perlfaq5.1

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.

\&
\&        my ($term, $oterm, $echo, $noecho, $fd_stdin);
\&
\&        $fd_stdin = fileno(STDIN);
\&
\&        $term     = POSIX::Termios\->new();
\&        $term\->getattr($fd_stdin);
\&        $oterm     = $term\->getlflag();
\&
\&        $echo     = ECHO | ECHOK | ICANON;
\&        $noecho   = $oterm & ~$echo;
\&
\&        sub cbreak {
\&                $term\->setlflag($noecho);
\&                $term\->setcc(VTIME, 1);
\&                $term\->setattr($fd_stdin, TCSANOW);
\&                }
\&
\&        sub cooked {
\&                $term\->setlflag($oterm);
\&                $term\->setcc(VTIME, 0);
\&                $term\->setattr($fd_stdin, TCSANOW);
\&                }
\&
\&        sub getone {
\&                my $key = \*(Aq\*(Aq;
\&                cbreak();
\&                sysread(STDIN, $key, 1);
\&                cooked();
\&                return $key;
\&                }
\&
\&        }
\&
\&        END { cooked() }
.Ve
.PP
The Term::ReadKey module from \s-1CPAN\s0 may be easier to use.  Recent versions
include also support for non-portable systems as well.
.PP
.Vb 8
\&        use Term::ReadKey;
\&        open(TTY, "</dev/tty");
\&        print "Gimme a char: ";
\&        ReadMode "raw";
\&        $key = ReadKey 0, *TTY;
\&        ReadMode "normal";
\&        printf "\enYou said %s, char number %03d\en",
\&                $key, ord $key;
.Ve
.Sh "How can I tell whether there's a character waiting on a filehandle?"
.IX Subsection "How can I tell whether there's a character waiting on a filehandle?"
The very first thing you should do is look into getting the Term::ReadKey
extension from \s-1CPAN\s0.  As we mentioned earlier, it now even has limited
support for non-portable (read: not open systems, closed, proprietary,
not \s-1POSIX\s0, not Unix, etc) systems.
.PP
You should also check out the Frequently Asked Questions list in
comp.unix.* for things like this: the answer is essentially the same.
It's very system dependent.  Here's one solution that works on \s-1BSD\s0
systems:
.PP
.Vb 5
\&        sub key_ready {
\&                my($rin, $nfd);
\&                vec($rin, fileno(STDIN), 1) = 1;
\&                return $nfd = select($rin,undef,undef,0);
\&                }
.Ve
.PP
If you want to find out how many characters are waiting, there's
also the \s-1FIONREAD\s0 ioctl call to be looked at.  The \fIh2ph\fR tool that
comes with Perl tries to convert C include files to Perl code, which
can be \f(CW\*(C`require\*(C'\fRd.  \s-1FIONREAD\s0 ends up defined as a function in the
\&\fIsys/ioctl.ph\fR file:
.PP
.Vb 1
\&        require \*(Aqsys/ioctl.ph\*(Aq;
\&
\&        $size = pack("L", 0);
\&        ioctl(FH, FIONREAD(), $size)    or die "Couldn\*(Aqt call ioctl: $!\en";
\&        $size = unpack("L", $size);
.Ve
.PP
If \fIh2ph\fR wasn't installed or doesn't work for you, you can
\&\fIgrep\fR the include files by hand:
.PP
.Vb 2
\&        % grep FIONREAD /usr/include/*/*
\&        /usr/include/asm/ioctls.h:#define FIONREAD      0x541B
.Ve
.PP
Or write a small C program using the editor of champions:
.PP
.Vb 9
\&        % cat > fionread.c
\&        #include <sys/ioctl.h>
\&        main() {
\&            printf("%#08x\en", FIONREAD);
\&        }
\&        ^D
\&        % cc \-o fionread fionread.c
\&        % ./fionread
\&        0x4004667f
.Ve
.PP
And then hard code it, leaving porting as an exercise to your successor.
.PP
.Vb 1
\&        $FIONREAD = 0x4004667f;         # XXX: opsys dependent
\&
\&        $size = pack("L", 0);
\&        ioctl(FH, $FIONREAD, $size)     or die "Couldn\*(Aqt call ioctl: $!\en";
\&        $size = unpack("L", $size);
.Ve
.PP
\&\s-1FIONREAD\s0 requires a filehandle connected to a stream, meaning that sockets,
pipes, and tty devices work, but \fInot\fR files.
.ie n .Sh "How do I do a ""tail \-f"" in perl?"
