-
=head1 NAME
perliol - C API for Perl's implementation of IO in Layers.
/* Defining a layer ... */
#include <perliol.h>
-
=head1 DESCRIPTION
This document describes the behavior and implementation of the PerlIO
The PerlIO abstraction was introduced in perl5.003_02 but languished as
just an abstraction until perl5.7.0. However during that time a number
-of perl extentions switched to using it, so the API is mostly fixed to
+of perl extensions switched to using it, so the API is mostly fixed to
maintain (source) compatibility.
The aim of the implementation is to provide the PerlIO API in a flexible
and platform neutral manner. It is also a trial of an "Object Oriented
-C, with vtables" approach which may be applied to perl6.
+C, with vtables" approach which may be applied to Perl 6.
+
+=head2 Basic Structure
+
+PerlIO is a stack of layers.
+
+The low levels of the stack work with the low-level operating system
+calls (file descriptors in C) getting bytes in and out, the higher
+layers of the stack buffer, filter, and otherwise manipulate the I/O,
+and return characters (or bytes) to Perl. Terms I<above> and I<below>
+are used to refer to the relative positioning of the stack layers.
+
+A layer contains a "vtable", the table of I/O operations (at C level
+a table of function pointers), and status flags. The functions in the
+vtable implement operations like "open", "read", and "write".
+
+When I/O, for example "read", is requested, the request goes from Perl
+first down the stack using "read" functions of each layer, then at the
+bottom the input is requested from the operating system services, then
+the result is returned up the stack, finally being interpreted as Perl
+data.
+
+The requests do not necessarily go always all the way down to the
+operating system: that's where PerlIO buffering comes into play.
+
+When you do an open() and specify extra PerlIO layers to be deployed,
+the layers you specify are "pushed" on top of the already existing
+default stack. One way to see it is that "operating system is
+on the left" and "Perl is on the right".
+
+What exact layers are in this default stack depends on a lot of
+things: your operating system, Perl version, Perl compile time
+configuration, and Perl runtime configuration. See L<PerlIO>,
+L<perlrun/PERLIO>, and L<open> for more information.
+
+binmode() operates similarly to open(): by default the specified
+layers are pushed on top of the existing stack.
+
+However, note that even as the specified layers are "pushed on top"
+for open() and binmode(), this doesn't mean that the effects are
+limited to the "top": PerlIO layers can be very 'active' and inspect
+and affect layers also deeper in the stack. As an example there
+is a layer called "raw" which repeatedly "pops" layers until
+it reaches the first layer that has declared itself capable of
+handling binary data. The "pushed" layers are processed in left-to-right
+order.
+
+sysopen() operates (unsurprisingly) at a lower level in the stack than
+open(). For example in Unix or Unix-like systems sysopen() operates
+directly at the level of file descriptors: in the terms of PerlIO
+layers, it uses only the "unix" layer, which is a rather thin wrapper
+on top of the Unix file descriptors.
=head2 Layers vs Disciplines
from "line disciplines" on Unix terminals. However, this document (and
the C code) uses the term "layer".
-This is, I hope, a natural term given the implementation, and should avoid
-connotations that are inherent in earlier uses of "discipline" for things
-which are rather different.
+This is, I hope, a natural term given the implementation, and should
+avoid connotations that are inherent in earlier uses of "discipline"
+for things which are rather different.
=head2 Data Structures
IV flags; /* Various flags for state */
};
-A C<PerlIOl *> is a pointer to to the struct, and the I<application> level
-C<PerlIO *> is a pointer to a C<PerlIOl *> - i.e. a pointer to a pointer to
-the struct. This allows the application level C<PerlIO *> to remain
-constant while the actual C<PerlIOl *> underneath changes. (Compare perl's
-C<SV *> which remains constant while its C<sv_any> field changes as the
-scalar's type changes.) An IO stream is then in general represented as a
-pointer to this linked-list of "layers".
+A C<PerlIOl *> is a pointer to the struct, and the I<application>
+level C<PerlIO *> is a pointer to a C<PerlIOl *> - i.e. a pointer
+to a pointer to the struct. This allows the application level C<PerlIO *>
+to remain constant while the actual C<PerlIOl *> underneath
+changes. (Compare perl's C<SV *> which remains constant while its
+C<sv_any> field changes as the scalar's type changes.) An IO stream is
+then in general represented as a pointer to this linked-list of
+"layers".
