=head1 DESCRIPTION
-This document describes the behavior and implementation of the PerlIO abstraction
-described in L<perlapio> when C<USE_PERLIO> is defined (and C<USE_SFIO> is not).
+This document describes the behavior and implementation of the PerlIO
+abstraction described in L<perlapio> when C<USE_PERLIO> is defined (and
+C<USE_SFIO> is not).
=head2 History and Background
-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 extenstions
-switch to using it, so the API is mostly fixed to maintain (source) compatibility.
+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 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.
+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.
=head2 Layers vs Disciplines
-Initial discussion of the ability to modify IO streams behaviour used the term
-"discipline" for the entities which were added. This came (I believe) from the use
-of the term in "sfio", which in turn borowed it 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 conotations
-that are inherent in earlier uses of "discipline" for things which are rather different.
+Initial discussion of the ability to modify IO streams behaviour used
+the term "discipline" for the entities which were added. This came (I
+believe) from the use of the term in "sfio", which in turn borrowed it
+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.
=head2 Data Structures
IV flags; /* Various flags for state */
};
-A PerlIOl * is a pointer to to the struct, and the I<application> level PerlIO *
-is a pointer to a PerlIOl * - i.e. a pointer to a pointer to the struct.
-This allows the application level PerlIO * to remain constant while the actual
-PerlIOl * underneath changes. (Compare perl's SV * which remains constant
-while its 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 PerlIO *,
-a &(perlio->next) "is" a PerlIO *, and so to some degree at least
-one layer can use the "standard" API on the next layer down.
+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
+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. The per-instance data for a particular handle.
+=item 2.
+
+The per-instance data for a particular handle.
=back
=head2 Functions and Attributes
-The functions and attributes are accessed via the "tab" (for table) member of
-PerlIOl. The functions (methods of the layer "class") are fixed, and are defined by the
-PerlIO_funcs type. They are broadly the same as the public PerlIO_xxxxx functions:
+The functions and attributes are accessed via the "tab" (for table)
+member of C<PerlIOl>. The functions (methods of the layer "class") are
+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 (*Pushed)(PerlIO *f,const char *mode,SV *arg);
+ IV (*Popped)(PerlIO *f);
+ 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);
+ SV * (*Getarg)(PerlIO *f);
+ IV (*Fileno)(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
+layer), then follow the functions which fall into four basic groups:
- 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);
- };
+=over 4
-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 layer), then follow the functions which fall into
-four basic groups:
+=item 1.
-=over 4
+Opening and setup functions
+
+=item 2.
-=item 1. Opening and setup functions
+Basic IO operations
-=item 2. Basic IO operations
+=item 3.
-=item 3. Stdio class buffering options.
+Stdio class buffering options.
-=item 4. Functions to support Perl's traditional "fast" access to the buffer.
+=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
+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
The above attempts to show how the layer scheme works in a simple case.
-The applications PerlIO * points to an entry in the table(s) representing open
-(allocated) handles. For example the first three slots 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.
+The application's C<PerlIO *> points to an entry in the table(s)
+representing open (allocated) handles. For example the first three slots
+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.
-The above is roughly equivalent to a "stdio" buffered stream, but with much more
-flexibility:
+The above is roughly equivalent to a "stdio" buffered stream, but with
+much more flexibility:
=over 4
=item *
-If Unix level read/write/lseek is not appropriate for (say) sockets then
-the "unix" layer can be replaced (at open time or even dynamically) with a
-"socket" layer.
+If Unix level C<read>/C<write>/C<lseek> is not appropriate for (say)
+sockets then the "unix" layer can be replaced (at open time or even
+dynamically) with a "socket" layer.
=item *
=item *
Extra layers can be inserted to process the data as it flows through.
-This was the driving need for including the scheme in perkl5.70+ - we needed a mechanism
-to allow data to be translated bewteen 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.
+This was the driving need for including the scheme in perl 5.7.0+ - we
+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 *
=head2 Per-instance flag bits
-The generic flag bits are a hybrid of O_XXXXX style flags deduced from
-the mode string passed to PerlIO_open() and state bits for typical buffer
+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.
-=over4
+=over 4
=item PERLIO_F_EOF
=item PERLIO_F_CANWRITE
-Writes are permited i.e. opened as "w" or "r+" or "a". etc.
+Writes are permitted, i.e. opened as "w" or "r+" or "a", etc.
=item PERLIO_F_CANREAD
-Reads are permited i.e. opened "r" or "w+" (or even "a+" - ick).
+Reads are permitted i.e. opened "r" or "w+" (or even "a+" - ick).
=item PERLIO_F_ERROR
-An error has occured (for PerlIO_error())
+An error has occurred (for C<PerlIO_error()>)
=item PERLIO_F_TRUNCATE
=item PERLIO_F_CRLF
-Layer is performing Win32-like "\n" => CR,LF for output and CR,LF => "\n" for
-input. Normally the provided "crlf" layer is only layer than need bother about
-this. PerlIO_binmode() will mess with this flag rather than add/remove layers
-if the PERLIO_K_CANCRLF bit is set for the layers class.
