g.:
403
404 sizeof(PerlIOAPR)
405
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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.
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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
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414=item kind
415
416 IV kind;
d4165bde 417
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418=over 4
419
420=item * PERLIO_K_BUFFERED
421
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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
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429=item * PERLIO_K_CANCRLF
430
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431Layer can translate between "\n" and CRLF line ends.
432
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433=item * PERLIO_K_FASTGETS
434
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435Layer allows buffer snooping.
436
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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
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443=back
444
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445=item Pushed
446
447 IV (*Pushed)(pTHX_ PerlIO *f,const char *mode, SV *arg);
50b80e25 448
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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
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459Returns 0 on success. On failure returns -1 and should set errno.
460
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461=item Popped
462
463 IV (*Popped)(pTHX_ PerlIO *f);
50b80e25 464
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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
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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
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479=item Open
480
481 PerlIO * (*Open)(...);
b76cc8ba 482
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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:
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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
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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
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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.)
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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
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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
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539Returns C<NULL> on failure.
540
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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
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551=item Getarg
552
553 SV * (*Getarg)(pTHX_ PerlIO *f,
554 CLONE_PARAMS *param, int flags);
b76cc8ba 555
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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
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562=item Fileno
563
564 IV (*Fileno)(pTHX_ PerlIO *f);
b76cc8ba 565
d1be9408 566Returns the Unix/Posix numeric file descriptor for the handle. Normally
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567C<PerlIOBase_fileno()> (which just asks next layer down) will suffice
568for this.
50b80e25 569
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570Returns -1 on error, which is considered to include the case where the
571layer cannot provide such a file descriptor.
d4165bde 572
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RB
573=item Dup
574
575 PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
576 CLONE_PARAMS *param, int flags);
d4165bde 577
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NIS
578XXX: Needs more docs.
579
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580Used as part of the "clone" process when a thread is spawned (in which
581case param will be non-NULL) and when a stream is being duplicated via
582'&' in the C<open>.
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583
584Similar to C<Open>, returns PerlIO* on success, C<NULL> on failure.
585
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586=item Read
587
588 SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
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589
590Basic read operation.
50b80e25 591
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592Typically will call C<Fill> and manipulate pointers (possibly via the
593API). C<PerlIOBuf_read()> may be suitable for derived classes which
594provide "fast gets" methods.
50b80e25 595
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596Returns actual bytes read, or -1 on an error.
597
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598=item Unread
599
600 SSize_t (*Unread)(pTHX_ PerlIO *f,
601 const void *vbuf, Size_t count);
50b80e25 602
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NIS
603A superset of stdio's C<ungetc()>. Should arrange for future reads to
604see the bytes in C<vbuf>. If there is no obviously better implementation
605then C<PerlIOBase_unread()> provides the function by pushing a "fake"
606"pending" layer above the calling layer.
50b80e25 607
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608Returns the number of unread chars.
609
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610=item Write
611
612 SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count);
50b80e25 613
d4165bde 614Basic write operation.
50b80e25 615
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616Returns bytes written or -1 on an error.
617
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618=item Seek
619
620 IV (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
50b80e25 621
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622Position the file pointer. Should normally call its own C<Flush>
623method and then the C<Seek> method of next layer down.
50b80e25 624
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625Returns 0 on success, -1 on failure.
626
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627=item Tell
628
629 Off_t (*Tell)(pTHX_ PerlIO *f);
50b80e25 630
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NIS
631Return the file pointer. May be based on layers cached concept of
632position to avoid overhead.
50b80e25 633
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634Returns -1 on failure to get the file pointer.
635
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636=item Close
637
638 IV (*Close)(pTHX_ PerlIO *f);
50b80e25 639
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NIS
640Close the stream. Should normally call C<PerlIOBase_close()> to flush
641itself and close layers below, and then deallocate any data structures
642(buffers, translation tables, ...) not held directly in the data
643structure.
50b80e25 644
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645Returns 0 on success, -1 on failure.
646
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RB
647=item Flush
648
649 IV (*Flush)(pTHX_ PerlIO *f);
50b80e25 650
9d799145
NIS
651Should make stream's state consistent with layers below. That is, any
652buffered write data should be written, and file position of lower layers
d1be9408 653adjusted for data read from below but not actually consumed.
b76cc8ba 654(Should perhaps C<Unread()> such data to the lower layer.)
50b80e25 655
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656Returns 0 on success, -1 on failure.