It should be noted that because of the double indirection in a C<PerlIO *>,
-a C<< &(perlio-
E<gt>next) >> "is" a C<PerlIO *>, and so to some degree
+a C<< &(perlio->next) >> "is" a C<PerlIO *>, and so to some degree
at least one layer can use the "standard" API on the next layer down.
A "layer" is composed of two parts:
=over 4
-=item 1. The functions and attributes of the "layer class".
+=item 1.
+
+The functions and attributes of the "layer class".
+
+=item 2.
-=item 2. The per-instance data for a particular handle.
+The per-instance data for a particular handle.
=back
fixed, and are defined by the C<PerlIO_funcs> type. They are broadly the
same as the public C<PerlIO_xxxxx> functions:
- struct _PerlIO_funcs
- {
- char * name;
- Size_t size;
- IV kind;
- IV (*Fileno)(PerlIO *f);
- PerlIO * (*Fdopen)(PerlIO_funcs *tab, int fd, const char *mode);
- PerlIO * (*Open)(PerlIO_funcs *tab, const char *path, const char *mode);
- int (*Reopen)(const char *path, const char *mode, PerlIO *f);
- IV (*Pushed)(PerlIO *f,const char *mode,const char *arg,STRLEN len);
- IV (*Popped)(PerlIO *f);
- /* Unix-like functions - cf sfio line disciplines */
- SSize_t (*Read)(PerlIO *f, void *vbuf, Size_t count);
- SSize_t (*Unread)(PerlIO *f, const void *vbuf, Size_t count);
- SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count);
- IV (*Seek)(PerlIO *f, Off_t offset, int whence);
- Off_t (*Tell)(PerlIO *f);
- IV (*Close)(PerlIO *f);
- /* Stdio-like buffered IO functions */
- IV (*Flush)(PerlIO *f);
- IV (*Fill)(PerlIO *f);
- IV (*Eof)(PerlIO *f);
- IV (*Error)(PerlIO *f);
- void (*Clearerr)(PerlIO *f);
- void (*Setlinebuf)(PerlIO *f);
- /* Perl's snooping functions */
- STDCHAR * (*Get_base)(PerlIO *f);
- Size_t (*Get_bufsiz)(PerlIO *f);
- STDCHAR * (*Get_ptr)(PerlIO *f);
- SSize_t (*Get_cnt)(PerlIO *f);
- void (*Set_ptrcnt)(PerlIO *f,STDCHAR *ptr,SSize_t cnt);
- };
-
-The first few members of the struct give a "name" for the layer, the
-size to C<malloc> for the per-instance data, and some flags which are
-attributes of the class as whole (such as whether it is a buffering
+ struct _PerlIO_funcs
+ {
+ Size_t fsize;
+ char * name;
+ Size_t size;
+ IV kind;
+ IV (*Pushed)(pTHX_ PerlIO *f,const char *mode,SV *arg, PerlIO_funcs *tab);
+ IV (*Popped)(pTHX_ PerlIO *f);
+ PerlIO * (*Open)(pTHX_ PerlIO_funcs *tab,
+ PerlIO_list_t *layers, IV n,
+ const char *mode,
+ int fd, int imode, int perm,
+ PerlIO *old,
+ int narg, SV **args);
+ IV (*Binmode)(pTHX_ PerlIO *f);
+ SV * (*Getarg)(pTHX_ PerlIO *f, CLONE_PARAMS *param, int flags)
+ IV (*Fileno)(pTHX_ PerlIO *f);
+ PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o, CLONE_PARAMS *param, int flags)
+ /* Unix-like functions - cf sfio line disciplines */
+ SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
+ SSize_t (*Unread)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
+ SSize_t (*Write)(pTHX_ PerlIO *f, const void *vbuf, Size_t count);
+ IV (*Seek)(pTHX_ PerlIO *f, Off_t offset, int whence);
+ Off_t (*Tell)(pTHX_ PerlIO *f);
+ IV (*Close)(pTHX_ PerlIO *f);
+ /* Stdio-like buffered IO functions */
+ IV (*Flush)(pTHX_ PerlIO *f);
+ IV (*Fill)(pTHX_ PerlIO *f);
+ IV (*Eof)(pTHX_ PerlIO *f);
+ IV (*Error)(pTHX_ PerlIO *f);
+ void (*Clearerr)(pTHX_ PerlIO *f);
+ void (*Setlinebuf)(pTHX_ PerlIO *f);
+ /* Perl's snooping functions */
+ STDCHAR * (*Get_base)(pTHX_ PerlIO *f);
+ Size_t (*Get_bufsiz)(pTHX_ PerlIO *f);
+ STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f);
+ SSize_t (*Get_cnt)(pTHX_ PerlIO *f);
+ void (*Set_ptrcnt)(pTHX_ PerlIO *f,STDCHAR *ptr,SSize_t cnt);
+ };
+
+The first few members of the struct give a function table size for
+compatibility check "name" for the layer, the size to C<malloc> for the per-instance data,
+and some flags which are attributes of the class as whole (such as whether it is a buffering
layer), then follow the functions which fall into four basic groups:
=over 4
-=item 1. Opening and setup functions
+=item 1.
