3 perliol - C API for Perl's implementation of IO in Layers.
7 /* Defining a layer ... */
12 This document describes the behavior and implementation of the PerlIO
13 abstraction described in L<perlapio> when C<USE_PERLIO> is defined (and
16 =head2 History and Background
18 The PerlIO abstraction was introduced in perl5.003_02 but languished as
19 just an abstraction until perl5.7.0. However during that time a number
20 of perl extensions switched to using it, so the API is mostly fixed to
21 maintain (source) compatibility.
23 The aim of the implementation is to provide the PerlIO API in a flexible
24 and platform neutral manner. It is also a trial of an "Object Oriented
25 C, with vtables" approach which may be applied to perl6.
27 =head2 Layers vs Disciplines
29 Initial discussion of the ability to modify IO streams behaviour used
30 the term "discipline" for the entities which were added. This came (I
31 believe) from the use of the term in "sfio", which in turn borrowed it
32 from "line disciplines" on Unix terminals. However, this document (and
33 the C code) uses the term "layer".
35 This is, I hope, a natural term given the implementation, and should
36 avoid connotations that are inherent in earlier uses of "discipline"
37 for things which are rather different.
39 =head2 Data Structures
41 The basic data structure is a PerlIOl:
43 typedef struct _PerlIO PerlIOl;
44 typedef struct _PerlIO_funcs PerlIO_funcs;
45 typedef PerlIOl *PerlIO;
49 PerlIOl * next; /* Lower layer */
50 PerlIO_funcs * tab; /* Functions for this layer */
51 IV flags; /* Various flags for state */
54 A C<PerlIOl *> is a pointer to the struct, and the I<application>
55 level C<PerlIO *> is a pointer to a C<PerlIOl *> - i.e. a pointer
56 to a pointer to the struct. This allows the application level C<PerlIO *>
57 to remain constant while the actual C<PerlIOl *> underneath
58 changes. (Compare perl's C<SV *> which remains constant while its
59 C<sv_any> field changes as the scalar's type changes.) An IO stream is
60 then in general represented as a pointer to this linked-list of
63 It should be noted that because of the double indirection in a C<PerlIO *>,
64 a C<< &(perlio->next) >> "is" a C<PerlIO *>, and so to some degree
65 at least one layer can use the "standard" API on the next layer down.
67 A "layer" is composed of two parts:
73 The functions and attributes of the "layer class".
77 The per-instance data for a particular handle.
81 =head2 Functions and Attributes
83 The functions and attributes are accessed via the "tab" (for table)
84 member of C<PerlIOl>. The functions (methods of the layer "class") are
85 fixed, and are defined by the C<PerlIO_funcs> type. They are broadly the
86 same as the public C<PerlIO_xxxxx> functions:
93 IV (*Pushed)(pTHX_ PerlIO *f,const char *mode,SV *arg);
94 IV (*Popped)(pTHX_ PerlIO *f);
95 PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab,
98 int fd, int imode, int perm,
100 int narg, SV **args);
101 IV (*Binmode)(pTHX_ PerlIO *f);
102 SV * (*Getarg)(pTHX_ PerlIO *f, CLONE_PARAMS *param, int flags)
103 IV (*Fileno)(pTHX_ PerlIO *f);
104 PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o, CLONE_PARAMS *param, int flags)
105 /* Unix-like functions - cf sfio line disciplines */
106 SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
107 SSize_t (*Unread)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
108 SSize_t (*Write)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
109 IV (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
110 Off_t (*Tell)(pTHX_ PerlIO *f);
111 IV (*Close)(pTHX_ PerlIO *f);
112 /* Stdio-like buffered IO functions */
113 IV (*Flush)(pTHX_ PerlIO *f);
114 IV (*Fill)(pTHX_ PerlIO *f);
115 IV (*Eof)(pTHX_ PerlIO *f);
116 IV (*Error)(pTHX_ PerlIO *f);
117 void (*Clearerr)(pTHX_ PerlIO *f);
118 void (*Setlinebuf)(pTHX_ PerlIO *f);
119 /* Perl's snooping functions */
120 STDCHAR * (*Get_base)(pTHX_ PerlIO *f);
121 Size_t (*Get_bufsiz)(pTHX_ PerlIO *f);
122 STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f);
123 SSize_t (*Get_cnt)(pTHX_ PerlIO *f);
124 void (*Set_ptrcnt)(pTHX_ PerlIO *f,STDCHAR *ptr,SSize_t cnt);
127 The first few members of the struct give a "name" for the layer, the
128 size to C<malloc> for the per-instance data, and some flags which are
129 attributes of the class as whole (such as whether it is a buffering
130 layer), then follow the functions which fall into four basic groups:
