perl56delta.pod

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=head1 NAME

perl56delta - what's new for perl v5.6.0

=head1 DESCRIPTION

This document describes differences between the 5.005 release and the 5.6.0
release.

=head1 Core Enhancements

=head2 Interpreter cloning, threads, and concurrency

Perl 5.6.0 introduces the beginnings of support for running multiple
interpreters concurrently in different threads.  In conjunction with
the perl_clone() API call, which can be used to selectively duplicate
the state of any given interpreter, it is possible to compile a
piece of code once in an interpreter, clone that interpreter
one or more times, and run all the resulting interpreters in distinct
threads.

On the Windows platform, this feature is used to emulate fork() at the
interpreter level.  See L<perlfork> for details about that.

This feature is still in evolution.  It is eventually meant to be used
to selectively clone a subroutine and data reachable from that
subroutine in a separate interpreter and run the cloned subroutine
in a separate thread.  Since there is no shared data between the
interpreters, little or no locking will be needed (unless parts of
the symbol table are explicitly shared).  This is obviously intended
to be an easy-to-use replacement for the existing threads support.

Support for cloning interpreters and interpreter concurrency can be
enabled using the -Dusethreads Configure option (see win32/Makefile for
how to enable it on Windows.)  The resulting perl executable will be
functionally identical to one that was built with -Dmultiplicity, but
the perl_clone() API call will only be available in the former.

-Dusethreads enables the cpp macro USE_ITHREADS by default, which in turn
enables Perl source code changes that provide a clear separation between
the op tree and the data it operates with.  The former is immutable, and
can therefore be shared between an interpreter and all of its clones,
while the latter is considered local to each interpreter, and is therefore
copied for each clone.

Note that building Perl with the -Dusemultiplicity Configure option
is adequate if you wish to run multiple B<independent> interpreters
concurrently in different threads.  -Dusethreads only provides the
additional functionality of the perl_clone() API call and other
support for running B<cloned> interpreters concurrently.

    NOTE: This is an experimental feature.  Implementation details are
    subject to change.

=head2 Lexically scoped warning categories

You can now control the granularity of warnings emitted by perl at a finer
level using the C<use warnings> pragma.  L<warnings> and L<perllexwarn>
have copious documentation on this feature.

=head2 Unicode and UTF-8 support

Perl now uses UTF-8 as its internal representation for character
strings.  The C<utf8> and C<bytes> pragmas are used to control this support
in the current lexical scope.  See L<perlunicode>, L<utf8> and L<bytes> for
more information.

This feature is expected to evolve quickly to support some form of I/O
disciplines that can be used to specify the kind of input and output data
(bytes or characters).  Until that happens, additional modules from CPAN
will be needed to complete the toolkit for dealing with Unicode.

    NOTE: This should be considered an experimental feature.  Implementation
    details are subject to change.

=head2 Support for interpolating named characters

The new C<\N> escape interpolates named characters within strings.
For example, C<"Hi! \N{WHITE SMILING FACE}"> evaluates to a string
with a unicode smiley face at the end.

=head2 "our" declarations

An "our" declaration introduces a value that can be best understood
as a lexically scoped symbolic alias to a global variable in the
package that was current where the variable was declared.  This is
mostly useful as an alternative to the C<vars> pragma, but also provides
the opportunity to introduce typing and other attributes for such
variables.  See L<perlfunc/our>.

=head2 Support for strings represented as a vector of ordinals

Literals of the form C<v1.2.3.4> are now parsed as a string composed
of characters with the specified ordinals.  This is an alternative, more
readable way to construct (possibly unicode) strings instead of
interpolating characters, as in C<"\x{1}\x{2}\x{3}\x{4}">.  The leading
C<v> may be omitted if there are more than two ordinals, so C<1.2.3> is
parsed the same as C<v1.2.3>.

Strings written in this form are also useful to represent version "numbers".
It is easy to compare such version "numbers" (which are really just plain
strings) using any of the usual string comparison operators C<eq>, C<ne>,
C<lt>, C<gt>, etc., or perform bitwise string operations on them using C<|>,
C<&>, etc.

