perlfaq3.1

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.

the current version of vi out of Berkeley, which incidentally can be built
with an embedded Perl interpreter\*(--see http://www.cpan.org/src/misc/ .
.Sh "Where can I get perl-mode for emacs?"
.IX Subsection "Where can I get perl-mode for emacs?"
Since Emacs version 19 patchlevel 22 or so, there have been both a
perl\-mode.el and support for the Perl debugger built in.  These should
come with the standard Emacs 19 distribution.
.PP
In the Perl source directory, you'll find a directory called \*(L"emacs\*(R",
which contains a cperl-mode that color-codes keywords, provides
context-sensitive help, and other nifty things.
.PP
Note that the perl-mode of emacs will have fits with \f(CW"main\*(Aqfoo"\fR
(single quote), and mess up the indentation and highlighting.  You
are probably using \f(CW"main::foo"\fR in new Perl code anyway, so this
shouldn't be an issue.
.Sh "How can I use curses with Perl?"
.IX Subsection "How can I use curses with Perl?"
The Curses module from \s-1CPAN\s0 provides a dynamically loadable object
module interface to a curses library.  A small demo can be found at the
directory http://www.cpan.org/authors/Tom_Christiansen/scripts/rep.gz ;
this program repeats a command and updates the screen as needed, rendering
\&\fBrep ps axu\fR similar to \fBtop\fR.
.Sh "How can I write a \s-1GUI\s0 (X, Tk, Gtk, etc.) in Perl?"
.IX Xref "GUI Tk Wx WxWidgets Gtk Gtk2 CamelBones Qt"
.IX Subsection "How can I write a GUI (X, Tk, Gtk, etc.) in Perl?"
(contributed by Ben Morrow)
.PP
There are a number of modules which let you write GUIs in Perl. Most
\&\s-1GUI\s0 toolkits have a perl interface: an incomplete list follows.
.IP "Tk" 4
.IX Item "Tk"
This works under Unix and Windows, and the current version doesn't
look half as bad under Windows as it used to. Some of the gui elements
still don't 'feel' quite right, though. The interface is very natural
and 'perlish', making it easy to use in small scripts that just need a
simple gui. It hasn't been updated in a while.
.IP "Wx" 4
.IX Item "Wx"
This is a Perl binding for the cross-platform wxWidgets toolkit 
<http://www.wxwidgets.org>. It works under Unix, Win32 and Mac \s-1OS\s0 X,
using native widgets (Gtk under Unix). The interface follows the \*(C+
interface closely, but the documentation is a little sparse for someone
who doesn't know the library, mostly just referring you to the \*(C+
documentation.
.IP "Gtk and Gtk2" 4
.IX Item "Gtk and Gtk2"
These are Perl bindings for the Gtk toolkit <http://www.gtk.org>. The
interface changed significantly between versions 1 and 2 so they have
separate Perl modules. It runs under Unix, Win32 and Mac \s-1OS\s0 X (currently
it requires an X server on Mac \s-1OS\s0, but a 'native' port is underway), and
the widgets look the same on every plaform: i.e., they don't match the
native widgets. As with Wx, the Perl bindings follow the C \s-1API\s0 closely,
and the documentation requires you to read the C documentation to
understand it.
.IP "Win32::GUI" 4
.IX Item "Win32::GUI"
This provides access to most of the Win32 \s-1GUI\s0 widgets from Perl.
Obviously, it only runs under Win32, and uses native widgets. The Perl
interface doesn't really follow the C interface: it's been made more
Perlish, and the documentation is pretty good. More advanced stuff may
require familiarity with the C Win32 APIs, or reference to \s-1MSDN\s0.
.IP "CamelBones" 4
.IX Item "CamelBones"
CamelBones <http://camelbones.sourceforge.net> is a Perl interface to
Mac \s-1OS\s0 X's Cocoa \s-1GUI\s0 toolkit, and as such can be used to produce native
GUIs on Mac \s-1OS\s0 X. It's not on \s-1CPAN\s0, as it requires frameworks that
\&\s-1CPAN\s0.pm doesn't know how to install, but installation is via the
standard \s-1OSX\s0 package installer. The Perl \s-1API\s0 is, again, very close to
the ObjC \s-1API\s0 it's wrapping, and the documentation just tells you how to
translate from one to the other.