136 Opening and setup functions
144 Stdio class buffering options.
148 Functions to support Perl's traditional "fast" access to the buffer.
152 A layer does not have to implement all the functions, but the whole
153 table has to be present. Unimplemented slots can be NULL (which will
154 result in an error when called) or can be filled in with stubs to
155 "inherit" behaviour from a "base class". This "inheritance" is fixed
156 for all instances of the layer, but as the layer chooses which stubs
157 to populate the table, limited "multiple inheritance" is possible.
159 =head2 Per-instance Data
161 The per-instance data are held in memory beyond the basic PerlIOl
162 struct, by making a PerlIOl the first member of the layer's struct
167 struct _PerlIO base; /* Base "class" info */
168 STDCHAR * buf; /* Start of buffer */
169 STDCHAR * end; /* End of valid part of buffer */
170 STDCHAR * ptr; /* Current position in buffer */
171 Off_t posn; /* Offset of buf into the file */
172 Size_t bufsiz; /* Real size of buffer */
173 IV oneword; /* Emergency buffer */
176 In this way (as for perl's scalars) a pointer to a PerlIOBuf can be
177 treated as a pointer to a PerlIOl.
179 =head2 Layers in action.
183 +-----------+ +----------+ +--------+
184 PerlIO ->| |--->| next |--->| NULL |
185 +-----------+ +----------+ +--------+
186 | | | buffer | | fd |
187 +-----------+ | | +--------+
191 The above attempts to show how the layer scheme works in a simple case.
192 The application's C<PerlIO *> points to an entry in the table(s)
193 representing open (allocated) handles. For example the first three slots
194 in the table correspond to C<stdin>,C<stdout> and C<stderr>. The table
195 in turn points to the current "top" layer for the handle - in this case
196 an instance of the generic buffering layer "perlio". That layer in turn
197 points to the next layer down - in this case the lowlevel "unix" layer.
199 The above is roughly equivalent to a "stdio" buffered stream, but with
200 much more flexibility:
206 If Unix level C<read>/C<write>/C<lseek> is not appropriate for (say)
207 sockets then the "unix" layer can be replaced (at open time or even
208 dynamically) with a "socket" layer.
212 Different handles can have different buffering schemes. The "top"
213 layer could be the "mmap" layer if reading disk files was quicker
214 using C<mmap> than C<read>. An "unbuffered" stream can be implemented
215 simply by not having a buffer layer.
219 Extra layers can be inserted to process the data as it flows through.
220 This was the driving need for including the scheme in perl 5.7.0+ - we
221 needed a mechanism to allow data to be translated between perl's
222 internal encoding (conceptually at least Unicode as UTF-8), and the
223 "native" format used by the system. This is provided by the
224 ":encoding(xxxx)" layer which typically sits above the buffering layer.
228 A layer can be added that does "\n" to CRLF translation. This layer
229 can be used on any platform, not just those that normally do such
234 =head2 Per-instance flag bits
236 The generic flag bits are a hybrid of C<O_XXXXX> style flags deduced
237 from the mode string passed to C<PerlIO_open()>, and state bits for
238 typical buffer layers.
246 =item PERLIO_F_CANWRITE
248 Writes are permitted, i.e. opened as "w" or "r+" or "a", etc.
250 =item PERLIO_F_CANREAD
252 Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick).
256 An error has occurred (for C<PerlIO_error()>).
258 =item PERLIO_F_TRUNCATE
260 Truncate file suggested by open mode.
262 =item PERLIO_F_APPEND
264 All writes should be appends.
268 Layer is performing Win32-like "\n" mapped to CR,LF for output and CR,LF
269 mapped to "\n" for input. Normally the provided "crlf" layer is the only
270 layer that need bother about this. C<PerlIO_binmode()> will mess with this
271 flag rather than add/remove layers if the C<PERLIO_K_CANCRLF> bit is set
272 for the layers class.
