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.

\&
\&        ReadMode(\*(Aqnoecho\*(Aq);
\&        $password = ReadLine(0);
.Ve
.Sh "How do I read and write the serial port?"
.IX Subsection "How do I read and write the serial port?"
This depends on which operating system your program is running on.  In
the case of Unix, the serial ports will be accessible through files in
/dev; on other systems, device names will doubtless differ.
Several problem areas common to all device interaction are the
following:
.IP "lockfiles" 4
.IX Item "lockfiles"
Your system may use lockfiles to control multiple access.  Make sure
you follow the correct protocol.  Unpredictable behavior can result
from multiple processes reading from one device.
.IP "open mode" 4
.IX Item "open mode"
If you expect to use both read and write operations on the device,
you'll have to open it for update (see \*(L"open\*(R" in perlfunc for
details).  You may wish to open it without running the risk of
blocking by using \fIsysopen()\fR and \f(CW\*(C`O_RDWR|O_NDELAY|O_NOCTTY\*(C'\fR from the
Fcntl module (part of the standard perl distribution).  See
\&\*(L"sysopen\*(R" in perlfunc for more on this approach.
.IP "end of line" 4
.IX Item "end of line"
Some devices will be expecting a \*(L"\er\*(R" at the end of each line rather
than a \*(L"\en\*(R".  In some ports of perl, \*(L"\er\*(R" and \*(L"\en\*(R" are different from
their usual (Unix) \s-1ASCII\s0 values of \*(L"\e012\*(R" and \*(L"\e015\*(R".  You may have to
give the numeric values you want directly, using octal (\*(L"\e015\*(R"), hex
(\*(L"0x0D\*(R"), or as a control-character specification (\*(L"\ecM\*(R").
.Sp
.Vb 2
\&        print DEV "atv1\e012";   # wrong, for some devices
\&        print DEV "atv1\e015";   # right, for some devices
.Ve
.Sp
Even though with normal text files a \*(L"\en\*(R" will do the trick, there is
still no unified scheme for terminating a line that is portable
between Unix, DOS/Win, and Macintosh, except to terminate \fI\s-1ALL\s0\fR line
ends with \*(L"\e015\e012\*(R", and strip what you don't need from the output.
This applies especially to socket I/O and autoflushing, discussed
next.
.IP "flushing output" 4
.IX Item "flushing output"
If you expect characters to get to your device when you \fIprint()\fR them,
you'll want to autoflush that filehandle.  You can use \fIselect()\fR
and the \f(CW$|\fR variable to control autoflushing (see \*(L"$|\*(R" in perlvar
and \*(L"select\*(R" in perlfunc, or perlfaq5, \*(L"How do I flush/unbuffer an
output filehandle?  Why must I do this?\*(R"):
.Sp
.Vb 3
\&        $oldh = select(DEV);
\&        $| = 1;
\&        select($oldh);
.Ve
.Sp
You'll also see code that does this without a temporary variable, as in
.Sp
.Vb 1
\&        select((select(DEV), $| = 1)[0]);
.Ve
.Sp
Or if you don't mind pulling in a few thousand lines
of code just because you're afraid of a little $| variable:
.Sp
.Vb 2
\&        use IO::Handle;
\&        DEV\->autoflush(1);
.Ve
.Sp
As mentioned in the previous item, this still doesn't work when using
socket I/O between Unix and Macintosh.  You'll need to hard code your
line terminators, in that case.
.IP "non-blocking input" 4
.IX Item "non-blocking input"
If you are doing a blocking \fIread()\fR or \fIsysread()\fR, you'll have to
arrange for an alarm handler to provide a timeout (see
\&\*(L"alarm\*(R" in perlfunc).  If you have a non-blocking open, you'll likely
have a non-blocking read, which means you may have to use a 4\-arg
\&\fIselect()\fR to determine whether I/O is ready on that device (see
\&\*(L"select\*(R" in perlfunc.
.PP
While trying to read from his caller-id box, the notorious Jamie Zawinski
\&\f(CW\*(C`<[e-mail]>\*(C'\fR, after much gnashing of teeth and fighting with sysread,
sysopen, \s-1POSIX\s0's tcgetattr business, and various other functions that
go bump in the night, finally came up with this:
.PP
.Vb 10
\&        sub open_modem {
\&                use IPC::Open2;
\&                my $stty = \`/bin/stty \-g\`;
\&                open2( \e*MODEM_IN, \e*MODEM_OUT, "cu \-l$modem_device \-s2400 2>&1");
\&                # starting cu hoses /dev/tty\*(Aqs stty settings, even when it has
\&                # been opened on a pipe...
\&                system("/bin/stty $stty");
\&                $_ = <MODEM_IN>;
\&                chomp;
\&                if ( !m/^Connected/ ) {
\&                        print STDERR "$0: cu printed \`$_\*(Aq instead of \`Connected\*(Aq\en";
\&                }
\&        }
.Ve
.Sh "How do I decode encrypted password files?"