657
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658=item Fill
659
660 IV (*Fill)(pTHX_ PerlIO *f);
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SB
661
662The buffer for this layer should be filled (for read) from layer
663below. When you "subclass" PerlIOBuf layer, you want to use its
664I<_read> method and to supply your own fill method, which fills the
665PerlIOBuf's buffer.
50b80e25 666
d4165bde 667Returns 0 on success, -1 on failure.
50b80e25 668
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RB
669=item Eof
670
671 IV (*Eof)(pTHX_ PerlIO *f);
50b80e25 672
9d799145 673Return end-of-file indicator. C<PerlIOBase_eof()> is normally sufficient.
50b80e25 674
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675Returns 0 on end-of-file, 1 if not end-of-file, -1 on error.
676
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RB
677=item Error
678
679 IV (*Error)(pTHX_ PerlIO *f);
50b80e25 680
9d799145 681Return error indicator. C<PerlIOBase_error()> is normally sufficient.
50b80e25 682
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683Returns 1 if there is an error (usually when C<PERLIO_F_ERROR> is set,
6840 otherwise.
685
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RB
686=item Clearerr
687
688 void (*Clearerr)(pTHX_ PerlIO *f);
50b80e25 689
9d799145
NIS
690Clear end-of-file and error indicators. Should call C<PerlIOBase_clearerr()>
691to set the C<PERLIO_F_XXXXX> flags, which may suffice.
50b80e25 692
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RB
693=item Setlinebuf
694
695 void (*Setlinebuf)(pTHX_ PerlIO *f);
50b80e25 696
b76cc8ba
NIS
697Mark the stream as line buffered. C<PerlIOBase_setlinebuf()> sets the
698PERLIO_F_LINEBUF flag and is normally sufficient.
50b80e25 699
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RB
700=item Get_base
701
702 STDCHAR * (*Get_base)(pTHX_ PerlIO *f);
50b80e25
NIS
703
704Allocate (if not already done so) the read buffer for this layer and
d4165bde 705return pointer to it. Return NULL on failure.
50b80e25 706
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RB
707=item Get_bufsiz
708
709 Size_t (*Get_bufsiz)(pTHX_ PerlIO *f);
50b80e25 710
9d799145 711Return the number of bytes that last C<Fill()> put in the buffer.
50b80e25 712
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713=item Get_ptr
714
715 STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f);
50b80e25 716
3039a93d 717Return the current read pointer relative to this layer's buffer.
50b80e25 718
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RB
719=item Get_cnt
720
721 SSize_t (*Get_cnt)(pTHX_ PerlIO *f);
50b80e25
NIS
722
723Return the number of bytes left to be read in the current buffer.
724
5cb3728c
RB
725=item Set_ptrcnt
726
727 void (*Set_ptrcnt)(pTHX_ PerlIO *f,
728 STDCHAR *ptr, SSize_t cnt);
50b80e25
NIS
729
730Adjust the read pointer and count of bytes to match C<ptr> and/or C<cnt>.
731The application (or layer above) must ensure they are consistent.
732(Checking is allowed by the paranoid.)
733
734=back
735
210e727c
JH
736=head2 Implementing PerlIO Layers
737
2535a4f7
JH
738If you find the implementation document unclear or not sufficient,
739look at the existing perlio layer implementations, which include:
740
741=over
742
743=item * C implementations
744
eae154c7
JH
745The F<perlio.c> and F<perliol.h> in the Perl core implement the
746"unix", "perlio", "stdio", "crlf", "utf8", "byte", "raw", "pending"
747layers, and also the "mmap" and "win32" layers if applicable.
748(The "win32" is currently unfinished and unused, to see what is used
749instead in Win32, see L<PerlIO/"Querying the layers of filehandles"> .)
750
2535a4f7
JH
751PerlIO::encoding, PerlIO::scalar, PerlIO::via in the Perl core.
752
753PerlIO::gzip and APR::PerlIO (mod_perl 2.0) on CPAN.
754
755=item * Perl implementations
756
757PerlIO::via::QuotedPrint in the Perl core and PerlIO::via::* on CPAN.
758
759=back
760
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JH
761If you are creating a PerlIO layer, you may want to be lazy, in other
762words, implement only the methods that interest you. The other methods
763you can either replace with the "blank" methods
764
765 PerlIOBase_noop_ok
766 PerlIOBase_noop_fail
767
768(which do nothing, and return zero and -1, respectively) or for
769certain methods you may assume a default behaviour by using a NULL
61bdadae
JH
770method. The Open method looks for help in the 'parent' layer.
