perldata.pod

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

    ($dev, $ino, undef, undef, $uid, $gid) = stat($file);

List assignment in scalar context returns the number of elements
produced by the expression on the right side of the assignment:

    $x = (($foo,$bar) = (3,2,1));       # set $x to 3, not 2
    $x = (($foo,$bar) = f());           # set $x to f()'s return count

This is handy when you want to do a list assignment in a Boolean
context, because most list functions return a null list when finished,
which when assigned produces a 0, which is interpreted as FALSE.

It's also the source of a useful idiom for executing a function or
performing an operation in list context and then counting the number of
return values, by assigning to an empty list and then using that
assignment in scalar context. For example, this code:

    $count = () = $string =~ /\d+/g;

will place into $count the number of digit groups found in $string.
This happens because the pattern match is in list context (since it
is being assigned to the empty list), and will therefore return a list
of all matching parts of the string. The list assignment in scalar
context will translate that into the number of elements (here, the
number of times the pattern matched) and assign that to $count. Note
that simply using

    $count = $string =~ /\d+/g;

would not have worked, since a pattern match in scalar context will
only return true or false, rather than a count of matches.

The final element of a list assignment may be an array or a hash:

    ($a, $b, @rest) = split;
    my($a, $b, %rest) = @_;

You can actually put an array or hash anywhere in the list, but the first one
in the list will soak up all the values, and anything after it will become
undefined.  This may be useful in a my() or local().

A hash can be initialized using a literal list holding pairs of
items to be interpreted as a key and a value:

    # same as map assignment above
    %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);

While literal lists and named arrays are often interchangeable, that's
not the case for hashes.  Just because you can subscript a list value like
a normal array does not mean that you can subscript a list value as a
hash.  Likewise, hashes included as parts of other lists (including
parameters lists and return lists from functions) always flatten out into
key/value pairs.  That's why it's good to use references sometimes.

It is often more readable to use the C<< => >> operator between key/value
pairs.  The C<< => >> operator is mostly just a more visually distinctive
synonym for a comma, but it also arranges for its left-hand operand to be
interpreted as a string -- if it's a bareword that would be a legal simple
identifier (C<< => >> doesn't quote compound identifiers, that contain
double colons). This makes it nice for initializing hashes:

    %map = (
                 red   => 0x00f,
                 blue  => 0x0f0,
                 green => 0xf00,
   );

or for initializing hash references to be used as records:

    $rec = {
                witch => 'Mable the Merciless',
                cat   => 'Fluffy the Ferocious',
                date  => '10/31/1776',
    };

or for using call-by-named-parameter to complicated functions:

   $field = $query->radio_group(
               name      => 'group_name',
               values    => ['eenie','meenie','minie'],
               default   => 'meenie',
               linebreak => 'true',
               labels    => \%labels
   );

Note that just because a hash is initialized in that order doesn't
mean that it comes out in that order.  See L<perlfunc/sort> for examples
of how to arrange for an output ordering.

=head2 Subscripts

An array is subscripted by specifying a dollar sign (C<$>), then the
name of the array (without the leading C<@>), then the subscript inside
square brackets.  For example:

    @myarray = (5, 50, 500, 5000);
    print "Element Number 2 is", $myarray[2], "\n";

The array indices start with 0. A negative subscript retrieves its 
value from the end.  In our example, C<$myarray[-1]> would have been 
5000, and C<$myarray[-2]> would have been 500.

Hash subscripts are similar, only instead of square brackets curly brackets
are used. For example:

    %scientists = 
    (
        "Newton" => "Isaac",
        "Einstein" => "Albert",
        "Darwin" => "Charles",
        "Feynman" => "Richard",
    );

    print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";

=head2 Slices
X<slice> X<array, slice> X<hash, slice>

A common way to access an array or a hash is one scalar element at a
time.  You can also subscript a list to get a single element from it.

    $whoami = $ENV{"USER"};             # one element from the hash
    $parent = $ISA[0];                  # one element from the array
    $dir    = (getpwnam("daemon"))[7];  # likewise, but with list

A slice accesses several elements of a list, an array, or a hash
simultaneously using a list of subscripts.  It's more convenient
than writing out the individual elements as a list of separate
scalar values.

    ($him, $her)   = @folks[0,-1];              # array slice
    @them          = @folks[0 .. 3];            # array slice
    ($who, $home)  = @ENV{"USER", "HOME"};      # hash slice
    ($uid, $dir)   = (getpwnam("daemon"))[2,7]; # list slice

Since you can assign to a list of variables, you can also assign to
an array or hash slice.

