perlsub.1

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.

.PP
Scalars are already passed by reference, so you can modify
scalar arguments without using this mechanism by referring explicitly
to \f(CW$_[0]\fR etc.  You can modify all the elements of an array by passing
all the elements as scalars, but you have to use the \f(CW\*(C`*\*(C'\fR mechanism (or
the equivalent reference mechanism) to \f(CW\*(C`push\*(C'\fR, \f(CW\*(C`pop\*(C'\fR, or change the size of
an array.  It will certainly be faster to pass the typeglob (or reference).
.PP
Even if you don't want to modify an array, this mechanism is useful for
passing multiple arrays in a single \s-1LIST\s0, because normally the \s-1LIST\s0
mechanism will merge all the array values so that you can't extract out
the individual arrays.  For more on typeglobs, see
\&\*(L"Typeglobs and Filehandles\*(R" in perldata.
.Sh "When to Still Use \fIlocal()\fP"
.IX Xref "local variable, local"
.IX Subsection "When to Still Use local()"
Despite the existence of \f(CW\*(C`my\*(C'\fR, there are still three places where the
\&\f(CW\*(C`local\*(C'\fR operator still shines.  In fact, in these three places, you
\&\fImust\fR use \f(CW\*(C`local\*(C'\fR instead of \f(CW\*(C`my\*(C'\fR.
.IP "1." 4
You need to give a global variable a temporary value, especially \f(CW$_\fR.
.Sp
The global variables, like \f(CW@ARGV\fR or the punctuation variables, must be 
\&\f(CW\*(C`local\*(C'\fRized with \f(CW\*(C`local()\*(C'\fR.  This block reads in \fI/etc/motd\fR, and splits
it up into chunks separated by lines of equal signs, which are placed
in \f(CW@Fields\fR.
.Sp
.Vb 6
\&    {
\&        local @ARGV = ("/etc/motd");
\&        local $/ = undef;
\&        local $_ = <>;  
\&        @Fields = split /^\es*=+\es*$/;
\&    }
.Ve
.Sp
It particular, it's important to \f(CW\*(C`local\*(C'\fRize \f(CW$_\fR in any routine that assigns
to it.  Look out for implicit assignments in \f(CW\*(C`while\*(C'\fR conditionals.
.IP "2." 4
You need to create a local file or directory handle or a local function.
.Sp
A function that needs a filehandle of its own must use
\&\f(CW\*(C`local()\*(C'\fR on a complete typeglob.   This can be used to create new symbol
table entries:
.Sp
.Vb 6
\&    sub ioqueue {
\&        local  (*READER, *WRITER);    # not my!
\&        pipe    (READER,  WRITER)     or die "pipe: $!";
\&        return (*READER, *WRITER);
\&    }
\&    ($head, $tail) = ioqueue();
.Ve
.Sp
See the Symbol module for a way to create anonymous symbol table
entries.
.Sp
Because assignment of a reference to a typeglob creates an alias, this
can be used to create what is effectively a local function, or at least,
a local alias.
.Sp
.Vb 6
\&    {
\&        local *grow = \e&shrink; # only until this block exists
\&        grow();                 # really calls shrink()
\&        move();                 # if move() grow()s, it shrink()s too
\&    }
\&    grow();                     # get the real grow() again
.Ve
.Sp
See \*(L"Function Templates\*(R" in perlref for more about manipulating
functions by name in this way.
.IP "3." 4
You want to temporarily change just one element of an array or hash.
.Sp
You can \f(CW\*(C`local\*(C'\fRize just one element of an aggregate.  Usually this
is done on dynamics:
.Sp
.Vb 5
\&    {
\&        local $SIG{INT} = \*(AqIGNORE\*(Aq;
\&        funct();                            # uninterruptible
\&    } 
\&    # interruptibility automatically restored here
.Ve
.Sp
But it also works on lexically declared aggregates.  Prior to 5.005,
this operation could on occasion misbehave.
.Sh "Pass by Reference"
.IX Xref "pass by reference pass-by-reference reference"
.IX Subsection "Pass by Reference"
If you want to pass more than one array or hash into a function\*(--or
return them from it\*(--and have them maintain their integrity, then
you're going to have to use an explicit pass-by-reference.  Before you
do that, you need to understand references as detailed in perlref.
This section may not make much sense to you otherwise.
.PP
Here are a few simple examples.  First, let's pass in several arrays
to a function and have it \f(CW\*(C`pop\*(C'\fR all of then, returning a new list
of all their former last elements:
.PP
.Vb 1
\&    @tailings = popmany ( \e@a, \e@b, \e@c, \e@d );
\&
\&    sub popmany {
\&        my $aref;
\&        my @retlist = ();
\&        foreach $aref ( @_ ) {
\&            push @retlist, pop @$aref;
\&        }
\&        return @retlist;
\&    }
.Ve
.PP
Here's how you might write a function that returns a
list of keys occurring in all the hashes passed to it:
.PP
.Vb 10
\&    @common = inter( \e%foo, \e%bar, \e%joe );
\&    sub inter {
\&        my ($k, $href, %seen); # locals
\&        foreach $href (@_) {
\&            while ( $k = each %$href ) {
\&                $seen{$k}++;
\&            }
\&        }
\&        return grep { $seen{$_} == @_ } keys %seen;
\&    }
.Ve
.PP
So far, we're using just the normal list return mechanism.
What happens if you want to pass or return a hash?  Well,
if you're using only one of them, or you don't mind them
concatenating, then the normal calling convention is ok, although
a little expensive.
.PP
Where people get into trouble is here:
.PP
.Vb 3
\&    (@a, @b) = func(@c, @d);
\&or
\&    (%a, %b) = func(%c, %d);
.Ve
.PP
That syntax simply won't work.  It sets just \f(CW@a\fR or \f(CW%a\fR and
clears the \f(CW@b\fR or \f(CW%b\fR.  Plus the function didn't get passed
into two separate arrays or hashes: it got one long list in \f(CW@_\fR,
as always.
.PP
If you can arrange for everyone to deal with this through references, it's
cleaner code, although not so nice to look at.  Here's a function that
takes two array references as arguments, returning the two array elements
in order of how many elements they have in them:
.PP
.Vb 10
\&    ($aref, $bref) = func(\e@c, \e@d);
\&    print "@$aref has more than @$bref\en";
\&    sub func {
\&        my ($cref, $dref) = @_;
\&        if (@$cref > @$dref) {
\&            return ($cref, $dref);
\&        } else {
\&            return ($dref, $cref);
\&        }
\&    }
.Ve
.PP
It turns out that you can actually do this also:
.PP
.Vb 10
\&    (*a, *b) = func(\e@c, \e@d);
\&    print "@a has more than @b\en";
\&    sub func {
\&        local (*c, *d) = @_;
\&        if (@c > @d) {
\&            return (\e@c, \e@d);
\&        } else {
\&            return (\e@d, \e@c);
\&        }
\&    }
.Ve
.PP
Here we're using the typeglobs to do symbol table aliasing.  It's
a tad subtle, though, and also won't work if you're using \f(CW\*(C`my\*(C'\fR
variables, because only globals (even in disguise as \f(CW\*(C`local\*(C'\fRs)
are in the symbol table.
.PP
If you're passing around filehandles, you could usually just use the bare
typeglob, like \f(CW*STDOUT\fR, but typeglobs references work, too.
For example:
.PP
.Vb 5
\&    splutter(\e*STDOUT);
\&    sub splutter {
\&        my $fh = shift;
\&        print $fh "her um well a hmmm\en";
\&    }
\&
\&    $rec = get_rec(\e*STDIN);
\&    sub get_rec {
\&        my $fh = shift;
\&        return scalar <$fh>;
\&    }
.Ve
.PP
If you're planning on generating new filehandles, you could do this.
Notice to pass back just the bare *FH, not its reference.
.PP
.Vb 5
\&    sub openit {
\&        my $path = shift;
\&        local *FH;
\&        return open (FH, $path) ? *FH : undef;
\&    }
.Ve
.Sh "Prototypes"
.IX Xref "prototype subroutine, prototype"
.IX Subsection "Prototypes"
Perl supports a very limited kind of compile-time argument checking
using function prototyping.  If you declare
.PP
.Vb 1
\&    sub mypush (\e@@)
.Ve
.PP
then \f(CW\*(C`mypush()\*(C'\fR takes arguments exactly like \f(CW\*(C`push()\*(C'\fR does.  The
function declaration must be visible at compile time.  The prototype
affects only interpretation of new-style calls to the function,
where new-style is defined as not using the \f(CW\*(C`&\*(C'\fR character.  In
other words, if you call it like a built-in function, then it behaves
like a built-in function.  If you call it like an old-fashioned
subroutine, then it behaves like an old-fashioned subroutine.  It
naturally falls out from this rule that prototypes have no influence
on subroutine references like \f(CW\*(C`\e&foo\*(C'\fR or on indirect subroutine
calls like \f(CW\*(C`&{$subref}\*(C'\fR or \f(CW\*(C`$subref\->()\*(C'\fR.
.PP
Method calls are not influenced by prototypes either, because the
function to be called is indeterminate at compile time, since
the exact code called depends on inheritance.
.PP
Because the intent of this feature is primarily to let you define
subroutines that work like built-in functions, here are prototypes
for some other functions that parse almost exactly like the
corresponding built-in.
.PP
.Vb 1
\&    Declared as                 Called as
\&
\&    sub mylink ($$)          mylink $old, $new
\&    sub myvec ($$$)          myvec $var, $offset, 1
\&    sub myindex ($$;$)       myindex &getstring, "substr"
\&    sub mysyswrite ($$$;$)   mysyswrite $buf, 0, length($buf) \- $off, $off
\&    sub myreverse (@)        myreverse $a, $b, $c
\&    sub myjoin ($@)          myjoin ":", $a, $b, $c
\&    sub mypop (\e@)           mypop @array
\&    sub mysplice (\e@$$@)     mysplice @array, @array, 0, @pushme
\&    sub mykeys (\e%)          mykeys %{$hashref}
\&    sub myopen (*;$)         myopen HANDLE, $name
\&    sub mypipe (**)          mypipe READHANDLE, WRITEHANDLE
\&    sub mygrep (&@)          mygrep { /foo/ } $a, $b, $c
\&    sub myrand (;$)          myrand 42
\&    sub mytime ()            mytime
.Ve
.PP
Any backslashed prototype character represents an actual argument
that absolutely must start with that character.  The value passed
as part of \f(CW@_\fR will be a reference to the actual argument given
in the subroutine call, obtained by applying \f(CW\*(C`\e\*(C'\fR to that argument.
.PP
You can also backslash several argument types simultaneously by using
the \f(CW\*(C`\e[]\*(C'\fR notation:
.PP
.Vb 1
\&    sub myref (\e[$@%&*])
.Ve
.PP
will allow calling \fImyref()\fR as
.PP
.Vb 5
\&    myref $var
\&    myref @array
\&    myref %hash
\&    myref &sub
\&    myref *glob
.Ve
.PP
and the first argument of \fImyref()\fR will be a reference to
a scalar, an array, a hash, a code, or a glob.
.PP
Unbackslashed prototype characters have special meanings.  Any
unbackslashed \f(CW\*(C`@\*(C'\fR or \f(CW\*(C`%\*(C'\fR eats all remaining arguments, and forces
list context.  An argument represented by \f(CW\*(C`$\*(C'\fR forces scalar context.  An
\&\f(CW\*(C`&\*(C'\fR requires an anonymous subroutine, which, if passed as the first
argument, does not require the \f(CW\*(C`sub\*(C'\fR keyword or a subsequent comma.
.PP
A \f(CW\*(C`*\*(C'\fR allows the subroutine to accept a bareword, constant, scalar expression,
typeglob, or a reference to a typeglob in that slot.  The value will be
available to the subroutine either as a simple scalar, or (in the latter
two cases) as a reference to the typeglob.  If you wish to always convert
such arguments to a typeglob reference, use \fISymbol::qualify_to_ref()\fR as
follows:
.PP
.Vb 1
\&    use Symbol \*(Aqqualify_to_ref\*(Aq;
\&
\&    sub foo (*) {
\&        my $fh = qualify_to_ref(shift, caller);
\&        ...
\&    }
.Ve
.PP
A semicolon (\f(CW\*(C`;\*(C'\fR) separates mandatory arguments from optional arguments.
It is redundant before \f(CW\*(C`@\*(C'\fR or \f(CW\*(C`%\*(C'\fR, which gobble up everything else.
.PP
As the last character of a prototype, or just before a semicolon, you can
use \f(CW\*(C`_\*(C'\fR in place of \f(CW\*(C`$\*(C'\fR: if this argument is not provided, \f(CW$_\fR will be
used instead.
.PP
Note how the last three examples in the table above are treated
specially by the parser.  \f(CW\*(C`mygrep()\*(C'\fR is parsed as a true list
operator, \f(CW\*(C`myrand()\*(C'\fR is parsed as a true unary operator with unary
precedence the same as \f(CW\*(C`rand()\*(C'\fR, and \f(CW\*(C`mytime()\*(C'\fR is truly without
arguments, just like \f(CW\*(C`time()\*(C'\fR.  That is, if you say
.PP
.Vb 1
\&    mytime +2;
.Ve
.PP
you'll get \f(CW\*(C`mytime() + 2\*(C'\fR, not \f(CWmytime(2)\fR, which is how it would be parsed
without a prototype.
.PP
The interesting thing about \f(CW\*(C`&\*(C'\fR is that you can generate new syntax with it,
provided it's in the initial position:
.IX Xref "&"
.PP
.Vb 9
\&    sub try (&@) {
\&        my($try,$catch) = @_;
\&        eval { &$try };