Add the 5.17.4 epigraph

[perl5.git] / pod / perldelta.pod

=encoding utf8

=head1 NAME

perldelta - what is new for perl v5.17.4

=head1 DESCRIPTION

This document describes differences between the 5.17.3 release and the 5.17.4
release.

If you are upgrading from an earlier release such as 5.17.2, first read
L<perl5173delta>, which describes differences between 5.17.2 and 5.17.3.

=head1 Core Enhancements

=head2 Latest Unicode 6.2 beta is included

This is supposed to be the final data for 6.2, unless glitches are
found.  The earlier experimental 6.2 beta data has been reverted, and
this used instead.  Not all the changes that were proposed for 6.2 and
that were in the earlier beta versions are actually going into 6.2.  In
particular, there are no changes from 6.1 in the General_Category of any
characters.  6.2 does revise the C<\X> handling for the REGIONAL
INDICATOR characters that were added in Unicode 6.0.  Perl now for the
first time fully handles this revision.

=head2 New DTrace probes

The following new DTrace probes have been added:

=over 4

=item C<op-entry>

=item C<loading-file>

=item C<loaded-file>

=back

=head2 C<${^LAST_FH}>

This new variable provides access to the filehandle that was last read.
This is the handle used by C<$.> and by C<tell> and C<eof> without
arguments.

=head2 Looser here-doc parsing

Here-doc terminators no longer require a terminating newline character when
they occur at the end of a file.  This was already the case at the end of a
string eval [perl #65838].

=head2 New mechanism for experimental features

Newly-added experimental features will now require this incantation:

    no warnings "experimental:feature_name";
    use feature "feature_name";  # would warn without the prev line

There is a new warnings category, called "experimental", containing
warnings that the L<feature> pragma emits when enabling experimental
features.

Newly-added experimental features will also be given special warning IDs,
which consist of "experimental:" followed by the name of the feature.  (The
plan is to extend this mechanism eventually to all warnings, to allow them
to be enabled or disabled individually, and not just by category.)

By saying

    no warnings "experimental:feature_name";

you are taking responsibility for any breakage that future changes to, or
removal of, the feature may cause.

=head2 Lexical subroutines

This new feature is still considered experimental.  To enable it, use the
mechanism described above:

    use 5.018;
    no warnings "experimental:lexical_subs";
    use feature "lexical_subs";

You can now declare subroutines with C<state sub foo>, C<my sub foo>, and
C<our sub foo>.  (C<state sub> requires that the "state" feature be
enabled, unless you write it as C<CORE::state sub foo>.)

C<state sub> creates a subroutine visible within the lexical scope in which
it is declared.  The subroutine is shared between calls to the outer sub.

C<my sub> declares a lexical subroutine that is created each time the
enclosing block is entered.  C<state sub> is generally slightly faster than
C<my sub>.

C<our sub> declares a lexical alias to the package subroutine of the same
name.

See L<perlsub/Lexical Subroutines>.

=head1 Incompatible Changes

=head2 Here-doc parsing

The body of a here-document inside a quote-like operator now always begins
on the line after the "<<foo" marker.  Previously, it was documented to
begin on the line following the containing quote-like operator, but that
was only sometimes the case [perl #114040].

=head2 Stricter parsing of substitution replacement

It is no longer possible to abuse the way the parser parses C<s///e> like
this:

    %_=(_,"Just another ");
    $_="Perl hacker,\n";
    s//_}->{_/e;print

=head2 Interaction of lexical and default warnings

Turning on any lexical warnings used first to disable all default warnings
if lexical warnings were not already enabled:

    $*; # deprecation warning
    use warnings "void";
    $#; # void warning; no deprecation warning

Now, the debugging, deprecated, glob, inplace and malloc warnings
categories are left on when turning on lexical warnings (unless they are
turned off by C<no warnings>, of course).

This may cause deprecation warnings to occur in code that used to be free
of warnings.

Those are the only categories consisting only of default warnings.  Default
warnings in other categories are still disabled by C<use warnings
"category">, as we do not yet have the infrastructure for controlling
individual warnings.

=head2 C<state sub> and C<our sub>

Due to an accident of history, C<state sub> and C<our sub> were equivalent
to a plain C<sub>, so one could even create an anonymous sub with
C<our sub { ... }>.  These are now disallowed outside of the "lexical_subs"
feature.  Under the "lexical_subs" feature they have new meanings described
in L<perlsub/Lexical Subroutines>.