.IP "Qt" 4
.IX Item "Qt"
There is a Perl interface to TrollTech's Qt toolkit, but it does not
appear to be maintained.
.IP "Athena" 4
.IX Item "Athena"
Sx is an interface to the Athena widget set which comes with X, but
again it appears not to be much used nowadays.
.Sh "How can I make my Perl program run faster?"
.IX Subsection "How can I make my Perl program run faster?"
The best way to do this is to come up with a better algorithm.  This
can often make a dramatic difference.  Jon Bentley's book
\&\fIProgramming Pearls\fR (that's not a misspelling!)  has some good tips
on optimization, too.  Advice on benchmarking boils down to: benchmark
and profile to make sure you're optimizing the right part, look for
better algorithms instead of microtuning your code, and when all else
fails consider just buying faster hardware.  You will probably want to
read the answer to the earlier question \*(L"How do I profile my Perl
programs?\*(R" if you haven't done so already.
.PP
A different approach is to autoload seldom-used Perl code.  See the
AutoSplit and AutoLoader modules in the standard distribution for
that.  Or you could locate the bottleneck and think about writing just
that part in C, the way we used to take bottlenecks in C code and
write them in assembler.  Similar to rewriting in C, modules that have
critical sections can be written in C (for instance, the \s-1PDL\s0 module
from \s-1CPAN\s0).
.PP
If you're currently linking your perl executable to a shared
\&\fIlibc.so\fR, you can often gain a 10\-25% performance benefit by
rebuilding it to link with a static libc.a instead.  This will make a
bigger perl executable, but your Perl programs (and programmers) may
thank you for it.  See the \fI\s-1INSTALL\s0\fR file in the source distribution
for more information.
.PP
The undump program was an ancient attempt to speed up Perl program by
storing the already-compiled form to disk.  This is no longer a viable
option, as it only worked on a few architectures, and wasn't a good
solution anyway.
.Sh "How can I make my Perl program take less memory?"
.IX Subsection "How can I make my Perl program take less memory?"
When it comes to time-space tradeoffs, Perl nearly always prefers to
throw memory at a problem.  Scalars in Perl use more memory than
strings in C, arrays take more than that, and hashes use even more.  While
there's still a lot to be done, recent releases have been addressing
these issues.  For example, as of 5.004, duplicate hash keys are
shared amongst all hashes using them, so require no reallocation.
.PP
In some cases, using \fIsubstr()\fR or \fIvec()\fR to simulate arrays can be
highly beneficial.  For example, an array of a thousand booleans will
take at least 20,000 bytes of space, but it can be turned into one
125\-byte bit vector\*(--a considerable memory savings.  The standard
Tie::SubstrHash module can also help for certain types of data
structure.  If you're working with specialist data structures
(matrices, for instance) modules that implement these in C may use
less memory than equivalent Perl modules.
.PP
Another thing to try is learning whether your Perl was compiled with
the system malloc or with Perl's builtin malloc.  Whichever one it
is, try using the other one and see whether this makes a difference.
Information about malloc is in the \fI\s-1INSTALL\s0\fR file in the source
distribution.  You can find out whether you are using perl's malloc by
typing \f(CW\*(C`perl \-V:usemymalloc\*(C'\fR.
.PP
Of course, the best way to save memory is to not do anything to waste
it in the first place. Good programming practices can go a long way
toward this:
.IP "\(bu" 4
Don't slurp!
.Sp
Don't read an entire file into memory if you can process it line
by line. Or more concretely, use a loop like this:
.Sp
.Vb 6
\&        #
\&        # Good Idea
\&        #
\&        while (<FILE>) {
\&           # ...
\&        }
.Ve
.Sp
instead of this:
.Sp
.Vb 7
\&        #
\&        # Bad Idea
\&        #
\&        @data = <FILE>;
\&        foreach (@data) {
\&            # ...
\&        }
.Ve
.Sp
When the files you're processing are small, it doesn't much matter which
way you do it, but it makes a huge difference when they start getting
larger.
.IP "\(bu" 4
Use map and grep selectively
.Sp
Remember that both map and grep expect a \s-1LIST\s0 argument, so doing this:
.Sp
.Vb 1
\&        @wanted = grep {/pattern/} <FILE>;
.Ve
.Sp
will cause the entire file to be slurped. For large files, it's better
to loop:
.Sp
.Vb 3
\&        while (<FILE>) {
\&                push(@wanted, $_) if /pattern/;
\&        }
.Ve
.IP "\(bu" 4
Avoid unnecessary quotes and stringification
.Sp
Don't quote large strings unless absolutely necessary:
.Sp
.Vb 1
\&        my $copy = "$large_string";
.Ve
.Sp
makes 2 copies of \f(CW$large_string\fR (one for \f(CW$copy\fR and another for the
quotes), whereas
.Sp
.Vb 1
\&        my $copy = $large_string;
.Ve
.Sp
only makes one copy.
.Sp
Ditto for stringifying large arrays:
.Sp
.Vb 4
\&        {
\&                local $, = "\en";
\&                print @big_array;
\&        }
.Ve
.Sp
is much more memory-efficient than either
.Sp
.Vb 1
\&        print join "\en", @big_array;
.Ve
.Sp
or
.Sp
.Vb 4
\&        {
\&                local $" = "\en";
\&                print "@big_array";
\&        }
.Ve
.IP "\(bu" 4
Pass by reference
.Sp
Pass arrays and hashes by reference, not by value. For one thing, it's
the only way to pass multiple lists or hashes (or both) in a single
call/return. It also avoids creating a copy of all the contents. This
requires some judgement, however, because any changes will be propagated
back to the original data. If you really want to mangle (er, modify) a
copy, you'll have to sacrifice the memory needed to make one.
.IP "\(bu" 4
Tie large variables to disk.
.Sp
For \*(L"big\*(R" data stores (i.e. ones that exceed available memory) consider
using one of the \s-1DB\s0 modules to store it on disk instead of in \s-1RAM\s0. This
will incur a penalty in access time, but that's probably better than
causing your hard disk to thrash due to massive swapping.
.Sh "Is it safe to return a reference to local or lexical data?"
.IX Subsection "Is it safe to return a reference to local or lexical data?"
Yes. Perl's garbage collection system takes care of this so
everything works out right.
.PP
.Vb 4
\&    sub makeone {
\&        my @a = ( 1 .. 10 );
\&        return \e@a;
\&    }
\&
\&    for ( 1 .. 10 ) {
\&        push @many, makeone();
\&    }
\&
\&    print $many[4][5], "\en";
\&
\&    print "@many\en";
.Ve
.Sh "How can I free an array or hash so my program shrinks?"
.IX Subsection "How can I free an array or hash so my program shrinks?"
(contributed by Michael Carman)
.PP
You usually can't. Memory allocated to lexicals (i.e. \fImy()\fR variables)
cannot be reclaimed or reused even if they go out of scope. It is
reserved in case the variables come back into scope. Memory allocated
to global variables can be reused (within your program) by using
\&\fIundef()\fRing and/or \fIdelete()\fR.
.PP
On most operating systems, memory allocated to a program can never be
returned to the system. That's why long-running programs sometimes re\-
exec themselves. Some operating systems (notably, systems that use
\&\fImmap\fR\|(2) for allocating large chunks of memory) can reclaim memory that
is no longer used, but on such systems, perl must be configured and
compiled to use the \s-1OS\s0's malloc, not perl's.
.PP
In general, memory allocation and de-allocation isn't something you can
or should be worrying about much in Perl.
.PP
See also \*(L"How can I make my Perl program take less memory?\*(R"
.Sh "How can I make my \s-1CGI\s0 script more efficient?"
.IX Subsection "How can I make my CGI script more efficient?"
Beyond the normal measures described to make general Perl programs
faster or smaller, a \s-1CGI\s0 program has additional issues.  It may be run
several times per second.  Given that each time it runs it will need
to be re-compiled and will often allocate a megabyte or more of system
memory, this can be a killer.  Compiling into C \fBisn't going to help
you\fR because the process start-up overhead is where the bottleneck is.