771The following table summarizes the behaviour:
210e727c
JH
772
773 method behaviour with NULL
774
775 Clearerr PerlIOBase_clearerr
776 Close PerlIOBase_close
61bdadae 777 Dup PerlIOBase_dup
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JH
778 Eof PerlIOBase_eof
779 Error PerlIOBase_error
780 Fileno PerlIOBase_fileno
781 Fill FAILURE
782 Flush SUCCESS
61bdadae 783 Getarg SUCCESS
210e727c
JH
784 Get_base FAILURE
785 Get_bufsiz FAILURE
786 Get_cnt FAILURE
787 Get_ptr FAILURE
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JH
788 Open INHERITED
789 Popped SUCCESS
790 Pushed SUCCESS
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JH
791 Read PerlIOBase_read
792 Seek FAILURE
793 Set_cnt FAILURE
794 Set_ptrcnt FAILURE
795 Setlinebuf PerlIOBase_setlinebuf
796 Tell FAILURE
797 Unread PerlIOBase_unread
798 Write FAILURE
50b80e25 799
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JH
800 FAILURE Set errno (to EINVAL in UNIXish, to LIB$_INVARG in VMS) and
801 return -1 (for numeric return values) or NULL (for pointers)
802 INHERITED Inherited from the layer below
803 SUCCESS Return 0 (for numeric return values) or a pointer
804
50b80e25
NIS
805=head2 Core Layers
806
807The file C<perlio.c> provides the following layers:
808
809=over 4
810
811=item "unix"
812
9d799145
NIS
813A basic non-buffered layer which calls Unix/POSIX C<read()>, C<write()>,
814C<lseek()>, C<close()>. No buffering. Even on platforms that distinguish
815between O_TEXT and O_BINARY this layer is always O_BINARY.
50b80e25
NIS
816
817=item "perlio"
818
9d799145
NIS
819A very complete generic buffering layer which provides the whole of
820PerlIO API. It is also intended to be used as a "base class" for other
1d11c889
JH
821layers. (For example its C<Read()> method is implemented in terms of
822the C<Get_cnt()>/C<Get_ptr()>/C<Set_ptrcnt()> methods).
50b80e25 823
9d799145
NIS
824"perlio" over "unix" provides a complete replacement for stdio as seen
825via PerlIO API. This is the default for USE_PERLIO when system's stdio
1d11c889
JH
826does not permit perl's "fast gets" access, and which do not
827distinguish between C<O_TEXT> and C<O_BINARY>.
50b80e25
NIS
828
829=item "stdio"
830
9d799145
NIS
831A layer which provides the PerlIO API via the layer scheme, but
832implements it by calling system's stdio. This is (currently) the default
833if system's stdio provides sufficient access to allow perl's "fast gets"
834access and which do not distinguish between C<O_TEXT> and C<O_BINARY>.
50b80e25
NIS
835
836=item "crlf"
837
9d799145
NIS
838A layer derived using "perlio" as a base class. It provides Win32-like
839"\n" to CR,LF translation. Can either be applied above "perlio" or serve
840as the buffer layer itself. "crlf" over "unix" is the default if system
841distinguishes between C<O_TEXT> and C<O_BINARY> opens. (At some point
842"unix" will be replaced by a "native" Win32 IO layer on that platform,
843as Win32's read/write layer has various drawbacks.) The "crlf" layer is
844a reasonable model for a layer which transforms data in some way.
50b80e25
NIS
845
846=item "mmap"
847
9d799145
NIS
848If Configure detects C<mmap()> functions this layer is provided (with
849"perlio" as a "base") which does "read" operations by mmap()ing the
850file. Performance improvement is marginal on modern systems, so it is
851mainly there as a proof of concept. It is likely to be unbundled from
852the core at some point. The "mmap" layer is a reasonable model for a
853minimalist "derived" layer.
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NIS
854
855=item "pending"
856
9d799145 857An "internal" derivative of "perlio" which can be used to provide
1d11c889
JH
858Unread() function for layers which have no buffer or cannot be
859bothered. (Basically this layer's C<Fill()> pops itself off the stack
860and so resumes reading from layer below.)
50b80e25
NIS
861
862=item "raw"
863
9d799145 864A dummy layer which never exists on the layer stack. Instead when
86e05cf2
NIS
865"pushed" it actually pops the stack removing itself, it then calls
866Binmode function table entry on all the layers in the stack - normally
867this (via PerlIOBase_binmode) removes any layers which do not have
868C<PERLIO_K_RAW> bit set. Layers can modify that behaviour by defining
869their own Binmode entry.
