applied patch (via private mail), modulo retrohunks in pod/perlfaq2.pod

	Subject: Re: docpatch 
	Message-Id: <199807051515.JAA03644@jhereg.perl.com>

p4raw-id: //depot/perl@1325

1 files changed, 310 insertions, 100 deletions

diff --git a/pod/perlfaq8.pod b/pod/perlfaq8.pod
index f4addd8c4c..272dd51229 100644
--- a/pod/perlfaq8.pod
+++ b/pod/perlfaq8.pod
@@ -1,6 +1,6 @@
 =head1 NAME
 
-perlfaq8 - System Interaction ($Revision: 1.21 $, $Date: 1997/04/24 22:44:19 $)
+perlfaq8 - System Interaction ($Revision: 1.25 $, $Date: 1998/07/05 15:07:20 $)
 
 =head1 DESCRIPTION
 
@@ -15,7 +15,7 @@ contain more detailed information on the vagaries of your perl.
 
 =head2 How do I find out which operating system I'm running under?
 
-The $^O variable ($OSTYPE if you use English) contains the operating
+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?
@@ -52,6 +52,162 @@ How you access/control keyboards, screens, and pointing devices
 
 =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
@@ -66,6 +222,11 @@ 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
@@ -112,18 +273,9 @@ next.
 =item flushing output
 
 If you expect characters to get to your device when you print() them,
-you'll want to autoflush that filehandle, as in the older
-
-    use FileHandle;
-    DEV->autoflush(1);
-
-and the newer
-
-    use IO::Handle;
-    DEV->autoflush(1);
-
-You can use select() and the C<$|> variable to control autoflushing
-(see L<perlvar/$|> and L<perlfunc/select>):
+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;
@@ -133,6 +285,12 @@ 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.
@@ -148,15 +306,38 @@ L<perlfunc/"select">.
 
 =back
 
+While trying to read from his caller-id box, the notorious Jamie Zawinski
+<jwz@netscape.com>, 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.  Programs like Crack can
-forcibly (and intelligently) try to guess passwords, but don't (can't)
-guarantee quick success.
+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
@@ -174,7 +355,7 @@ on a Unix-like system:
 
 =over 4
 
-=item STDIN, STDOUT and STDERR are shared
+=item STDIN, STDOUT, and STDERR are shared
 
 Both the main process and the backgrounded one (the "child" process)
 share the same STDIN, STDOUT and STDERR filehandles.  If both try to
@@ -204,9 +385,10 @@ Zombies are not an issue with C<system("prog &")>.
 
 =head2 How do I trap control characters/signals?
 
-You don't actually "trap" a control character.  Instead, that
-character generates a signal, which you then trap.  Signals are
-documented in L<perlipc/"Signals"> and chapter 6 of the Camel.
+You don't actually "trap" 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<perlipc/"Signals"> and chapter 6 of the Camel.
 
 Be warned that very few C libraries are re-entrant.  Therefore, if you
 attempt to print() in a handler that got invoked during another stdio
@@ -235,10 +417,11 @@ blocking flock() in L<perlipc/"Signals"> or chapter 6 of the Camel.
 
 =head2 How do I modify the shadow password file on a Unix system?
 
-If perl was installed correctly, the getpw*() functions described in
-L<perlfunc> provide (read-only) access to the shadow password file.
-To change the file, make a new shadow password file (the format varies
-from system to system - see L<passwd(5)> for specifics) and use
+If perl was installed correctly, and your shadow library was written
+properly, the getpw*() functions described in L<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 L<passwd(5)> for specifics) and use
 pwd_mkdb(8) to install it (see L<pwd_mkdb(5)> for more details).
 
 =head2 How do I set the time and date?
@@ -254,7 +437,7 @@ probably get away with setting an environment variable:
 
     $ENV{TZ} = "MST7MDT";		   # unixish
     $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
-    system "trn comp.lang.perl";
+    system "trn comp.lang.perl.misc";
 
 =head2 How can I sleep() or alarm() for under a second?
 
@@ -303,9 +486,17 @@ then you may be able to do something like this:
 
 Release 5 of Perl added the END block, which can be used to simulate
 atexit().  Each package's END block is called when the program or
-thread ends (see L<perlmod> manpage for more details).  It isn't
-called when untrapped signals kill the program, though, so if you use
-END blocks you should also use
+thread ends (see L<perlmod> manpage for more details).  
+
+For example, you can use this to make sure your filter program
+managed to finish its output without filling up the disk:
+
+    END {
+	close(STDOUT) || die "stdout close failed: $!";
+    } 
+
+The END block isn't called when untrapped signals kill the program, though, so if
+you use END blocks you should also use
 
 	use sigtrap qw(die normal-signals);
 
@@ -375,15 +566,21 @@ scripts inherently insecure.  Perl gives you a number of options
 =head2 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 pipe(), fork(), and exec()
-to do the job.  Make sure you read the deadlock warnings in its
-documentation, though (see L<IPC::Open2>).
+easy-to-use approach that internally uses pipe(), fork(), and exec() to do
+the job.  Make sure you read the deadlock warnings in its documentation,
+though (see L<IPC::Open2>).  See L<perlipc/"Bidirectional Communication
+with Another Process"> and L<perlipc/"Bidirectional Communication with
+Yourself">
+
+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 L<IPC::Open3>).
 
 =head2 Why can't I get the output of a command with system()?
 
 You're confusing the purpose of system() and backticks (``).  system()
 runs a command and returns exit status information (as a 16 bit value:
-the low 8 bits are the signal the process died from, if any, and
+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 STDOUT.
 
