perliol.pod

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Layer is line buffered. Write data should be passed to next layer down
whenever a "\n" is seen. Any data beyond the "\n" should then be
processed.

=item PERLIO_F_TEMP

File has been C<unlink()>ed, or should be deleted on C<close()>.

=item PERLIO_F_OPEN

Handle is open.

=item PERLIO_F_FASTGETS

This instance of this layer supports the "fast C<gets>" interface.
Normally set based on C<PERLIO_K_FASTGETS> for the class and by the
existence of the function(s) in the table. However a class that
normally provides that interface may need to avoid it on a
particular instance. The "pending" layer needs to do this when
it is pushed above a layer which does not support the interface.
(Perl's C<sv_gets()> does not expect the streams fast C<gets> behaviour
to change during one "get".)

=back

=head2 Methods in Detail

=over 4

=item fsize

	Size_t fsize;

Size of the function table. This is compared against the value PerlIO
code "knows" as a compatibility check. Future versions I<may> be able
to tolerate layers compiled against an old version of the headers.

=item name

	char * name;

The name of the layer whose open() method Perl should invoke on
open().  For example if the layer is called APR, you will call:

  open $fh, ">:APR", ...

and Perl knows that it has to invoke the PerlIOAPR_open() method
implemented by the APR layer.

=item size

	Size_t size;

The size of the per-instance data structure, e.g.:

  sizeof(PerlIOAPR)

If this field is zero then C<PerlIO_pushed> does not malloc anything
and assumes layer's Pushed function will do any required layer stack
manipulation - used to avoid malloc/free overhead for dummy layers.
If the field is non-zero it must be at least the size of C<PerlIOl>,
C<PerlIO_pushed> will allocate memory for the layer's data structures
and link new layer onto the stream's stack. (If the layer's Pushed
method returns an error indication the layer is popped again.)

=item kind

	IV kind;

=over 4

=item * PERLIO_K_BUFFERED

The layer is buffered.

=item * PERLIO_K_RAW

The layer is acceptable to have in a binmode(FH) stack - i.e. it does not
(or will configure itself not to) transform bytes passing through it.

=item * PERLIO_K_CANCRLF

Layer can translate between "\n" and CRLF line ends.

=item * PERLIO_K_FASTGETS

Layer allows buffer snooping.

=item * PERLIO_K_MULTIARG

Used when the layer's open() accepts more arguments than usual. The
extra arguments should come not before the C<MODE> argument. When this
flag is used it's up to the layer to validate the args.

=back

=item Pushed

	IV	(*Pushed)(pTHX_ PerlIO *f,const char *mode, SV *arg);

The only absolutely mandatory method. Called when the layer is pushed
onto the stack.  The C<mode> argument may be NULL if this occurs
post-open. The C<arg> will be non-C<NULL> if an argument string was
passed. In most cases this should call C<PerlIOBase_pushed()> to
convert C<mode> into the appropriate C<PERLIO_F_XXXXX> flags in
addition to any actions the layer itself takes.  If a layer is not
expecting an argument it need neither save the one passed to it, nor
provide C<Getarg()> (it could perhaps C<Perl_warn> that the argument
was un-expected).

Returns 0 on success. On failure returns -1 and should set errno.

=item Popped

	IV	(*Popped)(pTHX_ PerlIO *f);

Called when the layer is popped from the stack. A layer will normally
be popped after C<Close()> is called. But a layer can be popped
without being closed if the program is dynamically managing layers on
the stream. In such cases C<Popped()> should free any resources
(buffers, translation tables, ...) not held directly in the layer's
struct.  It should also C<Unread()> any unconsumed data that has been
read and buffered from the layer below back to that layer, so that it
can be re-provided to what ever is now above.

Returns 0 on success and failure.  If C<Popped()> returns I<true> then
I<perlio.c> assumes that either the layer has popped itself, or the
layer is super special and needs to be retained for other reasons.
In most cases it should return I<false>.

=item Open

	PerlIO *	(*Open)(...);

The C<Open()> method has lots of arguments because it combines the
functions of perl's C<open>, C<PerlIO_open>, perl's C<sysopen>,
C<PerlIO_fdopen> and C<PerlIO_reopen>.  The full prototype is as
follows:

 PerlIO *	(*Open)(pTHX_ PerlIO_funcs *tab,
			AV *layers, IV n,
			const char *mode,
			int fd, int imode, int perm,
			PerlIO *old,
			int narg, SV **args);

Open should (perhaps indirectly) call C<PerlIO_allocate()> to allocate
a slot in the table and associate it with the layers information for
the opened file, by calling C<PerlIO_push>.  The I<layers> AV is an
array of all the layers destined for the C<PerlIO *>, and any
arguments passed to them, I<n> is the index into that array of the
layer being called. The macro C<PerlIOArg> will return a (possibly
C<NULL>) SV * for the argument passed to the layer.

The I<mode> string is an "C<fopen()>-like" string which would match
the regular expression C</^[I#]?[rwa]\+?[bt]?$/>.

The C<'I'> prefix is used during creation of C<stdin>..C<stderr> via
special C<PerlIO_fdopen> calls; the C<'#'> prefix means that this is
C<sysopen> and that I<imode> and I<perm> should be passed to
C<PerlLIO_open3>; C<'r'> means B<r>ead, C<'w'> means B<w>rite and
C<'a'> means B<a>ppend. The C<'+'> suffix means that both reading and
writing/appending are permitted.  The C<'b'> suffix means file should
be binary, and C<'t'> means it is text. (Almost all layers should do
the IO in binary mode, and ignore the b/t bits. The C<:crlf> layer
should be pushed to handle the distinction.)