276 Data written to this layer should be UTF-8 encoded; data provided
277 by this layer should be considered UTF-8 encoded. Can be set on any layer
278 by ":utf8" dummy layer. Also set on ":encoding" layer.
282 Layer is unbuffered - i.e. write to next layer down should occur for
283 each write to this layer.
287 The buffer for this layer currently holds data written to it but not sent
292 The buffer for this layer currently holds unconsumed data read from
295 =item PERLIO_F_LINEBUF
297 Layer is line buffered. Write data should be passed to next layer down
298 whenever a "\n" is seen. Any data beyond the "\n" should then be
303 File has been C<unlink()>ed, or should be deleted on C<close()>.
309 =item PERLIO_F_FASTGETS
311 This instance of this layer supports the "fast C<gets>" interface.
312 Normally set based on C<PERLIO_K_FASTGETS> for the class and by the
313 existence of the function(s) in the table. However a class that
314 normally provides that interface may need to avoid it on a
315 particular instance. The "pending" layer needs to do this when
316 it is pushed above a layer which does not support the interface.
317 (Perl's C<sv_gets()> does not expect the streams fast C<gets> behaviour
318 to change during one "get".)
322 =head2 Methods in Detail
330 The name of the layer whose open() method Perl should invoke on
331 open(). For example if the layer is called APR, you will call:
333 open $fh, ">:APR", ...
335 and Perl knows that it has to invoke the PerlIOAPR_open() method
336 implemented by the APR layer.
342 The size of the per-instance data structure, e.g.:
352 =item * PERLIO_K_BUFFERED
354 The layer is buffered.
358 The layer is acceptable to have in a binmode(FH) stack - i.e. it does not
359 (or will configure itself not to) transform bytes passing through it.
361 =item * PERLIO_K_CANCRLF
363 Layer can translate between "\n" and CRLF line ends.
365 =item * PERLIO_K_FASTGETS
367 Layer allows buffer snooping.
369 =item * PERLIO_K_MULTIARG
371 Used when the layer's open() accepts more arguments than usual. The
372 extra arguments should come not before the C<MODE> argument. When this
373 flag is used it's up to the layer to validate the args.
379 IV (*Pushed)(pTHX_ PerlIO *f,const char *mode, SV *arg);
381 The only absolutely mandatory method. Called when the layer is pushed
382 onto the stack. The C<mode> argument may be NULL if this occurs
383 post-open. The C<arg> will be non-C<NULL> if an argument string was
384 passed. In most cases this should call C<PerlIOBase_pushed()> to
385 convert C<mode> into the appropriate C<PERLIO_F_XXXXX> flags in
386 addition to any actions the layer itself takes. If a layer is not
387 expecting an argument it need neither save the one passed to it, nor
388 provide C<Getarg()> (it could perhaps C<Perl_warn> that the argument
391 Returns 0 on success. On failure returns -1 and should set errno.
395 IV (*Popped)(pTHX_ PerlIO *f);
397 Called when the layer is popped from the stack. A layer will normally
398 be popped after C<Close()> is called. But a layer can be popped
399 without being closed if the program is dynamically managing layers on
400 the stream. In such cases C<Popped()> should free any resources
401 (buffers, translation tables, ...) not held directly in the layer's
402 struct. It should also C<Unread()> any unconsumed data that has been
403 read and buffered from the layer below back to that layer, so that it
404 can be re-provided to what ever is now above.
406 Returns 0 on success and failure.
410 PerlIO * (*Open)(...);
412 The C<Open()> method has lots of arguments because it combines the
413 functions of perl's C<open>, C<PerlIO_open>, perl's C<sysopen>,
414 C<PerlIO_fdopen> and C<PerlIO_reopen>. The full prototype is as
417 PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab,
420 int fd, int imode, int perm,
422 int narg, SV **args);
424 Open should (perhaps indirectly) call C<PerlIO_allocate()> to allocate
425 a slot in the table and associate it with the layers information for
426 the opened file, by calling C<PerlIO_push>. The I<layers> AV is an
427 array of all the layers destined for the C<PerlIO *>, and any
428 arguments passed to them, I<n> is the index into that array of the
429 layer being called. The macro C<PerlIOArg> will return a (possibly
430 C<NULL>) SV * for the argument passed to the layer.
432 The I<mode> string is an "C<fopen()>-like" string which would match
433 the regular expression C</^[I#]?[rwa]\+?[bt]?$/>.