+
+Opening and setup functions
-=item 2. Basic IO operations
+=item 2.
-=item 3. Stdio class buffering options.
+Basic IO operations
-=item 4. Functions to support Perl's traditional "fast" access to the buffer.
+=item 3.
+
+Stdio class buffering options.
+
+=item 4.
+
+Functions to support Perl's traditional "fast" access to the buffer.
=back
-A layer does not have to implement all the functions, but the whole table has
-to be present. Unimplemented slots can be NULL (which will will result in an error
-when called) or can be filled in with stubs to "inherit" behaviour from
-a "base class". This "inheritance" is fixed for all instances of the layer,
-but as the layer chooses which stubs to populate the table, limited
-"multiple inheritance" is possible.
+A layer does not have to implement all the functions, but the whole
+table has to be present. Unimplemented slots can be NULL (which will
+result in an error when called) or can be filled in with stubs to
+"inherit" behaviour from a "base class". This "inheritance" is fixed
+for all instances of the layer, but as the layer chooses which stubs
+to populate the table, limited "multiple inheritance" is possible.
=head2 Per-instance Data
-The per-instance data are held in memory beyond the basic PerlIOl struct,
-by making a PerlIOl the first member of the layer's struct thus:
+The per-instance data are held in memory beyond the basic PerlIOl
+struct, by making a PerlIOl the first member of the layer's struct
+thus:
typedef struct
{
IV oneword; /* Emergency buffer */
} PerlIOBuf;
-In this way (as for perl's scalars) a pointer to a PerlIOBuf can be treated
-as a pointer to a PerlIOl.
+In this way (as for perl's scalars) a pointer to a PerlIOBuf can be
+treated as a pointer to a PerlIOl.
=head2 Layers in action.
in the table correspond to C<stdin>,C<stdout> and C<stderr>. The table
in turn points to the current "top" layer for the handle - in this case
an instance of the generic buffering layer "perlio". That layer in turn
-points to the next layer down - in this case the lowlevel "unix" layer.
+points to the next layer down - in this case the low-level "unix" layer.
The above is roughly equivalent to a "stdio" buffered stream, but with
much more flexibility:
=item *
-Different handles can have different buffering schemes. The "top" layer
-could be the "mmap" layer if reading disk files was quicker using C<mmap>
-than C<read>. An "unbuffered" stream can be implemented simply by
-not having a buffer layer.
+Different handles can have different buffering schemes. The "top"
+layer could be the "mmap" layer if reading disk files was quicker
+using C<mmap> than C<read>. An "unbuffered" stream can be implemented
+simply by not having a buffer layer.
=item *
Extra layers can be inserted to process the data as it flows through.
This was the driving need for including the scheme in perl 5.7.0+ - we
-needed a mechanism to allow data to be translated bewteen perl's
+needed a mechanism to allow data to be translated between perl's
internal encoding (conceptually at least Unicode as UTF-8), and the
"native" format used by the system. This is provided by the
":encoding(xxxx)" layer which typically sits above the buffering layer.