If I<old> is not C<NULL> then this is a C<PerlIO_reopen>. Perl itself
does not use this (yet?) and semantics are a little vague.

If I<fd> not negative then it is the numeric file descriptor I<fd>,
which will be open in a manner compatible with the supplied mode
string, the call is thus equivalent to C<PerlIO_fdopen>. In this case
I<nargs> will be zero.

If I<nargs> is greater than zero then it gives the number of arguments
passed to C<open>, otherwise it will be 1 if for example
C<PerlIO_open> was called.  In simple cases SvPV_nolen(*args) is the
pathname to open.

Having said all that translation-only layers do not need to provide
C<Open()> at all, but rather leave the opening to a lower level layer
and wait to be "pushed".  If a layer does provide C<Open()> it should
normally call the C<Open()> method of next layer down (if any) and
then push itself on top if that succeeds.

If C<PerlIO_push> was performed and open has failed, it must
C<PerlIO_pop> itself, since if it's not, the layer won't be removed
and may cause bad problems.

Returns C<NULL> on failure.

=item Binmode

	IV        (*Binmode)(pTHX_ PerlIO *f);

Optional. Used when C<:raw> layer is pushed (explicitly or as a result
of binmode(FH)). If not present layer will be popped. If present
should configure layer as binary (or pop itself) and return 0.
If it returns -1 for error C<binmode> will fail with layer
still on the stack.

=item Getarg

	SV *      (*Getarg)(pTHX_ PerlIO *f,
			    CLONE_PARAMS *param, int flags);

Optional. If present should return an SV * representing the string
argument passed to the layer when it was
pushed. e.g. ":encoding(ascii)" would return an SvPV with value
"ascii". (I<param> and I<flags> arguments can be ignored in most
cases)

C<Dup> uses C<Getarg> to retrieve the argument originally passed to
C<Pushed>, so you must implement this function if your layer has an
extra argument to C<Pushed> and will ever be C<Dup>ed.

=item Fileno

	IV        (*Fileno)(pTHX_ PerlIO *f);

Returns the Unix/Posix numeric file descriptor for the handle. Normally
C<PerlIOBase_fileno()> (which just asks next layer down) will suffice
for this.

Returns -1 on error, which is considered to include the case where the
layer cannot provide such a file descriptor.

=item Dup

	PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
			CLONE_PARAMS *param, int flags);

XXX: Needs more docs.

Used as part of the "clone" process when a thread is spawned (in which
case param will be non-NULL) and when a stream is being duplicated via
'&' in the C<open>.

Similar to C<Open>, returns PerlIO* on success, C<NULL> on failure.

=item Read

	SSize_t	(*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);

Basic read operation.

Typically will call C<Fill> and manipulate pointers (possibly via the
API).  C<PerlIOBuf_read()> may be suitable for derived classes which
provide "fast gets" methods.

Returns actual bytes read, or -1 on an error.

=item	Unread

	SSize_t	(*Unread)(pTHX_ PerlIO *f,
			  const void *vbuf, Size_t count);

A superset of stdio's C<ungetc()>. Should arrange for future reads to
see the bytes in C<vbuf>. If there is no obviously better implementation
then C<PerlIOBase_unread()> provides the function by pushing a "fake"
"pending" layer above the calling layer.

Returns the number of unread chars.

=item Write

	SSize_t	(*Write)(PerlIO *f, const void *vbuf, Size_t count);

Basic write operation.

Returns bytes written or -1 on an error.

=item Seek

	IV	(*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);

Position the file pointer. Should normally call its own C<Flush>
method and then the C<Seek> method of next layer down.

Returns 0 on success, -1 on failure.

=item Tell

	Off_t	(*Tell)(pTHX_ PerlIO *f);

Return the file pointer. May be based on layers cached concept of
position to avoid overhead.

Returns -1 on failure to get the file pointer.

=item Close

	IV	(*Close)(pTHX_ PerlIO *f);

Close the stream. Should normally call C<PerlIOBase_close()> to flush
itself and close layers below, and then deallocate any data structures
(buffers, translation tables, ...) not  held directly in the data
structure.

Returns 0 on success, -1 on failure.

=item Flush

	IV	(*Flush)(pTHX_ PerlIO *f);

Should make stream's state consistent with layers below. That is, any
buffered write data should be written, and file position of lower layers
adjusted for data read from below but not actually consumed.
(Should perhaps C<Unread()> such data to the lower layer.)

Returns 0 on success, -1 on failure.

=item Fill

	IV	(*Fill)(pTHX_ PerlIO *f);

The buffer for this layer should be filled (for read) from layer
below.  When you "subclass" PerlIOBuf layer, you want to use its
I<_read> method and to supply your own fill method, which fills the
PerlIOBuf's buffer.

Returns 0 on success, -1 on failure.

=item Eof

	IV	(*Eof)(pTHX_ PerlIO *f);

Return end-of-file indicator. C<PerlIOBase_eof()> is normally sufficient.

Returns 0 on end-of-file, 1 if not end-of-file, -1 on error.

=item Error

	IV	(*Error)(pTHX_ PerlIO *f);

Return error indicator. C<PerlIOBase_error()> is normally sufficient.

Returns 1 if there is an error (usually when C<PERLIO_F_ERROR> is set,
0 otherwise.

=item  Clearerr

	void	(*Clearerr)(pTHX_ PerlIO *f);

Clear end-of-file and error indicators. Should call C<PerlIOBase_clearerr()>