=head2 C<gv_fetchmeth_*> and SUPER

The various C<gv_fetchmeth_*> XS functions used to treat a package whose
named ended with ::SUPER specially.  A method lookup on the Foo::SUPER
package would be treated as a SUPER method lookup on the Foo package.  This
is no longer the case.  To do a SUPER lookup, pass the Foo stash and the
GV_SUPER flag.

=head1 Performance Enhancements

=over 4

=item *

Speed up in regular expression matching against Unicode properties.  The
largest gain is for C<\X>, the Unicode "extended grapheme cluster".  The
gain for it is about 35% - 40%.  Bracketed character classes, e.g.,
C<[0-9\x{100}]> containing code points above 255 are also now faster.

=item *

On platforms supporting it, several former macros are now implemented as static
inline functions. This should speed things up slightly on non-GCC platforms.

=item *

Apply the optimisation of hashes in boolean context, such as in C<if> or C<and>,
to constructs in non-void context.

=item *

Extend the optimisation of hashes in boolean context to C<scalar(%hash)>,
C<%hash ? ... : ...>, and C<sub { %hash || ... }>.

=item *

When making a copy of the string being matched against (so that $1, $& et al
continue to show the correct value even if the original string is subsequently
modified), only copy that substring of the original string needed for the
capture variables, rather than copying the whole string.

This is a big win for code like

    $&;
    $_ = 'x' x 1_000_000;
    1 while /(.)/;

Also, when pessimizing if the code contains C<$`>, C<$&> or C<$'>, record the
presence of each variable separately, so that the determination of the substring
range is based on each variable separately. So performance-wise,

   $&; /x/

is now roughly equivalent to

   /(x)/

whereas previously it was like

   /^(.*)(x)(.*)$/

and

   $&; $'; /x/

is now roughly equivalent to

   /(x)(.*)$/

etc.

=back

=head1 Modules and Pragmata

=head2 Updated Modules and Pragmata

=over 4

=item *

L<Archive::Tar> has been upgraded from version 1.88 to 1.90.  This adds
documentation fixes.

=item *

L<B> has been upgraded from version 1.37 to 1.38.  This makes the module work
with the new pad API.

=item *

L<B::Concise> has been upgraded from version 0.92 to 0.93.  This adds support
for the new C<OpMAYBE_TRUEBOOL> and C<OPpTRUEBOOL> flags.

=item *

L<B::Deparse> has been upgraded from version 1.16 to 1.17.  This suppresses
trailing semicolons in formats.

=item *

L<CPANPLUS> has been upgraded from version 0.9130 to 0.9131.  This resolves
issues with the SQLite source engine.

=item *

L<DB_File> has been upgraded from version 1.826 to 1.827.  The main Perl module
no longer uses the C<"@_"> construct.

=item *

L<Devel::Peek> has been upgraded from version 1.09 to 1.10.  This fixes
compilation with C++ compilers and makes the module work with the new pad API.

=item *

L<DynaLoader> has been upgraded from version 1.15 to 1.16.  This fixes warnings
about using C<CODE> sections without an C<OUTPUT> section.

=item *

L<ExtUtils::ParseXS> has been upgraded from version 3.17 to 3.18.  This avoids a
bogus warning for initialised XSUB non-parameters [perl #112776].

=item *

L<File::Copy> has been upgraded from version 2.23 to 2.24.  C<copy()> no longer
zeros files when copying into the same directory, and also now fails (as it has
long been documented to do) when attempting to copy a file over itself.

=item *

L<File::Find> has been upgraded from version 1.21 to 1.22.  This fixes
inconsistent unixy path handling on VMS.

=item *

L<IPC::Open3> has been upgraded from version 1.12 to 1.13.  The C<open3()>
function no longer uses C<POSIX::close()> to close file descriptors since that
breaks the ref-counting of file descriptors done by PerlIO in cases where the
file descriptors are shared by PerlIO streams, leading to attempts to close the
file descriptors a second time when any such PerlIO streams are closed later on.

=item *

L<Locale::Codes> has been upgraded from version 3.22 to 3.23.  It includes some
new codes.

=item *

L<Module::CoreList> has been upgraded from version 2.71 to 2.73.  This restores
compatibility with older versions of perl and cleans up the corelist data for
various modules.

=item *

L<Opcode> has been upgraded from version 1.23 to 1.24 to reflect the removal of
the boolkeys opcode and the addition of the clonecv, introcv and padcv
opcodes.