.IX Subsection "How do I decode encrypted password files?"
You spend lots and lots of money on dedicated hardware, but this is
bound to get you talked about.
.PP
Seriously, you can't if they are Unix password files\*(--the Unix
password system employs one-way encryption.  It's more like hashing
than encryption.  The best you can do is check whether something else
hashes to the same string.  You can't turn a hash back into the
original string. Programs like Crack can forcibly (and intelligently)
try to guess passwords, but don't (can't) guarantee quick success.
.PP
If you're worried about users selecting bad passwords, you should
proactively check when they try to change their password (by modifying
\&\fIpasswd\fR\|(1), for example).
.Sh "How do I start a process in the background?"
.IX Subsection "How do I start a process in the background?"
Several modules can start other processes that do not block
your Perl program.  You can use IPC::Open3, Parallel::Jobs,
IPC::Run, and some of the \s-1POE\s0 modules.  See \s-1CPAN\s0 for more
details.
.PP
You could also use
.PP
.Vb 1
\&        system("cmd &")
.Ve
.PP
or you could use fork as documented in \*(L"fork\*(R" in perlfunc, with
further examples in perlipc.  Some things to be aware of, if you're
on a Unix-like system:
.IP "\s-1STDIN\s0, \s-1STDOUT\s0, and \s-1STDERR\s0 are shared" 4
.IX Item "STDIN, STDOUT, and STDERR are shared"
Both the main process and the backgrounded one (the \*(L"child\*(R" process)
share the same \s-1STDIN\s0, \s-1STDOUT\s0 and \s-1STDERR\s0 filehandles.  If both try to
access them at once, strange things can happen.  You may want to close
or reopen these for the child.  You can get around this with
\&\f(CW\*(C`open\*(C'\fRing a pipe (see \*(L"open\*(R" in perlfunc) but on some systems this
means that the child process cannot outlive the parent.
.IP "Signals" 4
.IX Item "Signals"
You'll have to catch the \s-1SIGCHLD\s0 signal, and possibly \s-1SIGPIPE\s0 too.
\&\s-1SIGCHLD\s0 is sent when the backgrounded process finishes.  \s-1SIGPIPE\s0 is
sent when you write to a filehandle whose child process has closed (an
untrapped \s-1SIGPIPE\s0 can cause your program to silently die).  This is
not an issue with \f(CW\*(C`system("cmd&")\*(C'\fR.
.IP "Zombies" 4
.IX Item "Zombies"
You have to be prepared to \*(L"reap\*(R" the child process when it finishes.
.Sp
.Vb 1
\&        $SIG{CHLD} = sub { wait };
\&
\&        $SIG{CHLD} = \*(AqIGNORE\*(Aq;
.Ve
.Sp
You can also use a double fork. You immediately \fIwait()\fR for your
first child, and the init daemon will \fIwait()\fR for your grandchild once
it exits.
.Sp
.Vb 8
\&        unless ($pid = fork) {
\&            unless (fork) {
\&                exec "what you really wanna do";
\&                die "exec failed!";
\&            }
\&            exit 0;
\&        }
\&        waitpid($pid, 0);
.Ve
.Sp
See \*(L"Signals\*(R" in perlipc for other examples of code to do this.
Zombies are not an issue with \f(CW\*(C`system("prog &")\*(C'\fR.
.Sh "How do I trap control characters/signals?"
.IX Subsection "How do I trap control characters/signals?"
You don't actually \*(L"trap\*(R" a control character.  Instead, that character
generates a signal which is sent to your terminal's currently
foregrounded process group, which you then trap in your process.
Signals are documented in \*(L"Signals\*(R" in perlipc and the
section on \*(L"Signals\*(R" in the Camel.
.PP
You can set the values of the \f(CW%SIG\fR hash to be the functions you want
to handle the signal.  After perl catches the signal, it looks in \f(CW%SIG\fR
for a key with the same name as the signal, then calls the subroutine
value for that key.
.PP
.Vb 1
\&        # as an anonymous subroutine
\&
\&        $SIG{INT} = sub { syswrite(STDERR, "ouch\en", 5 ) };
\&
\&        # or a reference to a function
\&
\&        $SIG{INT} = \e&ouch;
\&
\&        # or the name of the function as a string
\&
\&        $SIG{INT} = "ouch";
.Ve
.PP
Perl versions before 5.8 had in its C source code signal handlers which
would catch the signal and possibly run a Perl function that you had set
in \f(CW%SIG\fR.  This violated the rules of signal handling at that level
causing perl to dump core. Since version 5.8.0, perl looks at \f(CW%SIG\fR
*after* the signal has been caught, rather than while it is being caught.