+Layer is performing Win32-like "\n" mapped to CR,LF for output and CR,LF
+mapped to "\n" for input. Normally the provided "crlf" layer is the only
+layer that need bother about this. C<PerlIO_binmode()> will mess with this
+flag rather than add/remove layers if the C<PERLIO_K_CANCRLF> bit is set
+for the layers class.
=item PERLIO_F_UTF8
-Data for this written to this layer should be UTF-8 encoded, data provided
+Data written to this layer should be UTF-8 encoded; data provided
by this layer should be considered UTF-8 encoded. Can be set on any layer
by ":utf8" dummy layer. Also set on ":encoding" layer.
=item PERLIO_F_LINEBUF
-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.
+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 unlink()ed, or should be deleted on close().
+File has been C<unlink()>ed, or should be deleted on C<close()>.
=item PERLIO_F_OPEN
=item PERLIO_F_FASTGETS
-This instance of this layer supports the "fast gets" interface.
-Normally set based on PERLIO_K_FASTGETS for the class and by the
-existance of the function(s) in the table. However a class that
+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 an layer which does not support the interface.
-(Perls sv_gets() does not expect the steams fast gets behaviour
+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
=over 4
-=item IV (*Fileno)(PerlIO *f);
-
-Returns the Unix/Posix numeric file decriptor for the handle.
-Normally PerlIOBase_fileno() (which just asks next layer down) will suffice for this.
-
-=item PerlIO * (*Fdopen)(PerlIO_funcs *tab, int fd, const char *mode);
-
-Should (perhaps indirectly) call 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 PerlIO * (*Open)(PerlIO_funcs *tab, const char *path, const char *mode);
-
-Should attempt to open the given path and if that succeeds then (perhaps indirectly)
-call PerlIO_allocate() to allocate a slot in the table and associate it with the
-layers information for the opened file.
+=item IV (*Pushed)(PerlIO *f,const char *mode, SV *arg);
-=item int (*Reopen)(const char *path, const char *mode, PerlIO *f);
+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).
-Re-open the supplied 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.
-
-=item IV (*Pushed)(PerlIO *f,const char *mode,const char *arg,STRLEN len);
+=item IV (*Popped)(PerlIO *f);
-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 PerlIOBase_pushed() to conver C<mode> into
-the appropriate PERLIO_F_XXXXX flags in addition to any actions the layer itself takes.
+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.
+
+=item 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. (Binary/Text should be ignored by almost all layers and binary IO done,
+with PerlIO. 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(*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.
+
+=item SV * (*Getarg)(PerlIO *f);
+
+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".
-=item IV (*Popped)(PerlIO *f);
+=item IV (*Fileno)(PerlIO *f);
-Called when the layer is popped from the stack. A layer will normally be popped after
-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 Popped() should free
-any resources (buffers, translation tables, ...) not held directly in the layer's
-struct.
+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 SSize_t (*Read)(PerlIO *f, void *vbuf, Size_t count);
Basic read operation. Returns actual bytes read, or -1 on an error.
Typically will call Fill and manipulate pointers (possibly via the API).
-PerlIOBuf_read() may be suitable for derived classes which provide "fast gets" methods.
+C<PerlIOBuf_read()> may be suitable for derived classes which provide
+"fast gets" methods.
=item SSize_t (*Unread)(PerlIO *f, const void *vbuf, Size_t count);
-A superset of stdio's ungetc(). Should arrange for future reads to see the bytes in C<vbuf>.
-If there is no obviously better implementation then PerlIOBase_unread() provides
-the function by pushing a "fake" "pending" layer above the calling layer.
+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);
=item IV (*Seek)(PerlIO *f, Off_t offset, int whence);
-Position the file pointer. Should normally call its own Flush method and
-then the Seek method of next layer down.
+Position the file pointer. Should normally call its own C<Flush> method and
+then the C<Seek> method of next layer down.
=item Off_t (*Tell)(PerlIO *f);
-Return the file pointer. May be based on layers cached concept of position to
-avoid overhead.
+Return the file pointer. May be based on layers cached concept of
+position to avoid overhead.
=item IV (*Close)(PerlIO *f);
-Close the stream. Should normally call 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.
+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);
-Should make streams state consistent with layers below. That is any
-buffered write data should be written, and file position of lower layer
-adjusted for data read fron below but not actually consumed.
+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.)
=item IV (*Fill)(PerlIO *f);
=item IV (*Eof)(PerlIO *f);
-Return end-of-file indicator. PerlIOBase_eof() is normally sufficient.
+Return end-of-file indicator. C<PerlIOBase_eof()> is normally sufficient.
=item IV (*Error)(PerlIO *f);
-Return error indicator. PerlIOBase_error() is normally sufficient.