=item *

L<Socket> has been upgraded from version 2.004 to 2.006.
C<unpack_sockaddr_in()> and C<unpack_sockaddr_in6()> now return just the IP
address in scalar context, and C<inet_ntop()> now guards against incorrect
length scalars being passed in.

=item *

L<Storable> has been upgraded from version 2.38 to 2.39.  This contains various
bugfixes, including compatibility fixes for older versions of Perl and vstring
handling.

=item *

L<Sys::Syslog> has been upgraded from version 0.31 to 0.32.  This includes
several documentation and bug fixes.

=item *

L<threads::shared> has been upgraded from version 1.40 to 1.41.  This adds the
option to warn about or ignore attempts to clone structures that can't be
cloned, as opposed to just unconditionally dying in that case.

=item *

L<version> has been upgraded from version 0.99 to 0.9901.

=item *

L<XSLoader> has been upgraded from version 0.15 to 0.16.

=back

=head1 Diagnostics

The following additions or changes have been made to diagnostic output,
including warnings and fatal error messages.  For the complete list of
diagnostic messages, see L<perldiag>.

=head2 New Diagnostics

=head3 New Warnings

=over 4

=item *

L<Experimental "%s" subs not enabled|perldiag/"Experimental "%s" subs not enabled">

(F) To use lexical subs, you must first enable them:

    no warnings 'experimental:lexical_subs';
    use feature 'lexical_subs';
    my sub foo { ... }

=item *

L<Subroutine "&%s" is not available|perldiag/"Subroutine "&%s" is not available">

(W closure) During compilation, an inner named subroutine or eval is
attempting to capture an outer lexical subroutine that is not currently
available.  This can happen for one of two reasons.  First, the lexical
subroutine may be declared in an outer anonymous subroutine that has not
yet been created.  (Remember that named subs are created at compile time,
while anonymous subs are created at run-time.)  For example,

    sub { my sub a {...} sub f { \&a } }

At the time that f is created, it can't capture the current the "a" sub,
since the anonymous subroutine hasn't been created yet.  Conversely, the
following won't give a warning since the anonymous subroutine has by now
been created and is live:

    sub { my sub a {...} eval 'sub f { \&a }' }->();

The second situation is caused by an eval accessing a variable that has
gone out of scope, for example,

    sub f {
	my sub a {...}
	sub { eval '\&a' }
    }
    f()->();

Here, when the '\&a' in the eval is being compiled, f() is not currently
being executed, so its &a is not available for capture.

=item *

L<"%s" subroutine &%s masks earlier declaration in same %s|perldiag/"%s" subroutine &%s masks earlier declaration in same %s>

(W misc) A "my" or "state" subroutine has been redeclared in the
current scope or statement, effectively eliminating all access to
the previous instance.  This is almost always a typographical error.
Note that the earlier subroutine will still exist until the end of
the scope or until all closure references to it are destroyed.

=item *

L<The %s feature is experimental|perldiag/"The %s feature is experimental">

(S experimental) This warning is emitted if you enable an experimental
feature via C<use feature>.  Simply suppress the warning if you want
to use the feature, but know that in doing so you are taking the risk
of using an experimental feature which may change or be removed in a
future Perl version:

    no warnings "experimental:lexical_subs";
    use feature "lexical_subs";

=item *

L<sleep(%u) too large|perldiag/"sleep(%u) too large">

(W overflow) You called C<sleep> with a number that was larger than it can
reliably handle and C<sleep> probably slept for less time than requested.

=back

=head2 Changes to Existing Diagnostics

=over 4

=item *

L<vector argument not supported with alpha versions|perldiag/vector argument not supported with alpha versions>

This warning was not suppressable, even with C<no warnings>.  Now it is
suppressible, and has been moved from the "internal" category to the
"printf" category.

=item *

C<< Can't do {n,m} with n > m in regex; marked by <-- HERE in m/%s/ >>

This fatal error has been turned into a warning that reads:

L<< Quantifier {n,m} with n > m can't match in regex | perldiag/Quantifier {n,m} with n > m can't match in regex >>

(W regexp) Minima should be less than or equal to maxima.  If you really want
your regexp to match something 0 times, just put {0}.

=back

=head1 Configuration and Compilation

=over 4

=item *

F<Configure> will now correctly detect C<isblank()> when compiling with a C++
compiler.

=back

=head1 Platform Support

=head2 Discontinued Platforms

=over 4

=item VM/ESA

Support for VM/ESA has been removed. The port was tested on 2.3.0, which
IBM ended service on in March 2002. 2.4.0 ended service in June 2003, and
was superseded by Z/VM. The current version of Z/VM is V6.2.0, and scheduled
for end of service on 2015/04/30.

=back

=head2 Platform-Specific Notes

=over 4

=item Win32

Fixed a problem where perl could crash while cleaning up threads (including the
main thread) in threaded debugging builds on Win32 and possibly other platforms
[perl #114496].

A rare race condition that would lead to L<sleep|perlfunc/sleep> taking more
time than requested, and possibly even hanging, has been fixed [perl #33096].

=item Solaris

In Configure, avoid running sed commands with flags not supported on Solaris.

=item Darwin

Stop hardcoding an alignment on 8 byte boundaries to fix builds using
-Dusemorebits.

=item VMS

Fix linking on builds configured with -Dusemymalloc=y.

=back

=head1 Internal Changes

=over 4

=item *

The APIs for accessing lexical pads have changed considerably.

C<PADLIST>s are now longer C<AV>s, but their own type instead. C<PADLIST>s now
contain a C<PAD> and a C<PADNAMELIST> of C<PADNAME>s, rather than C<AV>s for the
pad and the list of pad names.  C<PAD>s, C<PADNAMELIST>s, and C<PADNAME>s are to
be accessed as such through the newly added pad API instead of the plain C<AV>
and C<SV> APIs.  See L<perlapi> for details.

=item *

In the regex API, the numbered capture callbacks are passed an index
indicating what match variable is being accessed. There are special
index values for the C<$`, $&, $&> variables. Previously the same three
values were used to retrieve C<${^PREMATCH}, ${^MATCH}, ${^POSTMATCH}>
too, but these have now been assigned three separate values. See
L<perlreapi/Numbered capture callbacks>.

=item *

C<PL_sawampersand> was previously a boolean indicating that any of
C<$`, $&, $&> had been seen; it now contains three one-bit flags
indicating the presence of each of the variables individually.

=back

=head1 Selected Bug Fixes

=over 4

=item *

The error "Can't localize through a reference" had disappeared in 5.16.0
when C<local %$ref> appeared on the last line of an lvalue subroutine.
This error disappeared for C<\local %$ref> in perl 5.8.1.  It has now
been restored.

=item *

The parsing of here-docs has been improved significantly, fixing several
parsing bugs and crashes and one memory leak, and correcting wrong
subsequent line numbers under certain conditions.

=item *

Inside an eval, the error message for an unterminated here-doc no longer
has a newline in the middle of it [perl #70836].

=item *

A substitution inside a substitution pattern (C<s/${s|||}//>) no longer
confuses the parser.

=item *

It may be an odd place to allow comments, but C<s//"" # hello/e> has
always worked, I<unless> there happens to be a null character before the
first #.  Now it works even in the presence of nulls.

=item *

An invalid range in C<tr///> or C<y///> no longer results in a memory leak.

=item *

String eval no longer treats a semicolon-delimited quote-like operator at
the very end (C<eval 'q;;'>) as a syntax error.

=item *

C<< warn {$_ => 1} + 1 >> is no longer a syntax error.  The parser used to
get confused with certain list operators followed by an anonymous hash and
then an infix operator that shares its form with a unary operator.

=item *

C<(caller $n)[6]> (which gives the text of the eval) used to return the
actual parser buffer.  Modifying it could result in crashes.  Now it always
returns a copy.  The string returned no longer has "\n;" tacked on to the
end.  The returned text also includes here-doc bodies, which used to be
omitted.

=item *

Reset the utf8 position cache when accessing magical variables to avoid the
string buffer and the utf8 position cache getting out of sync
[perl #114410].

=item *

Various cases of get magic being called twice for magical utf8 strings have been
fixed.

=item *

This code (when not in the presence of C<$&> etc)

    $_ = 'x' x 1_000_000;
    1 while /(.)/;

used to skip the buffer copy for performance reasons, but suffered from C<$1>
etc changing if the original string changed.  That's now been fixed.

=item *

Perl doesn't use PerlIO anymore to report out of memory messages, as PerlIO
might attempt to allocate more memory.

=item *

In a regular expression, if something is quantified with C<{n,m}>
where C<S<n E<gt> m>>, it can't possibly match.  Previously this was a fatal error,
but now is merely a warning (and that something won't match).  [perl #82954].