    @days[3..5]    = qw/Wed Thu Fri/;
    @colors{'red','blue','green'} 
                   = (0xff0000, 0x0000ff, 0x00ff00);
    @folks[0, -1]  = @folks[-1, 0];

The previous assignments are exactly equivalent to

    ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
    ($colors{'red'}, $colors{'blue'}, $colors{'green'})
                   = (0xff0000, 0x0000ff, 0x00ff00);
    ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);

Since changing a slice changes the original array or hash that it's
slicing, a C<foreach> construct will alter some--or even all--of the
values of the array or hash.

    foreach (@array[ 4 .. 10 ]) { s/peter/paul/ } 

    foreach (@hash{qw[key1 key2]}) {
        s/^\s+//;           # trim leading whitespace
        s/\s+$//;           # trim trailing whitespace
        s/(\w+)/\u\L$1/g;   # "titlecase" words
    }

A slice of an empty list is still an empty list.  Thus:

    @a = ()[1,0];           # @a has no elements
    @b = (@a)[0,1];         # @b has no elements
    @c = (0,1)[2,3];        # @c has no elements

But:

    @a = (1)[1,0];          # @a has two elements
    @b = (1,undef)[1,0,2];  # @b has three elements

This makes it easy to write loops that terminate when a null list
is returned:

    while ( ($home, $user) = (getpwent)[7,0]) {
        printf "%-8s %s\n", $user, $home;
    }

As noted earlier in this document, the scalar sense of list assignment
is the number of elements on the right-hand side of the assignment.
The null list contains no elements, so when the password file is
exhausted, the result is 0, not 2.

If you're confused about why you use an '@' there on a hash slice
instead of a '%', think of it like this.  The type of bracket (square
or curly) governs whether it's an array or a hash being looked at.
On the other hand, the leading symbol ('$' or '@') on the array or
hash indicates whether you are getting back a singular value (a
scalar) or a plural one (a list).

=head2 Typeglobs and Filehandles
X<typeglob> X<filehandle> X<*>

Perl uses an internal type called a I<typeglob> to hold an entire
symbol table entry.  The type prefix of a typeglob is a C<*>, because
it represents all types.  This used to be the preferred way to
pass arrays and hashes by reference into a function, but now that
we have real references, this is seldom needed.  

The main use of typeglobs in modern Perl is create symbol table aliases.
This assignment:

    *this = *that;

makes $this an alias for $that, @this an alias for @that, %this an alias
for %that, &this an alias for &that, etc.  Much safer is to use a reference.
This:

    local *Here::blue = \$There::green;

temporarily makes $Here::blue an alias for $There::green, but doesn't
make @Here::blue an alias for @There::green, or %Here::blue an alias for
%There::green, etc.  See L<perlmod/"Symbol Tables"> for more examples
of this.  Strange though this may seem, this is the basis for the whole
module import/export system.

Another use for typeglobs is to pass filehandles into a function or
to create new filehandles.  If you need to use a typeglob to save away
a filehandle, do it this way:

    $fh = *STDOUT;

or perhaps as a real reference, like this:

    $fh = \*STDOUT;

See L<perlsub> for examples of using these as indirect filehandles
in functions.

Typeglobs are also a way to create a local filehandle using the local()
operator.  These last until their block is exited, but may be passed back.
For example:

    sub newopen {
        my $path = shift;
        local  *FH;  # not my!
        open   (FH, $path)          or  return undef;
        return *FH;
    }
    $fh = newopen('/etc/passwd');

Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
for filehandle manipulations, although they're still needed to pass brand
new file and directory handles into or out of functions. That's because
C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
In other words, C<*FH> must be used to create new symbol table entries;
C<*foo{THING}> cannot.  When in doubt, use C<*FH>.

All functions that are capable of creating filehandles (open(),
opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
automatically create an anonymous filehandle 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
    }

Note that if an initialized scalar variable is used instead the
result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
to C<open( *{'zzz'}, ...)>.
C<use strict 'refs'> forbids such practice.

Another way to create anonymous filehandles is with the Symbol
module or with the IO::Handle module and its ilk.  These modules
have the advantage of not hiding different types of the same name
during the local().  See the bottom of L<perlfunc/open()> for an
example.

=head1 SEE ALSO

See L<perlvar> for a description of Perl's built-in variables and
a discussion of legal variable names.  See L<perlref>, L<perlsub>,
and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
the C<*foo{THING}> syntax.