Previous versions of this answer were incorrect.
.Sh "How do I modify the shadow password file on a Unix system?"
.IX Subsection "How do I modify the shadow password file on a Unix system?"
If perl was installed correctly and your shadow library was written
properly, the getpw*() functions described in perlfunc should in
theory provide (read-only) access to entries in the shadow password
file.  To change the file, make a new shadow password file (the format
varies from system to system\*(--see passwd for specifics) and use
\&\fIpwd_mkdb\fR\|(8) to install it (see pwd_mkdb for more details).
.Sh "How do I set the time and date?"
.IX Subsection "How do I set the time and date?"
Assuming you're running under sufficient permissions, you should be
able to set the system-wide date and time by running the \fIdate\fR\|(1)
program.  (There is no way to set the time and date on a per-process
basis.)  This mechanism will work for Unix, MS-DOS, Windows, and \s-1NT\s0;
the \s-1VMS\s0 equivalent is \f(CW\*(C`set time\*(C'\fR.
.PP
However, if all you want to do is change your time zone, you can
probably get away with setting an environment variable:
.PP
.Vb 3
\&        $ENV{TZ} = "MST7MDT";              # unixish
\&        $ENV{\*(AqSYS$TIMEZONE_DIFFERENTIAL\*(Aq}="\-5" # vms
\&        system "trn comp.lang.perl.misc";
.Ve
.Sh "How can I \fIsleep()\fP or \fIalarm()\fP for under a second?"
.IX Xref "Time::HiRes BSD::Itimer sleep select"
.IX Subsection "How can I sleep() or alarm() for under a second?"
If you want finer granularity than the 1 second that the \f(CW\*(C`sleep()\*(C'\fR
function provides, the easiest way is to use the \f(CW\*(C`select()\*(C'\fR function as
documented in \*(L"select\*(R" in perlfunc.  Try the \f(CW\*(C`Time::HiRes\*(C'\fR and
the \f(CW\*(C`BSD::Itimer\*(C'\fR modules (available from \s-1CPAN\s0, and starting from
Perl 5.8 \f(CW\*(C`Time::HiRes\*(C'\fR is part of the standard distribution).
.Sh "How can I measure time under a second?"
.IX Xref "Time::HiRes BSD::Itimer sleep select"
.IX Subsection "How can I measure time under a second?"
(contributed by brian d foy)
.PP
The \f(CW\*(C`Time::HiRes\*(C'\fR module (part of the standard distribution as of
Perl 5.8) measures time with the \f(CW\*(C`gettimeofday()\*(C'\fR system call, which
returns the time in microseconds since the epoch. If you can't install
\&\f(CW\*(C`Time::HiRes\*(C'\fR for older Perls and you are on a Unixish system, you
may be able to call \f(CWgettimeofday(2)\fR directly. See
\&\*(L"syscall\*(R" in perlfunc.
.Sh "How can I do an \fIatexit()\fP or \fIsetjmp()\fP/\fIlongjmp()\fP? (Exception handling)"
.IX Subsection "How can I do an atexit() or setjmp()/longjmp()? (Exception handling)"
Release 5 of Perl added the \s-1END\s0 block, which can be used to simulate
\&\fIatexit()\fR.  Each package's \s-1END\s0 block is called when the program or
thread ends (see perlmod manpage for more details).
.PP
For example, you can use this to make sure your filter program
managed to finish its output without filling up the disk:
.PP
.Vb 3
\&        END {
\&                close(STDOUT) || die "stdout close failed: $!";
\&        }
.Ve
.PP
The \s-1END\s0 block isn't called when untrapped signals kill the program,
though, so if you use \s-1END\s0 blocks you should also use
.PP
.Vb 1
\&        use sigtrap qw(die normal\-signals);
.Ve
.PP
Perl's exception-handling mechanism is its \fIeval()\fR operator.  You can
use \fIeval()\fR as setjmp and \fIdie()\fR as longjmp.  For details of this, see
the section on signals, especially the time-out handler for a blocking
\&\fIflock()\fR in \*(L"Signals\*(R" in perlipc or the section on \*(L"Signals\*(R" in
the Camel Book.
.PP
If exception handling is all you're interested in, try the
exceptions.pl library (part of the standard perl distribution).
.PP
If you want the \fIatexit()\fR syntax (and an \fIrmexit()\fR as well), try the
AtExit module available from \s-1CPAN\s0.
.ie n .Sh "Why doesn't my sockets program work under System V (Solaris)?  What does the error message ""Protocol not supported"" mean?"
.el .Sh "Why doesn't my sockets program work under System V (Solaris)?  What does the error message ``Protocol not supported'' mean?"
.IX Subsection "Why doesn't my sockets program work under System V (Solaris)?  What does the error message Protocol not supported mean?"