435 The C<'I'> prefix is used during creation of C<stdin>..C<stderr> via
436 special C<PerlIO_fdopen> calls; the C<'#'> prefix means that this is
437 C<sysopen> and that I<imode> and I<perm> should be passed to
438 C<PerlLIO_open3>; C<'r'> means B<r>ead, C<'w'> means B<w>rite and
439 C<'a'> means B<a>ppend. The C<'+'> suffix means that both reading and
440 writing/appending are permitted. The C<'b'> suffix means file should
441 be binary, and C<'t'> means it is text. (Binary/Text should be ignored
442 by almost all layers and binary IO done, with PerlIO. The C<:crlf>
443 layer should be pushed to handle the distinction.)
445 If I<old> is not C<NULL> then this is a C<PerlIO_reopen>. Perl itself
446 does not use this (yet?) and semantics are a little vague.
448 If I<fd> not negative then it is the numeric file descriptor I<fd>,
449 which will be open in a manner compatible with the supplied mode
450 string, the call is thus equivalent to C<PerlIO_fdopen>. In this case
451 I<nargs> will be zero.
453 If I<nargs> is greater than zero then it gives the number of arguments
454 passed to C<open>, otherwise it will be 1 if for example
455 C<PerlIO_open> was called. In simple cases SvPV_nolen(*args) is the
458 Having said all that translation-only layers do not need to provide
459 C<Open()> at all, but rather leave the opening to a lower level layer
460 and wait to be "pushed". If a layer does provide C<Open()> it should
461 normally call the C<Open()> method of next layer down (if any) and
462 then push itself on top if that succeeds.
464 Returns C<NULL> on failure.
468 IV (*Binmode)(pTHX_ PerlIO *f);
470 Optional. Used when C<:raw> layer is pushed (explicitly or as a result
471 of binmode(FH)). If not present layer will be popped. If present
472 should configure layer as binary (or pop itself) and return 0.
473 If it returns -1 for error C<binmode> will fail with layer
478 SV * (*Getarg)(pTHX_ PerlIO *f,
479 CLONE_PARAMS *param, int flags);
481 Optional. If present should return an SV * representing the string
482 argument passed to the layer when it was
483 pushed. e.g. ":encoding(ascii)" would return an SvPV with value
484 "ascii". (I<param> and I<flags> arguments can be ignored in most
489 IV (*Fileno)(pTHX_ PerlIO *f);
491 Returns the Unix/Posix numeric file descriptor for the handle. Normally
492 C<PerlIOBase_fileno()> (which just asks next layer down) will suffice
495 Returns -1 if the layer cannot provide such a file descriptor, or in
496 the case of the error.
498 XXX: two possible results end up in -1, one is an error the other is
503 PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
504 CLONE_PARAMS *param, int flags);
508 Similar to C<Open>, returns PerlIO* on success, C<NULL> on failure.
512 SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
514 Basic read operation.
516 Typically will call C<Fill> and manipulate pointers (possibly via the
517 API). C<PerlIOBuf_read()> may be suitable for derived classes which
518 provide "fast gets" methods.
520 Returns actual bytes read, or -1 on an error.
524 SSize_t (*Unread)(pTHX_ PerlIO *f,
525 const void *vbuf, Size_t count);
527 A superset of stdio's C<ungetc()>. Should arrange for future reads to
528 see the bytes in C<vbuf>. If there is no obviously better implementation
529 then C<PerlIOBase_unread()> provides the function by pushing a "fake"
530 "pending" layer above the calling layer.
532 Returns the number of unread chars.
536 SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count);
538 Basic write operation.
540 Returns bytes written or -1 on an error.
544 IV (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
546 Position the file pointer. Should normally call its own C<Flush>
547 method and then the C<Seek> method of next layer down.