=item *
-A layer can be added that does "\n" to CRLF translation. This layer can be used
-on any platform, not just those that normally do such things.
+A layer can be added that does "\n" to CRLF translation. This layer
+can be used on any platform, not just those that normally do such
+things.
=back
=head2 Per-instance flag bits
-The generic flag bits are a hybrid of C<O_XXXXX> style flags deduced from
-the mode string passed to C<PerlIO_open()>, and state bits for typical buffer
-layers.
+The generic flag bits are a hybrid of C<O_XXXXX> style flags deduced
+from the mode string passed to C<PerlIO_open()>, and state bits for
+typical buffer layers.
=over 4
=item PERLIO_F_ERROR
-An error has occured (for C<PerlIO_error()>)
+An error has occurred (for C<PerlIO_error()>).
=item PERLIO_F_TRUNCATE
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
-existance of the function(s) in the table. However a class that
+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 an layer which does not support the interface.
+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".)
=over 4
-=item IV (*Fileno)(PerlIO *f);
+=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
-Returns the Unix/Posix numeric file decriptor for the handle. Normally
+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,
+ PerlIO_list_t *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> 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.
+
+If a layer provides 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. C<PerlIOBase_open> is provided to do exactly that, so in
+most cases you don't have to write your own C<Open()> method. If this
+method is not defined, other layers may have difficulty pushing
+themselves on top of it during open.
+
+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.
-=item PerlIO * (*Fdopen)(PerlIO_funcs *tab, int fd, const char *mode);
+Returns -1 on error, which is considered to include the case where the
+layer cannot provide such a file descriptor.
-Should (perhaps indirectly) call C<PerlIO_allocate()> to allocate a slot
-in the table and associate it with the given numeric file descriptor,
-which will be open in an manner compatible with the supplied mode string.
+=item Dup
-=item PerlIO * (*Open)(PerlIO_funcs *tab, const char *path, const char *mode);
+ PerlIO * (*Dup)(pTHX_ PerlIO *f, PerlIO *o,
+ CLONE_PARAMS *param, int flags);
-Should attempt to open the given path and if that succeeds then (perhaps
-indirectly) call C<PerlIO_allocate()> to allocate a slot in the table and
-associate it with the layers information for the opened file.
+XXX: Needs more docs.
-=item int (*Reopen)(const char *path, const char *mode, PerlIO *f);
+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>.
-Re-open the supplied C<PerlIO *> to connect it to C<path> in C<mode>.
-Returns as success flag. Perl does not use this and L<perlapio> marks it
-as subject to change.
+Similar to C<Open>, returns PerlIO* on success, C<NULL> on failure.
-=item IV (*Pushed)(PerlIO *f,const char *mode,const char *arg,STRLEN len);
+=item Read
-Called when the layer is pushed onto the stack. The C<mode> argument may
-be NULL if this occurs post-open. The C<arg> and C<len> will be present
-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.
+ SSize_t (*Read)(pTHX_ PerlIO *f, void *vbuf, Size_t count);
-=item IV (*Popped)(PerlIO *f);
+Basic read operation.
-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.
+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.
-=item SSize_t (*Read)(PerlIO *f, void *vbuf, Size_t count);
+Returns actual bytes read, or -1 on an error.
-Basic read operation. Returns actual bytes read, or -1 on an error.
-Typically will call Fill and manipulate pointers (possibly via the API).
-C<PerlIOBuf_read()> may be suitable for derived classes which provide
-"fast gets" methods.
+=item Unread
-=item SSize_t (*Unread)(PerlIO *f, const void *vbuf, Size_t count);
+ 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.
-=item SSize_t (*Write)(PerlIO *f, const void *vbuf, Size_t count);
+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);
-Basic write operation. Returns bytes written or -1 on an error.
+Position the file pointer. Should normally call its own C<Flush>
+method and then the C<Seek> method of next layer down.
-=item IV (*Seek)(PerlIO *f, Off_t offset, int whence);
+Returns 0 on success, -1 on failure.
-Position the file pointer. Should normally call its own C<Flush> method and
-then the C<Seek> method of next layer down.
+=item Tell
-=item Off_t (*Tell)(PerlIO *f);
+ Off_t (*Tell)(pTHX_ PerlIO *f);
Return the file pointer. May be based on layers cached concept of
position to avoid overhead.
-=item IV (*Close)(PerlIO *f);
+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.
-=item IV (*Flush)(PerlIO *f);
+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 fron below but not actually consumed.
+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.
-=item IV (*Fill)(PerlIO *f);
+Returns 0 on success, -1 on failure.
-The buffer for this layer should be filled (for read) from layer below.
+=item Eof
-=item IV (*Eof)(PerlIO *f);
+ IV (*Eof)(pTHX_ PerlIO *f);
Return end-of-file indicator. C<PerlIOBase_eof()> is normally sufficient.
-=item IV (*Error)(PerlIO *f);
+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.
-=item void (*Clearerr)(PerlIO *f);
+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()>
to set the C<PERLIO_F_XXXXX> flags, which may suffice.
-=item void (*Setlinebuf)(PerlIO *f);
+=item Setlinebuf
-Mark the stream as line buffered.
+ void (*Setlinebuf)(pTHX_ PerlIO *f);
-=item STDCHAR * (*Get_base)(PerlIO *f);
+Mark the stream as line buffered. C<PerlIOBase_setlinebuf()> sets the
+PERLIO_F_LINEBUF flag and is normally sufficient.
+
+=item Get_base
+
+ STDCHAR * (*Get_base)(pTHX_ PerlIO *f);
Allocate (if not already done so) the read buffer for this layer and
-return pointer to it.
+return pointer to it. Return NULL on failure.
+
+=item Get_bufsiz
-=item Size_t (*Get_bufsiz)(PerlIO *f);
+ Size_t (*Get_bufsiz)(pTHX_ PerlIO *f);
Return the number of bytes that last C<Fill()> put in the buffer.
-=item STDCHAR * (*Get_ptr)(PerlIO *f);
+=item Get_ptr
+
+ STDCHAR * (*Get_ptr)(pTHX_ PerlIO *f);
Return the current read pointer relative to this layer's buffer.
-=item SSize_t (*Get_cnt)(PerlIO *f);
+=item Get_cnt
+
+ SSize_t (*Get_cnt)(pTHX_ PerlIO *f);
Return the number of bytes left to be read in the current buffer.
-=item void (*Set_ptrcnt)(PerlIO *f,STDCHAR *ptr,SSize_t cnt);
+=item Set_ptrcnt
+
+ void (*Set_ptrcnt)(pTHX_ PerlIO *f,
+ STDCHAR *ptr, SSize_t cnt);
Adjust the read pointer and count of bytes to match C<ptr> and/or C<cnt>.
The application (or layer above) must ensure they are consistent.
=back
+=head2 Utilities
+
+To ask for the next layer down use PerlIONext(PerlIO *f).
+
+To check that a PerlIO* is valid use PerlIOValid(PerlIO *f). (All
+this does is really just to check that the pointer is non-NULL and
+that the pointer behind that is non-NULL.)
+
+PerlIOBase(PerlIO *f) returns the "Base" pointer, or in other words,
+the C<PerlIOl*> pointer.
+
+PerlIOSelf(PerlIO* f, type) return the PerlIOBase cast to a type.
+
+Perl_PerlIO_or_Base(PerlIO* f, callback, base, failure, args) either
+calls the I<callback> from the functions of the layer I<f> (just by
+the name of the IO function, like "Read") with the I<args>, or if
+there is no such callback, calls the I<base> version of the callback
+with the same args, or if the f is invalid, set errno to EBADF and
+return I<failure>.
+
+Perl_PerlIO_or_fail(PerlIO* f, callback, failure, args) either calls
+the I<callback> of the functions of the layer I<f> with the I<args>,
+or if there is no such callback, set errno to EINVAL. Or if the f is
+invalid, set errno to EBADF and return I<failure>.
+
+Perl_PerlIO_or_Base_void(PerlIO* f, callback, base, args) either calls
+the I<callback> of the functions of the layer I<f> with the I<args>,
+or if there is no such callback, calls the I<base> version of the
+callback with the same args, or if the f is invalid, set errno to
+EBADF.
