=head1 DESCRIPTION
-Full-fledged support for I/O disciplines is now implemented provided perl is
-configured to use PerlIO as its IO system (which is now the default).
+Full-fledged support for I/O disciplines is now implemented provided
+Perl is configured to use PerlIO as its IO system (which is now the
+default).
The C<open> pragma serves as one of the interfaces to declare default
"layers" (aka disciplines) for all I/O.
The C<open> pragma is used to declare one or more default layers for
-I/O operations. Any open(), readpipe() (aka qx//) and similar operators
-found within the lexical scope of this pragma will use the declared defaults.
+I/O operations. Any open(), readpipe() (aka qx//) and similar
+operators found within the lexical scope of this pragma will use the
+declared defaults.
-When open() is given an explicit list of layers they are appended to the
-list declared using this pragma.
+When open() is given an explicit list of layers they are appended to
+the list declared using this pragma.
Directory handles may also support disciplines in future.
=head1 NONPERLIO FUNCTIONALITY
-If perl is not built to use PerlIO as its IO system then only the two pseudo-disciplines
-":raw" and ":crlf" are available.
+If Perl is not built to use PerlIO as its IO system then only the two
+pseudo-disciplines ":raw" and ":crlf" are available.
The ":raw" discipline corresponds to "binary mode" and the ":crlf"
discipline corresponds to "text mode" on platforms that distinguish
between the two modes when opening files (which is many DOS-like
-platforms, including Windows). These two disciplines are
-no-ops on platforms where binmode() is a no-op, but perform their
-functions everywhere if PerlIO is enabled.
+platforms, including Windows). These two disciplines are no-ops on
+platforms where binmode() is a no-op, but perform their functions
+everywhere if PerlIO is enabled.
=head1 IMPLEMENTATION DETAILS
-There are two package variables C<%layers> and C<@layers> which
-are mainly manipulated by C code in F<perlio.c>, but are visible
-to the nosy:
+There are two package variables C<%layers> and C<@layers> which are
+mainly manipulated by C code in F<perlio.c>, but are visible to the
+nosy:
print "Have ",join(',',keys %open::layers),"\n";
print "Using ",join(',',@open::layers),"\n";
-The C<%open::layers> hash is a record of the available "layers" that may be pushed
-onto a C<PerlIO> stream. The values of the hash are perl objects, of class C<PerlIO::Layer>
-which are created by the C code in F<perlio.c>. As yet there is nothing useful you
-can do with the objects at the perl level.
-
-The C<@open::layers> array is the current set of layers and their arguments.
-The array consists of layer => argument pairs and I<must> always have even number of
-entries and the even entries I<must> be C<PerlIO::Layer> objects or perl will "die"
-when it attempts to open a filehandle. In most cases the odd entry will be C<undef>,
-but in the case of (say) ":encoding(iso-8859-1)" it will be 'iso-8859-1'. These
+The C<%open::layers> hash is a record of the available "layers" that
+may be pushed onto a C<PerlIO> stream. The values of the hash are Perl
+objects, of class C<PerlIO::Layer> which are created by the C code in
+F<perlio.c>. As yet there is nothing useful you can do with the
+objects at the perl level.
+
+The C<@open::layers> array is the current set of layers and their
+arguments. The array consists of layer => argument pairs and I<must>
+always have even number of entries and the even entries I<must> be
+C<PerlIO::Layer> objects or Perl will "die" when it attempts to open a
+filehandle. In most cases the odd entry will be C<undef>, but in the
+case of (say) ":encoding(iso-8859-1)" it will be 'iso-8859-1'. These
argument entries are currently restricted to being strings.
-When a new C<PerlIO> stream is opened, the C code looks at the
-array to determine the default layers to be pushed. So with care it is possible
-to manipulate the default layer "stack":
+When a new C<PerlIO> stream is opened, the C code looks at the array
+to determine the default layers to be pushed. So with care it is
+possible to manipulate the default layer "stack":
splice(@PerlIO::layers,-2,2);
push(@PerlIO::layers,$PerlIO::layers{'stdio'} => undef);