perlfaq8.1

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
.Vb 3
\&        if (\-t STDIN && \-t STDOUT) {
\&                print "Now what? ";
\&                }
.Ve
.PP
On \s-1POSIX\s0 systems, you can test whether your own process group matches
the current process group of your controlling terminal as follows:
.PP
.Vb 1
\&        use POSIX qw/getpgrp tcgetpgrp/;
\&
\&        # Some POSIX systems, such as Linux, can be
\&        # without a /dev/tty at boot time.
\&        if (!open(TTY, "/dev/tty")) {
\&                print "no tty\en";
\&        } else {
\&                $tpgrp = tcgetpgrp(fileno(*TTY));
\&                $pgrp = getpgrp();
\&                if ($tpgrp == $pgrp) {
\&                        print "foreground\en";
\&                } else {
\&                        print "background\en";
\&                }
\&        }
.Ve
.Sh "How do I timeout a slow event?"
.IX Subsection "How do I timeout a slow event?"
Use the \fIalarm()\fR function, probably in conjunction with a signal
handler, as documented in \*(L"Signals\*(R" in perlipc and the section on
\&\*(L"Signals\*(R" in the Camel.  You may instead use the more flexible
Sys::AlarmCall module available from \s-1CPAN\s0.
.PP
The \fIalarm()\fR function is not implemented on all versions of Windows.
Check the documentation for your specific version of Perl.
.Sh "How do I set \s-1CPU\s0 limits?"
.IX Xref "BSD::Resource limit CPU"
.IX Subsection "How do I set CPU limits?"
(contributed by Xho)
.PP
Use the \f(CW\*(C`BSD::Resource\*(C'\fR module from \s-1CPAN\s0. As an example:
.PP
.Vb 2
\&        use BSD::Resource;
\&        setrlimit(RLIMIT_CPU,10,20) or die $!;
.Ve
.PP
This sets the soft and hard limits to 10 and 20 seconds, respectively.
After 10 seconds of time spent running on the \s-1CPU\s0 (not \*(L"wall\*(R" time),
the process will be sent a signal (\s-1XCPU\s0 on some systems) which, if not
trapped, will cause the process to terminate.  If that signal is
trapped, then after 10 more seconds (20 seconds in total) the process
will be killed with a non-trappable signal.
.PP
See the \f(CW\*(C`BSD::Resource\*(C'\fR and your systems documentation for the gory
details.
.Sh "How do I avoid zombies on a Unix system?"
.IX Subsection "How do I avoid zombies on a Unix system?"
Use the reaper code from \*(L"Signals\*(R" in perlipc to call \fIwait()\fR when a
\&\s-1SIGCHLD\s0 is received, or else use the double-fork technique described
in \*(L"How do I start a process in the background?\*(R" in perlfaq8.
.Sh "How do I use an \s-1SQL\s0 database?"
.IX Subsection "How do I use an SQL database?"
The \s-1DBI\s0 module provides an abstract interface to most database
servers and types, including Oracle, \s-1DB2\s0, Sybase, mysql, Postgresql,
\&\s-1ODBC\s0, and flat files.  The \s-1DBI\s0 module accesses each database type
through a database driver, or \s-1DBD\s0.  You can see a complete list of
available drivers on \s-1CPAN:\s0 http://www.cpan.org/modules/by\-module/DBD/ .
You can read more about \s-1DBI\s0 on http://dbi.perl.org .
.PP
Other modules provide more specific access: Win32::ODBC, Alzabo, iodbc,
and others found on \s-1CPAN\s0 Search: http://search.cpan.org .
.Sh "How do I make a \fIsystem()\fP exit on control-C?"
.IX Subsection "How do I make a system() exit on control-C?"
You can't.  You need to imitate the \fIsystem()\fR call (see perlipc for
sample code) and then have a signal handler for the \s-1INT\s0 signal that
passes the signal on to the subprocess.  Or you can check for it:
.PP
.Vb 2
\&        $rc = system($cmd);
\&        if ($rc & 127) { die "signal death" }
.Ve
.Sh "How do I open a file without blocking?"
.IX Subsection "How do I open a file without blocking?"
If you're lucky enough to be using a system that supports
non-blocking reads (most Unixish systems do), you need only to use the
O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with
\&\fIsysopen()\fR:
.PP
.Vb 3
\&        use Fcntl;
\&        sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
\&                or die "can\*(Aqt open /foo/somefile: $!":
.Ve
.Sh "How do I tell the difference between errors from the shell and perl?"
.IX Subsection "How do I tell the difference between errors from the shell and perl?"
(answer contributed by brian d foy)
.PP
When you run a Perl script, something else is running the script for you,
and that something else may output error messages.  The script might
emit its own warnings and error messages.  Most of the time you cannot
tell who said what.
.PP
You probably cannot fix the thing that runs perl, but you can change how
perl outputs its warnings by defining a custom warning and die functions.
.PP
Consider this script, which has an error you may not notice immediately.