.el .Sh "How do I do a \f(CWtail \-f\fP in perl?"
.IX Xref "tail IO::Handle File::Tail clearerr"
.IX Subsection "How do I do a tail -f in perl?"
First try
.PP
.Vb 1
\&        seek(GWFILE, 0, 1);
.Ve
.PP
The statement \f(CW\*(C`seek(GWFILE, 0, 1)\*(C'\fR doesn't change the current position,
but it does clear the end-of-file condition on the handle, so that the
next \f(CW\*(C`<GWFILE>\*(C'\fR makes Perl try again to read something.
.PP
If that doesn't work (it relies on features of your stdio implementation),
then you need something more like this:
.PP
.Vb 7
\&        for (;;) {
\&          for ($curpos = tell(GWFILE); <GWFILE>; $curpos = tell(GWFILE)) {
\&            # search for some stuff and put it into files
\&          }
\&          # sleep for a while
\&          seek(GWFILE, $curpos, 0);  # seek to where we had been
\&        }
.Ve
.PP
If this still doesn't work, look into the \f(CW\*(C`clearerr\*(C'\fR method
from \f(CW\*(C`IO::Handle\*(C'\fR, which resets the error and end-of-file states
on the handle.
.PP
There's also a \f(CW\*(C`File::Tail\*(C'\fR module from \s-1CPAN\s0.
.Sh "How do I \fIdup()\fP a filehandle in Perl?"
.IX Xref "dup"
.IX Subsection "How do I dup() a filehandle in Perl?"
If you check \*(L"open\*(R" in perlfunc, you'll see that several of the ways
to call \fIopen()\fR should do the trick.  For example:
.PP
.Vb 2
\&        open(LOG, ">>/foo/logfile");
\&        open(STDERR, ">&LOG");
.Ve
.PP
Or even with a literal numeric descriptor:
.PP
.Vb 2
\&   $fd = $ENV{MHCONTEXTFD};
\&   open(MHCONTEXT, "<&=$fd");   # like fdopen(3S)
.Ve
.PP
Note that \*(L"<&STDIN\*(R" makes a copy, but \*(L"<&=STDIN\*(R" make
an alias.  That means if you close an aliased handle, all
aliases become inaccessible.  This is not true with
a copied one.
.PP
Error checking, as always, has been left as an exercise for the reader.
.Sh "How do I close a file descriptor by number?"
.IX Xref "file, closing file descriptors POSIX close"
.IX Subsection "How do I close a file descriptor by number?"
If, for some reason, you have a file descriptor instead of a
filehandle (perhaps you used \f(CW\*(C`POSIX::open\*(C'\fR), you can use the
\&\f(CW\*(C`close()\*(C'\fR function from the \f(CW\*(C`POSIX\*(C'\fR module:
.PP
.Vb 1
\&        use POSIX ();
\&        
\&        POSIX::close( $fd );
.Ve
.PP
This should rarely be necessary, as the Perl \f(CW\*(C`close()\*(C'\fR function is to be
used for things that Perl opened itself, even if it was a dup of a
numeric descriptor as with \f(CW\*(C`MHCONTEXT\*(C'\fR above.  But if you really have
to, you may be able to do this:
.PP
.Vb 3
\&        require \*(Aqsys/syscall.ph\*(Aq;
\&        $rc = syscall(&SYS_close, $fd + 0);  # must force numeric
\&        die "can\*(Aqt sysclose $fd: $!" unless $rc == \-1;
.Ve
.PP
Or, just use the fdopen(3S) feature of \f(CW\*(C`open()\*(C'\fR:
.PP
.Vb 4
\&        {
\&        open my( $fh ), "<&=$fd" or die "Cannot reopen fd=$fd: $!";
\&        close $fh;
\&        }
.Ve
.ie n .Sh "Why can't I use ""C:\etemp\efoo"" in \s-1DOS\s0 paths?  Why doesn't `C:\etemp\efoo.exe` work?"
.el .Sh "Why can't I use ``C:\etemp\efoo'' in \s-1DOS\s0 paths?  Why doesn't `C:\etemp\efoo.exe` work?"
.IX Xref "filename, DOS issues"
.IX Subsection "Why can't I use C:tempfoo in DOS paths?  Why doesn't `C:tempfoo.exe` work?"
Whoops!  You just put a tab and a formfeed into that filename!