+Return error indicator. C<PerlIOBase_error()> is normally sufficient.
=item void (*Clearerr)(PerlIO *f);
-Clear end-of-file and error indicators. Should call PerlIOBase_clearerr()
-to set the PERLIO_F_XXXXX flags, which may suffice.
+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);
-Mark the stream as line buffered.
+Mark the stream as line buffered. C<PerlIOBase_setlinebuf()> sets the
+PERLIO_F_LINEBUF flag and is normally sufficient.
=item STDCHAR * (*Get_base)(PerlIO *f);
=item Size_t (*Get_bufsiz)(PerlIO *f);
-Return the number of bytes that last Fill() put in the buffer.
+Return the number of bytes that last C<Fill()> put in the buffer.
=item STDCHAR * (*Get_ptr)(PerlIO *f);
-Return the current read pointer relative to this layers buffer.
+Return the current read pointer relative to this layer's buffer.
=item SSize_t (*Get_cnt)(PerlIO *f);
=item "unix"
-A basic non-buffered layer which calls Unix/POSIX read(), write(), lseek(), close().
-No buffering. Even on platforms that distinguish between O_TEXT and O_BINARY
-this layer is always O_BINARY.
+A basic non-buffered layer which calls Unix/POSIX C<read()>, C<write()>,
+C<lseek()>, C<close()>. No buffering. Even on platforms that distinguish
+between O_TEXT and O_BINARY this layer is always O_BINARY.
=item "perlio"
-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 Read() method is implemented in terms of the Get_cnt()/Get_ptr()/Set_ptrcnt()
-methods).
+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).
-"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 O_TEXT and O_BINARY.
+"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>.
=item "stdio"
-A layer which provides the PerlIO API via the layer scheme, but implements it by calling
-system's stdio. This is (currently) the default if system's stdio provides sufficient
-access to allow perl's "fast gets" access and which do not distinguish between O_TEXT and
-O_BINARY.
+A layer which provides the PerlIO API via the layer scheme, but
+implements it by calling system's stdio. This is (currently) the default
+if system's stdio provides sufficient access to allow perl's "fast gets"
+access and which do not distinguish between C<O_TEXT> and C<O_BINARY>.
=item "crlf"
-A layer derived using "perlio" as a base class. It provides Win32-like "\n" to CR,LF
-translation. Can either be applied above "perlio" or serve as the buffer layer itself.
-"crlf" over "unix" is the default if system distinguishes between O_TEXT and O_BINARY
-opens. (At some point "unix" will be replaced by a "native" Win32 IO layer on that
-platform, as Win32's read/write layer has various drawbacks.)
-The "crlf" layer is a reasonable model for a layer which transforms data in some way.
+A layer derived using "perlio" as a base class. It provides Win32-like
+"\n" to CR,LF translation. Can either be applied above "perlio" or serve
+as the buffer layer itself. "crlf" over "unix" is the default if system
+distinguishes between C<O_TEXT> and C<O_BINARY> opens. (At some point
+"unix" will be replaced by a "native" Win32 IO layer on that platform,
+as Win32's read/write layer has various drawbacks.) The "crlf" layer is
+a reasonable model for a layer which transforms data in some way.
=item "mmap"
-If Configure detects C<mmap()> functions this layer is provided (with "perlio" as a
-"base") which does "read" operations by mmap()ing the file. Performance improvement
-is marginal on modern systems, so it is mainly there as a proof of concept.
-It is likely to be unbundled from the core at some point.
-The "mmap" layer is a reasonable model for a minimalist "derived" layer.
+If Configure detects C<mmap()> functions this layer is provided (with
+"perlio" as a "base") which does "read" operations by mmap()ing the
+file. Performance improvement is marginal on modern systems, so it is
+mainly there as a proof of concept. It is likely to be unbundled from
+the core at some point. The "mmap" layer is a reasonable model for a
+minimalist "derived" layer.
=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 Fill() pops itself off the stack and so resumes reading
-from layer below.)
+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.)
=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 PERLIO_K_RAW bit set.
+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.
=item "utf8"
-Another dummy layer. When pushed it pops itself and sets the PERLIO_F_UTF8 flag
-on the layer which was (and now is once more) the top of the stack.
+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.
=back
-In addition C<perlio.c> also provides a number of PerlIOBase_xxxx() functions
-which are intended to be used in the table slots of classes which do not need
-to do anything special for a particular method.
+In addition F<perlio.c> also provides a number of C<PerlIOBase_xxxx()>
+functions which are intended to be used in the table slots of classes
+which do not need to do anything special for a particular method.
=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)",$pathname)
-=back
+=item ":Scalar"
+
+Provides support for
+
+ open($fh,"...",\$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>.
+
+=item ":Object" or ":Perl"
+
+May be provided to allow layers to be implemented as perl code - implementation
+is being investigated.
+
+=back
=cut
projects / p5sagit/p5-mst-13.2.git / blobdiff