.PP
.Vb 1
\&        #!/usr/locl/bin/perl
\&
\&        print "Hello World\en";
.Ve
.PP
I get an error when I run this from my shell (which happens to be
bash).  That may look like perl forgot it has a \fIprint()\fR function,
but my shebang line is not the path to perl, so the shell runs the
script, and I get the error.
.PP
.Vb 2
\&        $ ./test
\&        ./test: line 3: print: command not found
.Ve
.PP
A quick and dirty fix involves a little bit of code, but this may be all
you need to figure out the problem.
.PP
.Vb 1
\&        #!/usr/bin/perl \-w
\&
\&        BEGIN {
\&        $SIG{_\|_WARN_\|_} = sub{ print STDERR "Perl: ", @_; };
\&        $SIG{_\|_DIE_\|_}  = sub{ print STDERR "Perl: ", @_; exit 1};
\&        }
\&
\&        $a = 1 + undef;
\&        $x / 0;
\&        _\|_END_\|_
.Ve
.PP
The perl message comes out with \*(L"Perl\*(R" in front.  The \s-1BEGIN\s0 block
works at compile time so all of the compilation errors and warnings
get the \*(L"Perl:\*(R" prefix too.
.PP
.Vb 7
\&        Perl: Useless use of division (/) in void context at ./test line 9.
\&        Perl: Name "main::a" used only once: possible typo at ./test line 8.
\&        Perl: Name "main::x" used only once: possible typo at ./test line 9.
\&        Perl: Use of uninitialized value in addition (+) at ./test line 8.
\&        Perl: Use of uninitialized value in division (/) at ./test line 9.
\&        Perl: Illegal division by zero at ./test line 9.
\&        Perl: Illegal division by zero at \-e line 3.
.Ve
.PP
If I don't see that \*(L"Perl:\*(R", it's not from perl.
.PP
You could also just know all the perl errors, and although there are
some people who may know all of them, you probably don't.  However, they
all should be in the perldiag manpage. If you don't find the error in
there, it probably isn't a perl error.
.PP
Looking up every message is not the easiest way, so let perl to do it
for you.  Use the diagnostics pragma with turns perl's normal messages
into longer discussions on the topic.
.PP
.Vb 1
\&        use diagnostics;
.Ve
.PP
If you don't get a paragraph or two of expanded discussion, it
might not be perl's message.
.Sh "How do I install a module from \s-1CPAN\s0?"
.IX Subsection "How do I install a module from CPAN?"
The easiest way is to have a module also named \s-1CPAN\s0 do it for you.
This module comes with perl version 5.004 and later.
.PP
.Vb 1
\&        $ perl \-MCPAN \-e shell
\&
\&        cpan shell \-\- CPAN exploration and modules installation (v1.59_54)
\&        ReadLine support enabled
\&
\&        cpan> install Some::Module
.Ve
.PP
To manually install the \s-1CPAN\s0 module, or any well-behaved \s-1CPAN\s0 module
for that matter, follow these steps:
.IP "1." 4
Unpack the source into a temporary area.
.IP "2." 4
.Vb 1
\&        perl Makefile.PL
.Ve
.IP "3." 4
.Vb 1
\&        make
.Ve
.IP "4." 4
.Vb 1
\&        make test
.Ve
.IP "5." 4
.Vb 1
\&        make install
.Ve
.PP
If your version of perl is compiled without dynamic loading, then you
just need to replace step 3 (\fBmake\fR) with \fBmake perl\fR and you will
get a new \fIperl\fR binary with your extension linked in.
.PP
See ExtUtils::MakeMaker for more details on building extensions.
See also the next question, \*(L"What's the difference between require
and use?\*(R".
.Sh "What's the difference between require and use?"
.IX Subsection "What's the difference between require and use?"
Perl offers several different ways to include code from one file into
another.  Here are the deltas between the various inclusion constructs:
.PP
.Vb 3
\&        1)  do $file is like eval \`cat $file\`, except the former
\&        1.1: searches @INC and updates %INC.
\&        1.2: bequeaths an *unrelated* lexical scope on the eval\*(Aqed code.
\&
\&        2)  require $file is like do $file, except the former
\&        2.1: checks for redundant loading, skipping already loaded files.
\&        2.2: raises an exception on failure to find, compile, or execute $file.
\&
\&        3)  require Module is like require "Module.pm", except the former
\&        3.1: translates each "::" into your system\*(Aqs directory separator.
\&        3.2: primes the parser to disambiguate class Module as an indirect object.
\&
\&        4)  use Module is like require Module, except the former
\&        4.1: loads the module at compile time, not run\-time.
\&        4.2: imports symbols and semantics from that package to the current one.
.Ve
.PP
In general, you usually want \f(CW\*(C`use\*(C'\fR and a proper Perl module.
.Sh "How do I keep my own module/library directory?"
.IX Subsection "How do I keep my own module/library directory?"
When you build modules, tell Perl where to install the modules.