Some Sys-V based systems, notably Solaris 2.X, redefined some of the
standard socket constants.  Since these were constant across all
architectures, they were often hardwired into perl code.  The proper
way to deal with this is to \*(L"use Socket\*(R" to get the correct values.
.PP
Note that even though SunOS and Solaris are binary compatible, these
values are different.  Go figure.
.Sh "How can I call my system's unique C functions from Perl?"
.IX Subsection "How can I call my system's unique C functions from Perl?"
In most cases, you write an external module to do it\*(--see the answer
to \*(L"Where can I learn about linking C with Perl? [h2xs, xsubpp]\*(R".
However, if the function is a system call, and your system supports
\&\fIsyscall()\fR, you can use the syscall function (documented in
perlfunc).
.PP
Remember to check the modules that came with your distribution, and
\&\s-1CPAN\s0 as well\*(--someone may already have written a module to do it. On
Windows, try Win32::API.  On Macs, try Mac::Carbon.  If no module
has an interface to the C function, you can inline a bit of C in your
Perl source with Inline::C.
.Sh "Where do I get the include files to do \fIioctl()\fP or \fIsyscall()\fP?"
.IX Subsection "Where do I get the include files to do ioctl() or syscall()?"
Historically, these would be generated by the h2ph tool, part of the
standard perl distribution.  This program converts \fIcpp\fR\|(1) directives
in C header files to files containing subroutine definitions, like
&SYS_getitimer, which you can use as arguments to your functions.
It doesn't work perfectly, but it usually gets most of the job done.
Simple files like \fIerrno.h\fR, \fIsyscall.h\fR, and \fIsocket.h\fR were fine,
but the hard ones like \fIioctl.h\fR nearly always need to hand-edited.
Here's how to install the *.ph files:
.PP
.Vb 3
\&        1.  become super\-user
\&        2.  cd /usr/include
\&        3.  h2ph *.h */*.h
.Ve
.PP
If your system supports dynamic loading, for reasons of portability and
sanity you probably ought to use h2xs (also part of the standard perl
distribution).  This tool converts C header files to Perl extensions.
See perlxstut for how to get started with h2xs.
.PP
If your system doesn't support dynamic loading, you still probably
ought to use h2xs.  See perlxstut and ExtUtils::MakeMaker for
more information (in brief, just use \fBmake perl\fR instead of a plain
\&\fBmake\fR to rebuild perl with a new static extension).
.Sh "Why do setuid perl scripts complain about kernel problems?"
.IX Subsection "Why do setuid perl scripts complain about kernel problems?"
Some operating systems have bugs in the kernel that make setuid
scripts inherently insecure.  Perl gives you a number of options
(described in perlsec) to work around such systems.
.Sh "How can I open a pipe both to and from a command?"
.IX Subsection "How can I open a pipe both to and from a command?"
The IPC::Open2 module (part of the standard perl distribution) is an
easy-to-use approach that internally uses \fIpipe()\fR, \fIfork()\fR, and \fIexec()\fR to do
the job.  Make sure you read the deadlock warnings in its documentation,
though (see IPC::Open2).  See
\&\*(L"Bidirectional Communication with Another Process\*(R" in perlipc and
\&\*(L"Bidirectional Communication with Yourself\*(R" in perlipc
.PP
You may also use the IPC::Open3 module (part of the standard perl
distribution), but be warned that it has a different order of
arguments from IPC::Open2 (see IPC::Open3).
.Sh "Why can't I get the output of a command with \fIsystem()\fP?"
.IX Subsection "Why can't I get the output of a command with system()?"
You're confusing the purpose of \fIsystem()\fR and backticks (``).  \fIsystem()\fR
runs a command and returns exit status information (as a 16 bit value:
the low 7 bits are the signal the process died from, if any, and
the high 8 bits are the actual exit value).  Backticks (``) run a
command and return what it sent to \s-1STDOUT\s0.
.PP
.Vb 2
\&        $exit_status   = system("mail\-users");
\&        $output_string = \`ls\`;
.Ve
.Sh "How can I capture \s-1STDERR\s0 from an external command?"
.IX Subsection "How can I capture STDERR from an external command?"
There are three basic ways of running external commands:
.PP
.Vb 3
\&        system $cmd;            # using system()
\&        $output = \`$cmd\`;               # using backticks (\`\`)
\&        open (PIPE, "cmd |");   # using open()
.Ve
.PP
With \fIsystem()\fR, both \s-1STDOUT\s0 and \s-1STDERR\s0 will go the same place as the
script's \s-1STDOUT\s0 and \s-1STDERR\s0, unless the \fIsystem()\fR command redirects them.
Backticks and \fIopen()\fR read \fBonly\fR the \s-1STDOUT\s0 of your command.
.PP
You can also use the \fIopen3()\fR function from IPC::Open3.  Benjamin
Goldberg provides some sample code: