perl5004delta.pod

MSYS在windows下模拟了一个类unix的终端

=head1 NAME

perldelta - what's new for perl5.004

=head1 DESCRIPTION

This document describes differences between the 5.003 release (as
documented in I<Programming Perl>, second edition--the Camel Book) and
this one.

=head1 Supported Environments

Perl5.004 builds out of the box on Unix, Plan 9, LynxOS, VMS, OS/2,
QNX, AmigaOS, and Windows NT.  Perl runs on Windows 95 as well, but it
cannot be built there, for lack of a reasonable command interpreter.

=head1 Core Changes

Most importantly, many bugs were fixed, including several security
problems.  See the F<Changes> file in the distribution for details.

=head2 List assignment to %ENV works

C<%ENV = ()> and C<%ENV = @list> now work as expected (except on VMS
where it generates a fatal error).

=head2 Change to "Can't locate Foo.pm in @INC" error

The error "Can't locate Foo.pm in @INC" now lists the contents of @INC
for easier debugging.

=head2 Compilation option: Binary compatibility with 5.003

There is a new Configure question that asks if you want to maintain
binary compatibility with Perl 5.003.  If you choose binary
compatibility, you do not have to recompile your extensions, but you
might have symbol conflicts if you embed Perl in another application,
just as in the 5.003 release.  By default, binary compatibility
is preserved at the expense of symbol table pollution.

=head2 $PERL5OPT environment variable

You may now put Perl options in the $PERL5OPT environment variable.
Unless Perl is running with taint checks, it will interpret this
variable as if its contents had appeared on a "#!perl" line at the
beginning of your script, except that hyphens are optional.  PERL5OPT
may only be used to set the following switches: B<-[DIMUdmw]>.

=head2 Limitations on B<-M>, B<-m>, and B<-T> options

The C<-M> and C<-m> options are no longer allowed on the C<#!> line of
a script.  If a script needs a module, it should invoke it with the
C<use> pragma.

The B<-T> option is also forbidden on the C<#!> line of a script,
unless it was present on the Perl command line.  Due to the way C<#!>
works, this usually means that B<-T> must be in the first argument.
Thus:

    #!/usr/bin/perl -T -w

will probably work for an executable script invoked as C<scriptname>,
while:

    #!/usr/bin/perl -w -T

will probably fail under the same conditions.  (Non-Unix systems will
probably not follow this rule.)  But C<perl scriptname> is guaranteed
to fail, since then there is no chance of B<-T> being found on the
command line before it is found on the C<#!> line.

=head2 More precise warnings

If you removed the B<-w> option from your Perl 5.003 scripts because it
made Perl too verbose, we recommend that you try putting it back when
you upgrade to Perl 5.004.  Each new perl version tends to remove some
undesirable warnings, while adding new warnings that may catch bugs in
your scripts.

=head2 Deprecated: Inherited C<AUTOLOAD> for non-methods

Before Perl 5.004, C<AUTOLOAD> functions were looked up as methods
(using the C<@ISA> hierarchy), even when the function to be autoloaded
was called as a plain function (e.g. C<Foo::bar()>), not a method
(e.g. C<< Foo->bar() >> or C<< $obj->bar() >>).

Perl 5.005 will use method lookup only for methods' C<AUTOLOAD>s.
However, there is a significant base of existing code that may be using
the old behavior.  So, as an interim step, Perl 5.004 issues an optional
warning when a non-method uses an inherited C<AUTOLOAD>.

The simple rule is:  Inheritance will not work when autoloading
non-methods.  The simple fix for old code is:  In any module that used to
depend on inheriting C<AUTOLOAD> for non-methods from a base class named
C<BaseClass>, execute C<*AUTOLOAD = \&BaseClass::AUTOLOAD> during startup.

=head2 Previously deprecated %OVERLOAD is no longer usable

Using %OVERLOAD to define overloading was deprecated in 5.003.
Overloading is now defined using the overload pragma. %OVERLOAD is
still used internally but should not be used by Perl scripts. See
L<overload> for more details.

=head2 Subroutine arguments created only when they're modified

In Perl 5.004, nonexistent array and hash elements used as subroutine
parameters are brought into existence only if they are actually
assigned to (via C<@_>).

Earlier versions of Perl vary in their handling of such arguments.
Perl versions 5.002 and 5.003 always brought them into existence.
Perl versions 5.000 and 5.001 brought them into existence only if
they were not the first argument (which was almost certainly a bug).
Earlier versions of Perl never brought them into existence.

For example, given this code:

     undef @a; undef %a;
     sub show { print $_[0] };
     sub change { $_[0]++ };
     show($a[2]);
     change($a{b});

After this code executes in Perl 5.004, $a{b} exists but $a[2] does
not.  In Perl 5.002 and 5.003, both $a{b} and $a[2] would have existed
(but $a[2]'s value would have been undefined).

=head2 Group vector changeable with C<$)>

The C<$)> special variable has always (well, in Perl 5, at least)
reflected not only the current effective group, but also the group list
as returned by the C<getgroups()> C function (if there is one).
However, until this release, there has not been a way to call the
C<setgroups()> C function from Perl.

In Perl 5.004, assigning to C<$)> is exactly symmetrical with examining
it: The first number in its string value is used as the effective gid;
if there are any numbers after the first one, they are passed to the
C<setgroups()> C function (if there is one).

=head2 Fixed parsing of $$<digit>, &$<digit>, etc.

Perl versions before 5.004 misinterpreted any type marker followed by
"$" and a digit.  For example, "$$0" was incorrectly taken to mean
"${$}0" instead of "${$0}".  This bug is (mostly) fixed in Perl 5.004.

However, the developers of Perl 5.004 could not fix this bug completely,
because at least two widely-used modules depend on the old meaning of
"$$0" in a string.  So Perl 5.004 still interprets "$$<digit>" in the
old (broken) way inside strings; but it generates this message as a
warning.  And in Perl 5.005, this special treatment will cease.

=head2 Fixed localization of $<digit>, $&, etc.

Perl versions before 5.004 did not always properly localize the
regex-related special variables.  Perl 5.004 does localize them, as
the documentation has always said it should.  This may result in $1,
$2, etc. no longer being set where existing programs use them.

=head2 No resetting of $. on implicit close

The documentation for Perl 5.0 has always stated that C<$.> is I<not>
reset when an already-open file handle is reopened with no intervening
call to C<close>.  Due to a bug, perl versions 5.000 through 5.003
I<did> reset C<$.> under that circumstance; Perl 5.004 does not.

=head2 C<wantarray> may return undef

The C<wantarray> operator returns true if a subroutine is expected to
return a list, and false otherwise.  In Perl 5.004, C<wantarray> can
also return the undefined value if a subroutine's return value will
not be used at all, which allows subroutines to avoid a time-consuming
calculation of a return value if it isn't going to be used.

=head2 C<eval EXPR> determines value of EXPR in scalar context

Perl (version 5) used to determine the value of EXPR inconsistently,
sometimes incorrectly using the surrounding context for the determination.
Now, the value of EXPR (before being parsed by eval) is always determined in
a scalar context.  Once parsed, it is executed as before, by providing
the context that the scope surrounding the eval provided.  This change
makes the behavior Perl4 compatible, besides fixing bugs resulting from
the inconsistent behavior.  This program:

    @a = qw(time now is time);
    print eval @a;
    print '|', scalar eval @a;

used to print something like "timenowis881399109|4", but now (and in perl4)
prints "4|4".

=head2 Changes to tainting checks

A bug in previous versions may have failed to detect some insecure
conditions when taint checks are turned on.  (Taint checks are used
in setuid or setgid scripts, or when explicitly turned on with the
C<-T> invocation option.)  Although it's unlikely, this may cause a
previously-working script to now fail -- which should be construed
as a blessing, since that indicates a potentially-serious security
hole was just plugged.

The new restrictions when tainting include:

=over 4

=item No glob() or <*>

These operators may spawn the C shell (csh), which cannot be made
safe.  This restriction will be lifted in a future version of Perl
when globbing is implemented without the use of an external program.

=item No spawning if tainted $CDPATH, $ENV, $BASH_ENV

These environment variables may alter the behavior of spawned programs
(especially shells) in ways that subvert security.  So now they are
treated as dangerous, in the manner of $IFS and $PATH.

=item No spawning if tainted $TERM doesn't look like a terminal name

Some termcap libraries do unsafe things with $TERM.  However, it would be
unnecessarily harsh to treat all $TERM values as unsafe, since only shell
metacharacters can cause trouble in $TERM.  So a tainted $TERM is
considered to be safe if it contains only alphanumerics, underscores,
dashes, and colons, and unsafe if it contains other characters (including
whitespace).

=back

=head2 New Opcode module and revised Safe module

A new Opcode module supports the creation, manipulation and
application of opcode masks.  The revised Safe module has a new API
and is implemented using the new Opcode module.  Please read the new
Opcode and Safe documentation.

=head2 Embedding improvements

In older versions of Perl it was not possible to create more than one
Perl interpreter instance inside a single process without leaking like a
sieve and/or crashing.  The bugs that caused this behavior have all been
fixed.  However, you still must take care when embedding Perl in a C
program.  See the updated perlembed manpage for tips on how to manage
your interpreters.

=head2 Internal change: FileHandle class based on IO::* classes

File handles are now stored internally as type IO::Handle.  The
FileHandle module is still supported for backwards compatibility, but
it is now merely a front end to the IO::* modules -- specifically,
IO::Handle, IO::Seekable, and IO::File.  We suggest, but do not
require, that you use the IO::* modules in new code.

In harmony with this change, C<*GLOB{FILEHANDLE}> is now just a
backward-compatible synonym for C<*GLOB{IO}>.

=head2 Internal change: PerlIO abstraction interface

It is now possible to build Perl with AT&T's sfio IO package
instead of stdio.  See L<perlapio> for more details, and
the F<INSTALL> file for how to use it.

=head2 New and changed syntax

=over 4

=item $coderef->(PARAMS)

A subroutine reference may now be suffixed with an arrow and a
(possibly empty) parameter list.  This syntax denotes a call of the
referenced subroutine, with the given parameters (if any).

This new syntax follows the pattern of S<C<< $hashref->{FOO} >>> and
S<C<< $aryref->[$foo] >>>: You may now write S<C<&$subref($foo)>> as
S<C<< $subref->($foo) >>>.  All these arrow terms may be chained;
thus, S<C<< &{$table->{FOO}}($bar) >>> may now be written
S<C<< $table->{FOO}->($bar) >>>.

=back

=head2 New and changed builtin constants

=over 4

=item __PACKAGE__

The current package name at compile time, or the undefined value if
there is no current package (due to a C<package;> directive).  Like
C<__FILE__> and C<__LINE__>, C<__PACKAGE__> does I<not> interpolate
into strings.

=back

=head2 New and changed builtin variables

=over 4

=item $^E

Extended error message on some platforms.  (Also known as
$EXTENDED_OS_ERROR if you C<use English>).

=item $^H

The current set of syntax checks enabled by C<use strict>.  See the
documentation of C<strict> for more details.  Not actually new, but
newly documented.
Because it is intended for internal use by Perl core components,
there is no C<use English> long name for this variable.

=item $^M

By default, running out of memory it is not trappable.  However, if
compiled for this, Perl may use the contents of C<$^M> as an emergency
pool after die()ing with this message.  Suppose that your Perl were
compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.  Then

    $^M = 'a' x (1<<16);

would allocate a 64K buffer for use when in emergency.
See the F<INSTALL> file for information on how to enable this option.
As a disincentive to casual use of this advanced feature,
there is no C<use English> long name for this variable.

=back

=head2 New and changed builtin functions

=over 4

=item delete on slices

This now works.  (e.g. C<delete @ENV{'PATH', 'MANPATH'}>)

=item flock

is now supported on more platforms, prefers fcntl to lockf when
emulating, and always flushes before (un)locking.

=item printf and sprintf

Perl now implements these functions itself; it doesn't use the C
library function sprintf() any more, except for floating-point
numbers, and even then only known flags are allowed.  As a result, it
is now possible to know which conversions and flags will work, and
what they will do.

The new conversions in Perl's sprintf() are:

   %i	a synonym for %d
   %p	a pointer (the address of the Perl value, in hexadecimal)
   %n	special: *stores* the number of characters output so far
        into the next variable in the parameter list 

The new flags that go between the C<%> and the conversion are:

   #	prefix octal with "0", hex with "0x"
   h	interpret integer as C type "short" or "unsigned short"
   V	interpret integer as Perl's standard integer type

Also, where a number would appear in the flags, an asterisk ("*") may
be used instead, in which case Perl uses the next item in the
parameter list as the given number (that is, as the field width or
precision).  If a field width obtained through "*" is negative, it has
the same effect as the '-' flag: left-justification.

See L<perlfunc/sprintf> for a complete list of conversion and flags.

=item keys as an lvalue

As an lvalue, C<keys> allows you to increase the number of hash buckets
allocated for the given hash.  This can gain you a measure of efficiency if
you know the hash is going to get big.  (This is similar to pre-extending
an array by assigning a larger number to $#array.)  If you say

    keys %hash = 200;

then C<%hash> will have at least 200 buckets allocated for it.  These
buckets will be retained even if you do C<%hash = ()>; use C<undef
%hash> if you want to free the storage while C<%hash> is still in scope.
You can't shrink the number of buckets allocated for the hash using
C<keys> in this way (but you needn't worry about doing this by accident,
as trying has no effect).

=item my() in Control Structures

You can now use my() (with or without the parentheses) in the control
expressions of control structures such as:

    while (defined(my $line = <>)) {
        $line = lc $line;
    } continue {
        print $line;
    }

    if ((my $answer = <STDIN>) =~ /^y(es)?$/i) {
        user_agrees();
    } elsif ($answer =~ /^n(o)?$/i) {
        user_disagrees();
    } else {
        chomp $answer;
        die "`$answer' is neither `yes' nor `no'";
    }

Also, you can declare a foreach loop control variable as lexical by