perlfaq8.pod

ARM上的如果你对底层感兴趣

=head1 NAME

perlfaq8 - System Interaction ($Revision: 1.26 $, $Date: 1998/08/05 12:20:28 $)

=head1 DESCRIPTION

This section of the Perl FAQ covers questions involving operating
system interaction.  This involves interprocess communication (IPC),
control over the user-interface (keyboard, screen and pointing
devices), and most anything else not related to data manipulation.

Read the FAQs and documentation specific to the port of perl to your
operating system (eg, L<perlvms>, L<perlplan9>, ...).  These should
contain more detailed information on the vagaries of your perl.

=head2 How do I find out which operating system I'm running under?

The $^O variable ($OSNAME if you use English) contains the operating
system that your perl binary was built for.

=head2 How come exec() doesn't return?

Because that's what it does: it replaces your currently running
program with a different one.  If you want to keep going (as is
probably the case if you're asking this question) use system()
instead.

=head2 How do I do fancy stuff with the keyboard/screen/mouse?

How you access/control keyboards, screens, and pointing devices
("mice") is system-dependent.  Try the following modules:

=over 4

=item Keyboard

    Term::Cap			Standard perl distribution
    Term::ReadKey		CPAN
    Term::ReadLine::Gnu		CPAN
    Term::ReadLine::Perl	CPAN
    Term::Screen		CPAN

=item Screen

    Term::Cap			Standard perl distribution
    Curses			CPAN
    Term::ANSIColor		CPAN

=item Mouse

    Tk				CPAN

=back

Some of these specific cases are shown below.

=head2 How do I print something out in color?

In general, you don't, because you don't know whether
the recipient has a color-aware display device.  If you
know that they have an ANSI terminal that understands
color, you can use the Term::ANSIColor module from CPAN:

    use Term::ANSIColor;
    print color("red"), "Stop!\n", color("reset");
    print color("green"), "Go!\n", color("reset");

Or like this:

    use Term::ANSIColor qw(:constants);
    print RED, "Stop!\n", RESET;
    print GREEN, "Go!\n", RESET;

=head2 How do I read just one key without waiting for a return key?

Controlling input buffering is a remarkably system-dependent matter.
If most systems, you can just use the B<stty> command as shown in
L<perlfunc/getc>, but as you see, that's already getting you into
portability snags.  

    open(TTY, "+</dev/tty") or die "no tty: $!";
    system "stty  cbreak </dev/tty >/dev/tty 2>&1";
    $key = getc(TTY);		# perhaps this works
    # OR ELSE
    sysread(TTY, $key, 1);	# probably this does
    system "stty -cbreak </dev/tty >/dev/tty 2>&1";

The Term::ReadKey module from CPAN offers an easy-to-use interface that
should be more efficient than shelling out to B<stty> for each key.
It even includes limited support for Windows.

    use Term::ReadKey;
    ReadMode('cbreak');
    $key = ReadKey(0);
    ReadMode('normal');

However, that requires that you have a working C compiler and can use it
to build and install a CPAN module.  Here's a solution using
the standard POSIX module, which is already on your systems (assuming
your system supports POSIX).

    use HotKey;
    $key = readkey();

And here's the HotKey module, which hides the somewhat mystifying calls
to manipulate the POSIX termios structures.

    # HotKey.pm
    package HotKey;

    @ISA = qw(Exporter);
    @EXPORT = qw(cbreak cooked readkey);

    use strict;
    use POSIX qw(:termios_h);
    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);  # ok, so i don't want echo either
        $term->setcc(VTIME, 1);
        $term->setattr($fd_stdin, TCSANOW);
    }

    sub cooked {
        $term->setlflag($oterm);
        $term->setcc(VTIME, 0);
        $term->setattr($fd_stdin, TCSANOW);
    }

    sub readkey {
        my $key = '';
        cbreak();
        sysread(STDIN, $key, 1);
        cooked();
        return $key;
    }

    END { cooked() }

    1;

=head2 How do I check whether input is ready on the keyboard?

The easiest way to do this is to read a key in nonblocking mode with the
Term::ReadKey module from CPAN, passing it an argument of -1 to indicate
not to block:

    use Term::ReadKey;

    ReadMode('cbreak');

    if (defined ($char = ReadKey(-1)) ) {
        # input was waiting and it was $char
    } else {
        # no input was waiting
    }

    ReadMode('normal');                  # restore normal tty settings

=head2 How do I clear the screen?