Remember that within double quoted strings (\*(L"like\ethis\*(R"), the
backslash is an escape character.  The full list of these is in
\&\*(L"Quote and Quote-like Operators\*(R" in perlop.  Unsurprisingly, you don't
have a file called \*(L"c:(tab)emp(formfeed)oo\*(R" or
\&\*(L"c:(tab)emp(formfeed)oo.exe\*(R" on your legacy \s-1DOS\s0 filesystem.
.PP
Either single-quote your strings, or (preferably) use forward slashes.
Since all \s-1DOS\s0 and Windows versions since something like MS-DOS 2.0 or so
have treated \f(CW\*(C`/\*(C'\fR and \f(CW\*(C`\e\*(C'\fR the same in a path, you might as well use the
one that doesn't clash with Perl\*(--or the \s-1POSIX\s0 shell, \s-1ANSI\s0 C and \*(C+,
awk, Tcl, Java, or Python, just to mention a few.  \s-1POSIX\s0 paths
are more portable, too.
.ie n .Sh "Why doesn't glob(""*.*"") get all the files?"
.el .Sh "Why doesn't glob(``*.*'') get all the files?"
.IX Xref "glob"
.IX Subsection "Why doesn't glob(*.*) get all the files?"
Because even on non-Unix ports, Perl's glob function follows standard
Unix globbing semantics.  You'll need \f(CW\*(C`glob("*")\*(C'\fR to get all (non-hidden)
files.  This makes \fIglob()\fR portable even to legacy systems.  Your
port may include proprietary globbing functions as well.  Check its
documentation for details.
.ie n .Sh "Why does Perl let me delete read-only files?  Why does ""\-i"" clobber protected files?  Isn't this a bug in Perl?"
.el .Sh "Why does Perl let me delete read-only files?  Why does \f(CW\-i\fP clobber protected files?  Isn't this a bug in Perl?"
.IX Subsection "Why does Perl let me delete read-only files?  Why does -i clobber protected files?  Isn't this a bug in Perl?"
This is elaborately and painstakingly described in the
\&\fIfile-dir-perms\fR article in the \*(L"Far More Than You Ever Wanted To
Know\*(R" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .
.PP
The executive summary: learn how your filesystem works.  The
permissions on a file say what can happen to the data in that file.
The permissions on a directory say what can happen to the list of
files in that directory.  If you delete a file, you're removing its
name from the directory (so the operation depends on the permissions
of the directory, not of the file).  If you try to write to the file,
the permissions of the file govern whether you're allowed to.
.Sh "How do I select a random line from a file?"
.IX Xref "file, selecting a random line"
.IX Subsection "How do I select a random line from a file?"
Here's an algorithm from the Camel Book:
.PP
.Vb 2
\&        srand;
\&        rand($.) < 1 && ($line = $_) while <>;
.Ve
.PP
This has a significant advantage in space over reading the whole file
in.  You can find a proof of this method in \fIThe Art of Computer
Programming\fR, Volume 2, Section 3.4.2, by Donald E. Knuth.
.PP
You can use the File::Random module which provides a function
for that algorithm:
.PP
.Vb 2
\&        use File::Random qw/random_line/;
\&        my $line = random_line($filename);
.Ve
.PP
Another way is to use the Tie::File module, which treats the entire
file as an array.  Simply access a random array element.
.Sh "Why do I get weird spaces when I print an array of lines?"
.IX Subsection "Why do I get weird spaces when I print an array of lines?"
Saying
.PP
.Vb 1
\&        print "@lines\en";
.Ve
.PP
joins together the elements of \f(CW@lines\fR with a space between them.
If \f(CW@lines\fR were \f(CW\*(C`("little", "fluffy", "clouds")\*(C'\fR then the above
statement would print
.PP
.Vb 1
\&        little fluffy clouds
.Ve
.PP
but if each element of \f(CW@lines\fR was a line of text, ending a newline
character \f(CW\*(C`("little\en", "fluffy\en", "clouds\en")\*(C'\fR then it would print:
.PP
.Vb 3
\&        little
\&         fluffy
\&         clouds
.Ve
.PP
If your array contains lines, just print them:
.PP
.Vb 1
\&        print @lines;
.Ve
.SH "REVISION"
.IX Header "REVISION"
Revision: \f(CW$Revision:\fR 10126 $
.PP
Date: \f(CW$Date:\fR 2007\-10\-27 21:29:20 +0200 (Sat, 27 Oct 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 here are in the public
domain.  You are permitted and encouraged to use this code and any
derivatives thereof in your own programs for fun or for profit as you
see fit.  A simple comment in the code giving credit to the \s-1FAQ\s0 would
be courteous but is not required.