+
+Perl_PerlIO_or_fail_void(PerlIO* f, callback, args) either calls the
+I<callback> of the functions of the layer I<f> with the I<args>, or if
+there is no such callback, set errno to EINVAL. Or if the f is
+invalid, set errno to EBADF.
+
+=head2 Implementing PerlIO Layers
+
+If you find the implementation document unclear or not sufficient,
+look at the existing PerlIO layer implementations, which include:
+
+=over
+
+=item * C implementations
+
+The F<perlio.c> and F<perliol.h> in the Perl core implement the
+"unix", "perlio", "stdio", "crlf", "utf8", "byte", "raw", "pending"
+layers, and also the "mmap" and "win32" layers if applicable.
+(The "win32" is currently unfinished and unused, to see what is used
+instead in Win32, see L<PerlIO/"Querying the layers of filehandles"> .)
+
+PerlIO::encoding, PerlIO::scalar, PerlIO::via in the Perl core.
+
+PerlIO::gzip and APR::PerlIO (mod_perl 2.0) on CPAN.
+
+=item * Perl implementations
+
+PerlIO::via::QuotedPrint in the Perl core and PerlIO::via::* on CPAN.
+
+=back
+
+If you are creating a PerlIO layer, you may want to be lazy, in other
+words, implement only the methods that interest you. The other methods
+you can either replace with the "blank" methods
+
+ PerlIOBase_noop_ok
+ PerlIOBase_noop_fail
+
+(which do nothing, and return zero and -1, respectively) or for
+certain methods you may assume a default behaviour by using a NULL
+method. The Open method looks for help in the 'parent' layer.
+The following table summarizes the behaviour:
+
+ method behaviour with NULL
+
+ Clearerr PerlIOBase_clearerr
+ Close PerlIOBase_close
+ Dup PerlIOBase_dup
+ Eof PerlIOBase_eof
+ Error PerlIOBase_error
+ Fileno PerlIOBase_fileno
+ Fill FAILURE
+ Flush SUCCESS
+ Getarg SUCCESS
+ Get_base FAILURE
+ Get_bufsiz FAILURE
+ Get_cnt FAILURE
+ Get_ptr FAILURE
+ Open INHERITED
+ Popped SUCCESS
+ Pushed SUCCESS
+ Read PerlIOBase_read
+ Seek FAILURE
+ Set_cnt FAILURE
+ Set_ptrcnt FAILURE
+ Setlinebuf PerlIOBase_setlinebuf
+ Tell FAILURE
+ Unread PerlIOBase_unread
+ Write FAILURE
+
+ FAILURE Set errno (to EINVAL in Unixish, to LIB$_INVARG in VMS) and
+ return -1 (for numeric return values) or NULL (for pointers)
+ INHERITED Inherited from the layer below
+ SUCCESS Return 0 (for numeric return values) or a pointer
=head2 Core Layers
A very complete generic buffering layer which provides the whole of
PerlIO API. It is also intended to be used as a "base class" for other
-layers. (For example its C<Read()> method is implemented in terms of the
-C<Get_cnt()>/C<Get_ptr()>/C<Set_ptrcnt()> methods).
+layers. (For example its C<Read()> method is implemented in terms of
+the C<Get_cnt()>/C<Get_ptr()>/C<Set_ptrcnt()> methods).
"perlio" over "unix" provides a complete replacement for stdio as seen
via PerlIO API. This is the default for USE_PERLIO when system's stdio
-does not permit perl's "fast gets" access, and which do not distinguish
-between C<O_TEXT> and C<O_BINARY>.
+does not permit perl's "fast gets" access, and which do not
+distinguish between C<O_TEXT> and C<O_BINARY>.
=item "stdio"
=item "pending"
An "internal" derivative of "perlio" which can be used to provide
-Unread() function for layers which have no buffer or cannot be bothered.
-(Basically this layer's C<Fill()> pops itself off the stack and so resumes
-reading from layer below.)
+Unread() function for layers which have no buffer or cannot be
+bothered. (Basically this layer's C<Fill()> pops itself off the stack
+and so resumes reading from layer below.)