In conjunction with the new C<$^V> magic variable (which contains
the perl version as a string), such literals can be used as a readable way
to check if you're running a particular version of Perl:

    # this will parse in older versions of Perl also
    if ($^V and $^V gt v5.6.0) {
        # new features supported
    }

C<require> and C<use> also have some special magic to support such
literals, but this particular usage should be avoided because it leads to
misleading error messages under versions of Perl which don't support vector
strings.  Using a true version number will ensure correct behavior in all
versions of Perl:

    require 5.006;    # run time check for v5.6
    use 5.006_001;    # compile time check for v5.6.1

Also, C<sprintf> and C<printf> support the Perl-specific format flag C<%v>
to print ordinals of characters in arbitrary strings:

    printf "v%vd", $^V;		# prints current version, such as "v5.5.650"
    printf "%*vX", ":", $addr;	# formats IPv6 address
    printf "%*vb", " ", $bits;	# displays bitstring

See L<perldata/"Scalar value constructors"> for additional information.

=head2 Improved Perl version numbering system

Beginning with Perl version 5.6.0, the version number convention has been
changed to a "dotted integer" scheme that is more commonly found in open
source projects.

Maintenance versions of v5.6.0 will be released as v5.6.1, v5.6.2 etc.
The next development series following v5.6.0 will be numbered v5.7.x,
beginning with v5.7.0, and the next major production release following
v5.6.0 will be v5.8.0.

The English module now sets $PERL_VERSION to $^V (a string value) rather
than C<$]> (a numeric value).  (This is a potential incompatibility.
Send us a report via perlbug if you are affected by this.)

The v1.2.3 syntax is also now legal in Perl.
See L<Support for strings represented as a vector of ordinals> for more on that.

To cope with the new versioning system's use of at least three significant
digits for each version component, the method used for incrementing the
subversion number has also changed slightly.  We assume that versions older
than v5.6.0 have been incrementing the subversion component in multiples of
10.  Versions after v5.6.0 will increment them by 1.  Thus, using the new
notation, 5.005_03 is the "same" as v5.5.30, and the first maintenance
version following v5.6.0 will be v5.6.1 (which should be read as being
equivalent to a floating point value of 5.006_001 in the older format,
stored in C<$]>).

=head2 New syntax for declaring subroutine attributes

Formerly, if you wanted to mark a subroutine as being a method call or
as requiring an automatic lock() when it is entered, you had to declare
that with a C<use attrs> pragma in the body of the subroutine.
That can now be accomplished with declaration syntax, like this:

    sub mymethod : locked method;
    ...
    sub mymethod : locked method {
	...
    }

    sub othermethod :locked :method;
    ...
    sub othermethod :locked :method {
	...
    }


(Note how only the first C<:> is mandatory, and whitespace surrounding
the C<:> is optional.)

F<AutoSplit.pm> and F<SelfLoader.pm> have been updated to keep the attributes
with the stubs they provide.  See L<attributes>.

=head2 File and directory handles can be autovivified

Similar to how constructs such as C<< $x->[0] >> autovivify a reference,
handle constructors (open(), opendir(), pipe(), socketpair(), sysopen(),
socket(), and accept()) now autovivify a file or directory handle
if the handle passed to them is an uninitialized scalar variable.  This
allows the constructs such as C<open(my $fh, ...)> and C<open(local $fh,...)>
to be used to create filehandles that will conveniently be closed
automatically when the scope ends, provided there are no other references
to them.  This largely eliminates the need for typeglobs when opening
filehandles that must be passed around, as in the following example:

    sub myopen {
        open my $fh, "@_"
	     or die "Can't open '@_': $!";
	return $fh;
    }

    {
        my $f = myopen("</etc/motd");
	print <$f>;
	# $f implicitly closed here
    }

=head2 open() with more than two arguments

If open() is passed three arguments instead of two, the second argument
is used as the mode and the third argument is taken to be the file name.
This is primarily useful for protecting against unintended magic behavior
of the traditional two-argument form.  See L<perlfunc/open>.

=head2 64-bit support

Any platform that has 64-bit integers either

	(1) natively as longs or ints
	(2) via special compiler flags
	(3) using long long or int64_t

is able to use "quads" (64-bit integers) as follows:

=over 4

=item *

constants (decimal, hexadecimal, octal, binary) in the code 

=item *

arguments to oct() and hex()

=item *

arguments to print(), printf() and sprintf() (flag prefixes ll, L, q)

=item *

printed as such

=item *

pack() and unpack() "q" and "Q" formats

=item *

in basic arithmetics: + - * / % (NOTE: operating close to the limits
of the integer values may produce surprising results)

=item *

in bit arithmetics: & | ^ ~ << >> (NOTE: these used to be forced 
to be 32 bits wide but now operate on the full native width.)