549 Returns 0 on success, -1 on failure.
553 Off_t (*Tell)(pTHX_ PerlIO *f);
555 Return the file pointer. May be based on layers cached concept of
556 position to avoid overhead.
558 Returns -1 on failure to get the file pointer.
562 IV (*Close)(pTHX_ PerlIO *f);
564 Close the stream. Should normally call C<PerlIOBase_close()> to flush
565 itself and close layers below, and then deallocate any data structures
566 (buffers, translation tables, ...) not held directly in the data
569 Returns 0 on success, -1 on failure.
573 IV (*Flush)(pTHX_ PerlIO *f);
575 Should make stream's state consistent with layers below. That is, any
576 buffered write data should be written, and file position of lower layers
577 adjusted for data read from below but not actually consumed.
578 (Should perhaps C<Unread()> such data to the lower layer.)
580 Returns 0 on success, -1 on failure.
584 IV (*Fill)(pTHX_ PerlIO *f);
586 The buffer for this layer should be filled (for read) from layer
587 below. When you "subclass" PerlIOBuf layer, you want to use its
588 I<_read> method and to supply your own fill method, which fills the
591 Returns 0 on success, -1 on failure.
595 IV (*Eof)(pTHX_ PerlIO *f);
597 Return end-of-file indicator. C<PerlIOBase_eof()> is normally sufficient.
599 Returns 0 on end-of-file, 1 if not end-of-file, -1 on error.
603 IV (*Error)(pTHX_ PerlIO *f);
605 Return error indicator. C<PerlIOBase_error()> is normally sufficient.
607 Returns 1 if there is an error (usually when C<PERLIO_F_ERROR> is set,
612 void (*Clearerr)(pTHX_ PerlIO *f);
614 Clear end-of-file and error indicators. Should call C<PerlIOBase_clearerr()>
615 to set the C<PERLIO_F_XXXXX> flags, which may suffice.
619 void (*Setlinebuf)(pTHX_ PerlIO *f);
621 Mark the stream as line buffered. C<PerlIOBase_setlinebuf()> sets the
622 PERLIO_F_LINEBUF flag and is normally sufficient.
626 STDCHAR * (*Get_base)(pTHX_ PerlIO *f);
628 Allocate (if not already done so) the read buffer for this layer and
629 return pointer to it. Return NULL on failure.
633 Size_t (*Get_bufsiz)(pTHX_ PerlIO *f);
635 Return the number of bytes that last C<Fill()> put in the buffer.
639 STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f);
641 Return the current read pointer relative to this layer's buffer.
645 SSize_t (*Get_cnt)(pTHX_ PerlIO *f);
647 Return the number of bytes left to be read in the current buffer.
651 void (*Set_ptrcnt)(pTHX_ PerlIO *f,
652 STDCHAR *ptr, SSize_t cnt);
654 Adjust the read pointer and count of bytes to match C<ptr> and/or C<cnt>.
655 The application (or layer above) must ensure they are consistent.
656 (Checking is allowed by the paranoid.)
663 The file C<perlio.c> provides the following layers:
669 A basic non-buffered layer which calls Unix/POSIX C<read()>, C<write()>,
670 C<lseek()>, C<close()>. No buffering. Even on platforms that distinguish
671 between O_TEXT and O_BINARY this layer is always O_BINARY.
675 A very complete generic buffering layer which provides the whole of
676 PerlIO API. It is also intended to be used as a "base class" for other
677 layers. (For example its C<Read()> method is implemented in terms of
678 the C<Get_cnt()>/C<Get_ptr()>/C<Set_ptrcnt()> methods).
680 "perlio" over "unix" provides a complete replacement for stdio as seen
681 via PerlIO API. This is the default for USE_PERLIO when system's stdio
682 does not permit perl's "fast gets" access, and which do not
683 distinguish between C<O_TEXT> and C<O_BINARY>.
687 A layer which provides the PerlIO API via the layer scheme, but
688 implements it by calling system's stdio. This is (currently) the default
689 if system's stdio provides sufficient access to allow perl's "fast gets"
690 access and which do not distinguish between C<O_TEXT> and C<O_BINARY>.