50b80e25
NIS
870
871=item "utf8"
872
9d799145 873Another dummy layer. When pushed it pops itself and sets the
1d11c889
JH
874C<PERLIO_F_UTF8> flag on the layer which was (and now is once more)
875the top of the stack.
50b80e25
NIS
876
877=back
878
9d799145
NIS
879In addition F<perlio.c> also provides a number of C<PerlIOBase_xxxx()>
880functions which are intended to be used in the table slots of classes
881which do not need to do anything special for a particular method.
50b80e25
NIS
882
883=head2 Extension Layers
884
1d11c889
JH
885Layers can made available by extension modules. When an unknown layer
886is encountered the PerlIO code will perform the equivalent of :
b76cc8ba
NIS
887
888 use PerlIO 'layer';
889
1d11c889 890Where I<layer> is the unknown layer. F<PerlIO.pm> will then attempt to:
b76cc8ba
NIS
891
892 require PerlIO::layer;
893
1d11c889
JH
894If after that process the layer is still not defined then the C<open>
895will fail.
b76cc8ba
NIS
896
897The following extension layers are bundled with perl:
50b80e25
NIS
898
899=over 4
900
b76cc8ba 901=item ":encoding"
50b80e25
NIS
902
903 use Encoding;
904
1d11c889
JH
905makes this layer available, although F<PerlIO.pm> "knows" where to
906find it. It is an example of a layer which takes an argument as it is
907called thus:
50b80e25 908
b31b80f9 909 open( $fh, "<:encoding(iso-8859-7)", $pathname );
50b80e25 910
385e1f9f 911=item ":scalar"
b76cc8ba 912
b31b80f9 913Provides support for reading data from and writing data to a scalar.
b76cc8ba 914
385e1f9f 915 open( $fh, "+<:scalar", \$scalar );
50b80e25 916
1d11c889
JH
917When a handle is so opened, then reads get bytes from the string value
918of I<$scalar>, and writes change the value. In both cases the position
919in I<$scalar> starts as zero but can be altered via C<seek>, and
920determined via C<tell>.
b76cc8ba 921
385e1f9f
EM
922Please note that this layer is implied when calling open() thus:
923
924 open( $fh, "+<", \$scalar );
925
926=item ":via"
b76cc8ba 927
4f7853f4
GA
928Provided to allow layers to be implemented as Perl code. For instance:
929
e934609f 930 use PerlIO::via::StripHTML;
385e1f9f 931 open( my $fh, "<:via(StripHTML)", "index.html" );
4f7853f4 932
e934609f 933See L<PerlIO::via> for details.
b76cc8ba
NIS
934
935=back
50b80e25 936
d4165bde
SB
937=head1 TODO
938
939Things that need to be done to improve this document.
940
941=over
942
943=item *
944
945Explain how to make a valid fh without going through open()(i.e. apply
946a layer). For example if the file is not opened through perl, but we
947want to get back a fh, like it was opened by Perl.
948
949How PerlIO_apply_layera fits in, where its docs, was it made public?
950
951Currently the example could be something like this:
952
953 PerlIO *foo_to_PerlIO(pTHX_ char *mode, ...)
954 {
955 char *mode; /* "w", "r", etc */
956 const char *layers = ":APR"; /* the layer name */
957 PerlIO *f = PerlIO_allocate(aTHX);
958 if (!f) {
959 return NULL;
960 }
961
962 PerlIO_apply_layers(aTHX_ f, mode, layers);
963
964 if (f) {
965 PerlIOAPR *st = PerlIOSelf(f, PerlIOAPR);
966 /* fill in the st struct, as in _open() */
967 st->file = file;
968 PerlIOBase(f)->flags |= PERLIO_F_OPEN;
969
970 return f;
971 }
972 return NULL;
973 }
974
975=item *
976
977fix/add the documentation in places marked as XXX.
978
979=item *
980
981The handling of errors by the layer is not specified. e.g. when $!
982should be set explicitly, when the error handling should be just
983delegated to the top layer.
984
985Probably give some hints on using SETERRNO() or pointers to where they
986can be found.
987
988=item *
989
990I think it would help to give some concrete examples to make it easier
991to understand the API. Of course I agree that the API has to be
992concise, but since there is no second document that is more of a
993guide, I think that it'd make it easier to start with the doc which is
994an API, but has examples in it in places where things are unclear, to
995a person who is not a PerlIO guru (yet).
996
997=back
998
50b80e25 999=cut