=item *

vec()

=back

Note that unless you have the case (a) you will have to configure
and compile Perl using the -Duse64bitint Configure flag.

    NOTE: The Configure flags -Duselonglong and -Duse64bits have been
    deprecated.  Use -Duse64bitint instead.

There are actually two modes of 64-bitness: the first one is achieved
using Configure -Duse64bitint and the second one using Configure
-Duse64bitall.  The difference is that the first one is minimal and
the second one maximal.  The first works in more places than the second.

The C<use64bitint> does only as much as is required to get 64-bit
integers into Perl (this may mean, for example, using "long longs")
while your memory may still be limited to 2 gigabytes (because your
pointers could still be 32-bit).  Note that the name C<64bitint> does
not imply that your C compiler will be using 64-bit C<int>s (it might,
but it doesn't have to): the C<use64bitint> means that you will be
able to have 64 bits wide scalar values.

The C<use64bitall> goes all the way by attempting to switch also
integers (if it can), longs (and pointers) to being 64-bit.  This may
create an even more binary incompatible Perl than -Duse64bitint: the
resulting executable may not run at all in a 32-bit box, or you may
have to reboot/reconfigure/rebuild your operating system to be 64-bit
aware.

Natively 64-bit systems like Alpha and Cray need neither -Duse64bitint
nor -Duse64bitall.

Last but not least: note that due to Perl's habit of always using
floating point numbers, the quads are still not true integers.
When quads overflow their limits (0...18_446_744_073_709_551_615 unsigned,
-9_223_372_036_854_775_808...9_223_372_036_854_775_807 signed), they
are silently promoted to floating point numbers, after which they will
start losing precision (in their lower digits).

    NOTE: 64-bit support is still experimental on most platforms.
    Existing support only covers the LP64 data model.  In particular, the
    LLP64 data model is not yet supported.  64-bit libraries and system
    APIs on many platforms have not stabilized--your mileage may vary.

=head2 Large file support

If you have filesystems that support "large files" (files larger than
2 gigabytes), you may now also be able to create and access them from
Perl.

    NOTE: The default action is to enable large file support, if
    available on the platform.

If the large file support is on, and you have a Fcntl constant
O_LARGEFILE, the O_LARGEFILE is automatically added to the flags
of sysopen().

Beware that unless your filesystem also supports "sparse files" seeking
to umpteen petabytes may be inadvisable.

Note that in addition to requiring a proper file system to do large
files you may also need to adjust your per-process (or your
per-system, or per-process-group, or per-user-group) maximum filesize
limits before running Perl scripts that try to handle large files,
especially if you intend to write such files.

Finally, in addition to your process/process group maximum filesize
limits, you may have quota limits on your filesystems that stop you
(your user id or your user group id) from using large files.

Adjusting your process/user/group/file system/operating system limits
is outside the scope of Perl core language.  For process limits, you
may try increasing the limits using your shell's limits/limit/ulimit
command before running Perl.  The BSD::Resource extension (not
included with the standard Perl distribution) may also be of use, it
offers the getrlimit/setrlimit interface that can be used to adjust
process resource usage limits, including the maximum filesize limit.

=head2 Long doubles

In some systems you may be able to use long doubles to enhance the
range and precision of your double precision floating point numbers
(that is, Perl's numbers).  Use Configure -Duselongdouble to enable
this support (if it is available).

=head2 "more bits"

You can "Configure -Dusemorebits" to turn on both the 64-bit support
and the long double support.

=head2 Enhanced support for sort() subroutines

Perl subroutines with a prototype of C<($$)>, and XSUBs in general, can
now be used as sort subroutines.  In either case, the two elements to
be compared are passed as normal parameters in @_.  See L<perlfunc/sort>.

For unprototyped sort subroutines, the historical behavior of passing 
the elements to be compared as the global variables $a and $b remains
unchanged.

=head2 C<sort $coderef @foo> allowed

sort() did not accept a subroutine reference as the comparison
function in earlier versions.  This is now permitted.