694 A layer derived using "perlio" as a base class. It provides Win32-like
695 "\n" to CR,LF translation. Can either be applied above "perlio" or serve
696 as the buffer layer itself. "crlf" over "unix" is the default if system
697 distinguishes between C<O_TEXT> and C<O_BINARY> opens. (At some point
698 "unix" will be replaced by a "native" Win32 IO layer on that platform,
699 as Win32's read/write layer has various drawbacks.) The "crlf" layer is
700 a reasonable model for a layer which transforms data in some way.
704 If Configure detects C<mmap()> functions this layer is provided (with
705 "perlio" as a "base") which does "read" operations by mmap()ing the
706 file. Performance improvement is marginal on modern systems, so it is
707 mainly there as a proof of concept. It is likely to be unbundled from
708 the core at some point. The "mmap" layer is a reasonable model for a
709 minimalist "derived" layer.
713 An "internal" derivative of "perlio" which can be used to provide
714 Unread() function for layers which have no buffer or cannot be
715 bothered. (Basically this layer's C<Fill()> pops itself off the stack
716 and so resumes reading from layer below.)
720 A dummy layer which never exists on the layer stack. Instead when
721 "pushed" it actually pops the stack removing itself, it then calls
722 Binmode function table entry on all the layers in the stack - normally
723 this (via PerlIOBase_binmode) removes any layers which do not have
724 C<PERLIO_K_RAW> bit set. Layers can modify that behaviour by defining
725 their own Binmode entry.
729 Another dummy layer. When pushed it pops itself and sets the
730 C<PERLIO_F_UTF8> flag on the layer which was (and now is once more)
731 the top of the stack.
735 In addition F<perlio.c> also provides a number of C<PerlIOBase_xxxx()>
736 functions which are intended to be used in the table slots of classes
737 which do not need to do anything special for a particular method.
739 =head2 Extension Layers
741 Layers can made available by extension modules. When an unknown layer
742 is encountered the PerlIO code will perform the equivalent of :
746 Where I<layer> is the unknown layer. F<PerlIO.pm> will then attempt to:
748 require PerlIO::layer;
750 If after that process the layer is still not defined then the C<open>
753 The following extension layers are bundled with perl:
761 makes this layer available, although F<PerlIO.pm> "knows" where to
762 find it. It is an example of a layer which takes an argument as it is
765 open($fh,"<:encoding(iso-8859-7)",$pathname)
771 open($fh,"...",\$scalar)
773 When a handle is so opened, then reads get bytes from the string value
774 of I<$scalar>, and writes change the value. In both cases the position
775 in I<$scalar> starts as zero but can be altered via C<seek>, and
776 determined via C<tell>.
780 Provided to allow layers to be implemented as Perl code. For instance:
782 use MIME::QuotedPrint;
783 open(my $fh, ">Via(MIME::QuotedPrint)", "qp");
785 See L<PerlIO::Via> for details.
791 Things that need to be done to improve this document.
797 Explain how to make a valid fh without going through open()(i.e. apply
798 a layer). For example if the file is not opened through perl, but we
799 want to get back a fh, like it was opened by Perl.
801 How PerlIO_apply_layera fits in, where its docs, was it made public?
803 Currently the example could be something like this:
805 PerlIO *foo_to_PerlIO(pTHX_ char *mode, ...)
807 char *mode; /* "w", "r", etc */
808 const char *layers = ":APR"; /* the layer name */
809 PerlIO *f = PerlIO_allocate(aTHX);
814 PerlIO_apply_layers(aTHX_ f, mode, layers);
817 PerlIOAPR *st = PerlIOSelf(f, PerlIOAPR);
818 /* fill in the st struct, as in _open() */
820 PerlIOBase(f)->flags |= PERLIO_F_OPEN;
829 fix/add the documentation in places marked as XXX.
833 The handling of errors by the layer is not specified. e.g. when $!
834 should be set explicitly, when the error handling should be just
835 delegated to the top layer.
837 Probably give some hints on using SETERRNO() or pointers to where they
842 I think it would help to give some concrete examples to make it easier
843 to understand the API. Of course I agree that the API has to be
844 concise, but since there is no second document that is more of a
845 guide, I think that it'd make it easier to start with the doc which is
846 an API, but has examples in it in places where things are unclear, to
847 a person who is not a PerlIO guru (yet).