=item *

It used to be possible for formats defined in subroutines that have
subsequently been undefined and redefined to close over variables in the
wrong pad (the newly-defined enclosing sub), resulting in crashes or
"Bizarre copy" errors.

=item *

Redefinition of XSUBs at run time could produce warnings with the wrong
line number.

=item *

The %vd sprintf format does not support version objects for alpha versions.
It used to output the format itself (%vd) when passed an alpha version, and
also emit an "Invalid conversion in printf" warning.  It no longer does,
but produces the empty string in the output.  It also no longer leaks
memory in this case.

=item *

A bug fix in an earlier 5.17.x release caused C<no a a 3> (a syntax error)
to result in a bad read or assertion failure, because an op was being freed
twice.

=item *

C<< $obj->SUPER::method >> calls in the main package could fail if the
SUPER package had already been accessed by other means.

=item *

Stash aliasing (C<*foo:: = *bar::>) no longer causes SUPER calls to ignore
changes to methods or @ISA or use the wrong package.

=item *

Method calls on packages whose names end in ::SUPER are no longer treated
as SUPER method calls, resulting in failure to find the method.
Furthermore, defining subroutines in such packages no longer causes them to
be found by SUPER method calls on the containing package [perl #114924].

=back

=head1 Known Problems

=over 4

=item *

Changes in the lexical pad API break some CPAN modules.

To avoid having to patch those modules again later if we change pads from AVs
into their own types, APIs for accessing the contents of pads have been added.

=back

=head1 Acknowledgements

Perl 5.17.4 represents approximately 4 weeks of development since Perl 5.17.3
and contains approximately 82,000 lines of changes across 360 files from 37
authors.

Perl continues to flourish into its third decade thanks to a vibrant community
of users and developers. The following people are known to have contributed the
improvements that became Perl 5.17.4:

Abhijit Menon-Sen, Andy Dougherty, Aristotle Pagaltzis, Chris 'BinGOs'
Williams, Colin Kuskie, Craig A. Berry, Daniel Dragan, David Golden, David
Leadbeater, David Mitchell, David Nicol, Dominic Hargreaves, Father
Chrysostomos, Florian Ragwitz, H.Merijn Brand, James E Keenan, Jerry D. Hedden,
Jesse Luehrs, John Peacock, Karen Etheridge, Karl Williamson, Leon Timmermans,
Michael G Schwern, Nicholas Clark, Peter Martini, Rafael Garcia-Suarez, Ricardo
Signes, Shawn M Moore, Shlomi Fish, Steffen Müller, Steve Hay, Sullivan Beck,
Sébastien Aperghis-Tramoni, Tony Cook, Vincent Pit, Yves Orton.

The list above is almost certainly incomplete as it is automatically generated
from version control history. In particular, it does not include the names of
the (very much appreciated) contributors who reported issues to the Perl bug
tracker.

Many of the changes included in this version originated in the CPAN modules
included in Perl's core. We're grateful to the entire CPAN community for
helping Perl to flourish.

For a more complete list of all of Perl's historical contributors, please see
the F<AUTHORS> file in the Perl source distribution.

=head1 Reporting Bugs

If you find what you think is a bug, you might check the articles recently
posted to the comp.lang.perl.misc newsgroup and the perl bug database at
http://rt.perl.org/perlbug/ .  There may also be information at
http://www.perl.org/ , the Perl Home Page.

If you believe you have an unreported bug, please run the L<perlbug> program
included with your release.  Be sure to trim your bug down to a tiny but
sufficient test case.  Your bug report, along with the output of C<perl -V>,
will be sent off to perlbug@perl.org to be analysed by the Perl porting team.

If the bug you are reporting has security implications, which make it
inappropriate to send to a publicly archived mailing list, then please send it
to perl5-security-report@perl.org.  This points to a closed subscription
unarchived mailing list, which includes all the core committers, who will be
able to help assess the impact of issues, figure out a resolution, and help
co-ordinate the release of patches to mitigate or fix the problem across all
platforms on which Perl is supported.  Please only use this address for
security issues in the Perl core, not for modules independently distributed on
CPAN.

=head1 SEE ALSO

The F<Changes> file for an explanation of how to view exhaustive details on
what changed.

The F<INSTALL> file for how to build Perl.

The F<README> file for general stuff.

The F<Artistic> and F<Copying> files for copyright information.

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