=item "raw"
A dummy layer which never exists on the layer stack. Instead when
-"pushed" it actually pops the stack(!), removing itself, and any other
-layers until it reaches a layer with the class C<PERLIO_K_RAW> bit set.
+"pushed" it actually pops the stack removing itself, it then calls
+Binmode function table entry on all the layers in the stack - normally
+this (via PerlIOBase_binmode) removes any layers which do not have
+C<PERLIO_K_RAW> bit set. Layers can modify that behaviour by defining
+their own Binmode entry.
=item "utf8"
Another dummy layer. When pushed it pops itself and sets the
-C<PERLIO_F_UTF8> flag on the layer which was (and now is once more) the top
-of the stack.
+C<PERLIO_F_UTF8> flag on the layer which was (and now is once more)
+the top of the stack.
=back
=head2 Extension Layers
-Layers can made available by extension modules.
+Layers can made available by extension modules. When an unknown layer
+is encountered the PerlIO code will perform the equivalent of :
+
+ use PerlIO 'layer';
+
+Where I<layer> is the unknown layer. F<PerlIO.pm> will then attempt to:
+
+ require PerlIO::layer;
+
+If after that process the layer is still not defined then the C<open>
+will fail.
+
+The following extension layers are bundled with perl:
=over 4
-=item "encoding"
+=item ":encoding"
use Encoding;
-makes this layer available. It is an example of a layer which takes an argument.
-as it is called as:
+makes this layer available, although F<PerlIO.pm> "knows" where to
+find it. It is an example of a layer which takes an argument as it is
+called thus:
+
+ open( $fh, "<:encoding(iso-8859-7)", $pathname );
+
+=item ":scalar"
- open($fh,"<:encoding(iso-8859-7)",$pathname)
+Provides support for reading data from and writing data to a scalar.
+
+ open( $fh, "+<:scalar", \$scalar );
+
+When a handle is so opened, then reads get bytes from the string value
+of I<$scalar>, and writes change the value. In both cases the position
+in I<$scalar> starts as zero but can be altered via C<seek>, and
+determined via C<tell>.
+
+Please note that this layer is implied when calling open() thus:
+
+ open( $fh, "+<", \$scalar );
+
+=item ":via"
+
+Provided to allow layers to be implemented as Perl code. For instance:
+
+ use PerlIO::via::StripHTML;
+ open( my $fh, "<:via(StripHTML)", "index.html" );
+
+See L<PerlIO::via> for details.
=back
+=head1 TODO
-=cut
+Things that need to be done to improve this document.
+
+=over
+
+=item *
+
+Explain how to make a valid fh without going through open()(i.e. apply
+a layer). For example if the file is not opened through perl, but we
+want to get back a fh, like it was opened by Perl.
+
+How PerlIO_apply_layera fits in, where its docs, was it made public?
+
+Currently the example could be something like this:
+ PerlIO *foo_to_PerlIO(pTHX_ char *mode, ...)
+ {
+ char *mode; /* "w", "r", etc */
+ const char *layers = ":APR"; /* the layer name */
+ PerlIO *f = PerlIO_allocate(aTHX);
+ if (!f) {
+ return NULL;
+ }
+ PerlIO_apply_layers(aTHX_ f, mode, layers);
+ if (f) {
+ PerlIOAPR *st = PerlIOSelf(f, PerlIOAPR);
+ /* fill in the st struct, as in _open() */
+ st->file = file;
+ PerlIOBase(f)->flags |= PERLIO_F_OPEN;
+
+ return f;
+ }
+ return NULL;
+ }
+
+=item *
+
+fix/add the documentation in places marked as XXX.
+
+=item *
+
+The handling of errors by the layer is not specified. e.g. when $!
+should be set explicitly, when the error handling should be just
+delegated to the top layer.
+
+Probably give some hints on using SETERRNO() or pointers to where they
+can be found.
+
+=item *
+
+I think it would help to give some concrete examples to make it easier
+to understand the API. Of course I agree that the API has to be
+concise, but since there is no second document that is more of a
+guide, I think that it'd make it easier to start with the doc which is
+an API, but has examples in it in places where things are unclear, to
+a person who is not a PerlIO guru (yet).
+
+=back
+
+=cut
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