If you only have to so infrequently, use C<system>:

    system("clear");

If you have to do this a lot, save the clear string
so you can print it 100 times without calling a program
100 times:

    $clear_string = `clear`;
    print $clear_string;

If you're planning on doing other screen manipulations, like cursor
positions, etc, you might wish to use Term::Cap module:

    use Term::Cap;
    $terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
    $clear_string = $terminal->Tputs('cl');

=head2 How do I get the screen size?

If you have Term::ReadKey module installed from CPAN, 
you can use it to fetch the width and height in characters
and in pixels:

    use Term::ReadKey;
    ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();

This is more portable than the raw C<ioctl>, but not as 
illustrative:

    require 'sys/ioctl.ph';
    die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
    open(TTY, "+</dev/tty")                     or die "No tty: $!";
    unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) {
        die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ;
    }
    ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
    print "(row,col) = ($row,$col)";
    print "  (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
    print "\n";

=head2 How do I ask the user for a password?

(This question has nothing to do with the web.  See a different
FAQ for that.)

There's an example of this in L<perlfunc/crypt>).  First, you put
the terminal into "no echo" mode, then just read the password
normally.  You may do this with an old-style ioctl() function, POSIX
terminal control (see L<POSIX>, and Chapter 7 of the Camel), or a call
to the B<stty> program, with varying degrees of portability.

You can also do this for most systems using the Term::ReadKey module
from CPAN, which is easier to use and in theory more portable.

    use Term::ReadKey;

    ReadMode('noecho');
    $password = ReadLine(0);

=head2 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, the devices names will doubtless differ.
Several problem areas common to all device interaction are the
following

=over 4

=item lockfiles

Your system may use lockfiles to control multiple access.  Make sure
you follow the correct protocol.  Unpredictable behaviour can result
from multiple processes reading from one device.

=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<perlfunc/"open"> for
details).  You may wish to open it without running the risk of
blocking by using sysopen() and C<O_RDWR|O_NDELAY|O_NOCTTY> from the
Fcntl module (part of the standard perl distribution).  See
L<perlfunc/"sysopen"> for more on this approach.

=item end of line

Some devices will be expecting a "\r" at the end of each line rather
than a "\n".  In some ports of perl, "\r" and "\n" are different from
their usual (Unix) ASCII values of "\012" and "\015".  You may have to
give the numeric values you want directly, using octal ("\015"), hex
("0x0D"), or as a control-character specification ("\cM").

    print DEV "atv1\012";	# wrong, for some devices
    print DEV "atv1\015";	# right, for some devices

Even though with normal text files, a "\n" 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 I<ALL> line
ends with "\015\012", and strip what you don't need from the output.
This applies especially to socket I/O and autoflushing, discussed
next.

=item flushing output

If you expect characters to get to your device when you print() them,
you'll want to autoflush that filehandle.  You can use select()
and the C<$|> variable to control autoflushing (see L<perlvar/$|>
and L<perlfunc/select>):

    $oldh = select(DEV);
    $| = 1;
    select($oldh);

You'll also see code that does this without a temporary variable, as in

    select((select(DEV), $| = 1)[0]);

Or if you don't mind pulling in a few thousand lines
of code just because you're afraid of a little $| variable:

    use IO::Handle;
    DEV->autoflush(1);

As mentioned in the previous item, this still doesn't work when using
socket I/O between Unix and Macintosh.  You'll need to hardcode your
line terminators, in that case.

=item non-blocking input

If you are doing a blocking read() or sysread(), you'll have to
arrange for an alarm handler to provide a timeout (see
L<perlfunc/alarm>).  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
select() to determine whether I/O is ready on that device (see
L<perlfunc/"select">.

=back

While trying to read from his caller-id box, the notorious Jamie Zawinski
<[e-mail]>, after much gnashing of teeth and fighting with sysread,
sysopen, POSIX's tcgetattr business, and various other functions that
go bump in the night, finally came up with this:

    sub open_modem {
	use IPC::Open2;
	my $stty = `/bin/stty -g`;
	open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
	# starting cu hoses /dev/tty's stty settings, even when it has
	# been opened on a pipe...
	system("/bin/stty $stty");
	$_ = <MODEM_IN>;
	chop;
	if ( !m/^Connected/ ) {
	    print STDERR "$0: cu printed `$_' instead of `Connected'\n";
	}
    }


=head2 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.

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 check is 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.

If you're worried about users selecting bad passwords, you should
proactively check when they try to change their password (by modifying
passwd(1), for example).

=head2 How do I start a process in the background?

You could use

    system("cmd &")

or you could use fork as documented in L<perlfunc/"fork">, with
further examples in L<perlipc>.  Some things to be aware of, if you're
on a Unix-like system:

=over 4

=item STDIN, STDOUT, and STDERR are shared