.PP
For \f(CW\*(C`Makefile.PL\*(C'\fR\-based distributions, use the \s-1PREFIX\s0 and \s-1LIB\s0 options
when generating Makefiles:
.PP
.Vb 1
\&        perl Makefile.PL PREFIX=/mydir/perl LIB=/mydir/perl/lib
.Ve
.PP
You can set this in your \s-1CPAN\s0.pm configuration so modules automatically install
in your private library directory when you use the \s-1CPAN\s0.pm shell:
.PP
.Vb 3
\&        % cpan
\&        cpan> o conf makepl_arg PREFIX=/mydir/perl,LIB=/mydir/perl/lib
\&        cpan> o conf commit
.Ve
.PP
For \f(CW\*(C`Build.PL\*(C'\fR\-based distributions, use the \-\-install_base option:
.PP
.Vb 1
\&        perl Build.PL \-\-install_base /mydir/perl
.Ve
.PP
You can configure \s-1CPAN\s0.pm to automatically use this option too:
.PP
.Vb 3
\&        % cpan
\&        cpan> o conf mbuild_arg \-\-install_base /mydir/perl
\&        cpan> o conf commit
.Ve
.Sh "How do I add the directory my program lives in to the module/library search path?"
.IX Subsection "How do I add the directory my program lives in to the module/library search path?"
(contributed by brian d foy)
.PP
If you know the directory already, you can add it to \f(CW@INC\fR as you would
for any other directory. You might <use lib> if you know the directory
at compile time:
.PP
.Vb 1
\&        use lib $directory;
.Ve
.PP
The trick in this task is to find the directory. Before your script does
anything else (such as a \f(CW\*(C`chdir\*(C'\fR), you can get the current working
directory with the \f(CW\*(C`Cwd\*(C'\fR module, which comes with Perl:
.PP
.Vb 4
\&        BEGIN {
\&                use Cwd;
\&                our $directory = cwd;
\&                }
\&        
\&        use lib $directory;
.Ve
.PP
You can do a similar thing with the value of \f(CW$0\fR, which holds the
script name. That might hold a relative path, but \f(CW\*(C`rel2abs\*(C'\fR can turn
it into an absolute path. Once you have the
.PP
.Vb 3
\&        BEGIN { 
\&                use File::Spec::Functions qw(rel2abs);
\&                use File::Basename qw(dirname);
\&                
\&                my $path   = rel2abs( $0 );
\&                our $directory = dirname( $path );
\&                }
\&                
\&        use lib $directory;
.Ve
.PP
The \f(CW\*(C`FindBin\*(C'\fR module, which comes with Perl, might work. It searches
through \f(CW$ENV{PATH}\fR (so your script has to be in one of those
directories). You can then use that directory (in \f(CW$FindBin::Bin\fR)
to locate nearby directories you want to add:
.PP
.Vb 2
\&        use FindBin;
\&        use lib "$FindBin::Bin/../lib";
.Ve
.Sh "How do I add a directory to my include path (@INC) at runtime?"
.IX Subsection "How do I add a directory to my include path (@INC) at runtime?"
Here are the suggested ways of modifying your include path, including
environment variables, run-time switches, and in-code statements:
.IP "the \s-1PERLLIB\s0 environment variable" 4
.IX Item "the PERLLIB environment variable"
.Vb 2
\&        $ export PERLLIB=/path/to/my/dir
\&        $ perl program.pl
.Ve
.IP "the \s-1PERL5LIB\s0 environment variable" 4
.IX Item "the PERL5LIB environment variable"
.Vb 2
\&        $ export PERL5LIB=/path/to/my/dir
\&        $ perl program.pl
.Ve
.IP "the perl \-Idir command line flag" 4
.IX Item "the perl -Idir command line flag"
.Vb 1
\&        $ perl \-I/path/to/my/dir program.pl
.Ve
.IP "the use lib pragma:" 4
.IX Item "the use lib pragma:"
.Vb 1
\&        use lib "$ENV{HOME}/myown_perllib";
.Ve
.PP
The last is particularly useful because it knows about machine
dependent architectures.  The lib.pm pragmatic module was first
included with the 5.002 release of Perl.
.Sh "What is socket.ph and where do I get it?"
.IX Subsection "What is socket.ph and where do I get it?"
It's a Perl 4 style file defining values for system networking
constants.  Sometimes it is built using h2ph when Perl is installed,
but other times it is not.  Modern programs \f(CW\*(C`use Socket;\*(C'\fR instead.
.SH "REVISION"
.IX Header "REVISION"
Revision: \f(CW$Revision:\fR 10183 $
.PP
Date: \f(CW$Date:\fR 2007\-11\-07 09:35:12 +0100 (Wed, 07 Nov 2007) $
.PP
See perlfaq for source control details and availability.
.SH "AUTHOR AND COPYRIGHT"
.IX Header "AUTHOR AND COPYRIGHT"
Copyright (c) 1997\-2007 Tom Christiansen, Nathan Torkington, and
other authors as noted. All rights reserved.
.PP
This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.
.PP
Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain.  You are permitted and
encouraged to use this code in your own programs for fun
or for profit as you see fit.  A simple comment in the code giving
credit would be courteous but is not required.