@@ -434,15 +631,51 @@ Note that you I<must> use Bourne shell (sh(1)) redirection syntax in
 backticks, not csh(1)!  Details on why Perl's system() and backtick
 and pipe opens all use the Bourne shell are in
 http://www.perl.com/CPAN/doc/FMTEYEWTK/versus/csh.whynot .
+To capture a command's STDERR and STDOUT together:
 
-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 L<IPC::Open3>).
+    $output = `cmd 2>&1`;                       # either with backticks
+    $pid = open(PH, "cmd 2>&1 |");              # or with an open pipe
+    while (<PH>) { }                            #    plus a read
+
+To capture a command's STDOUT but discard its STDERR:
+
+    $output = `cmd 2>/dev/null`;                # either with backticks
+    $pid = open(PH, "cmd 2>/dev/null |");       # or with an open pipe
+    while (<PH>) { }                            #    plus a read
+
+To capture a command's STDERR but discard its STDOUT:
+
+    $output = `cmd 2>&1 1>/dev/null`;           # either with backticks
+    $pid = open(PH, "cmd 2>&1 1>/dev/null |");  # or with an open pipe
+    while (<PH>) { }                            #    plus a read
+
+To exchange a command's STDOUT and STDERR in order to capture the STDERR
+but leave its STDOUT to come out our old STDERR:
+
+    $output = `cmd 3>&1 1>&2 2>&3 3>&-`;        # either with backticks
+    $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
+    while (<PH>) { }                            #    plus a read
+
+To read both a command's STDOUT and its STDERR separately, it's easiest
+and safest to redirect them separately to files, and then read from those
+files when the program is done:
+
+    system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
+
+Ordering is important in all these examples.  That's because the shell
+processes file descriptor redirections in strictly left to right order.
+
+    system("prog args 1>tmpfile 2>&1");
+    system("prog args 2>&1 1>tmpfile");
+
+The first command sends both standard out and standard error to the
+temporary file.  The second command sends only the old standard output
+there, and the old standard error shows up on the old standard out.
 
 =head2 Why doesn't open() return an error when a pipe open fails?
 
 It does, but probably not how you expect it to.  On systems that
-follow the standard fork()/exec() paradigm (eg, Unix), it works like
+follow the standard fork()/exec() paradigm (such as Unix), it works like
 this: open() causes a fork().  In the parent, open() returns with the
 process ID of the child.  The child exec()s the command to be piped
 to/from.  The parent can't know whether the exec() was successful or
@@ -487,7 +720,7 @@ In most cases, this could and probably should be written as
 	or die "cat program failed!";
 
 Which will get the output quickly (as its generated, instead of only
-at the end ) and also check the return value.
+at the end) and also check the return value.
 
 system() also provides direct control over whether shell wildcard
 processing may take place, whereas backticks do not.
@@ -513,6 +746,8 @@ You have to do this:
 
 Just as with system(), no shell escapes happen when you exec() a list.
 
+There are more examples of this L<perlipc/"Safe Pipe Opens">.
+
 =head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
 
 Because some stdio's set error and eof flags that need clearing.  The
@@ -582,9 +817,10 @@ approach will suffice:
 =head2 How can I write expect in Perl?
 
 Once upon a time, there was a library called chat2.pl (part of the
-standard perl distribution), which never really got finished.  These
-days, your best bet is to look at the Comm.pl library available from
-CPAN.
+standard perl distribution), which never really got finished.  If you
+find it somewhere, I<don't use it>.  These days, your best bet is to
+look at the Expect module available from CPAN, which also requires two
+other modules from CPAN, IO::Pty and IO::Stty.
 
 =head2 Is there a way to hide perl's command line from programs such as "ps"?
 
@@ -614,10 +850,6 @@ created after the change.  There is shell magic that may allow you to
 fake it by eval()ing the script's output in your shell; check out the
 comp.unix.questions FAQ for details.
 
-=item VMS
-
-Change to %ENV persist after Perl exits, but directory changes do not.
-
 =back
 
 =head2 How do I close a process's filehandle without waiting for it to complete?
@@ -638,7 +870,8 @@ module for other solutions.
 =item *
 
 Open /dev/tty and use the the TIOCNOTTY ioctl on it.  See L<tty(4)>
-for details.
+for details.  Or better yet, you can just use the POSIX::setsid()
+function, so you don't have to worry about process groups.
 
 =item *
 
@@ -705,12 +938,17 @@ in L<perlfunc/fork>.
 There are a number of excellent interfaces to SQL databases.  See the
 DBD::* modules available from
 http://www.perl.com/CPAN/modules/dbperl/DBD .
+A lot of information on this can be found at 
+http://www.hermetica.com/technologia/perl/DBI/index.html .
 
 =head2 How do I make a system() exit on control-C?
 
 You can't.  You need to imitate the system() call (see L<perlipc> for
 sample code) and then have a signal handler for the INT signal that
-passes the signal on to the subprocess.
+passes the signal on to the subprocess.  Or you can check for it:
+
+    $rc = system($cmd);
+    if ($rc & 127) { die "signal death" } 
 
 =head2 How do I open a file without blocking?
 
@@ -758,9 +996,31 @@ If your version of perl is compiled without dynamic loading, then you
 just need to replace step 3 (B<make>) with B<make perl> and you will
 get a new F<perl> binary with your extension linked in.
 
-See L<ExtUtils::MakeMaker> for more details on building extensions
-and an answer to the question "How do I keep my own module/library
-directory?"
+See L<ExtUtils::MakeMaker> for more details on building extensions.
+See also the next question.
+
+=head2 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:
+
+    1)  do $file is like eval `cat $file`, except the former:
+	1.1: searches @INC.
+	1.2: bequeaths an *unrelated* lexical scope on the eval'ed 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's 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.
+
+In general, you usually want C<use> and a proper Perl module.
 
 =head2 How do I keep my own module/library directory?
 
@@ -796,56 +1056,6 @@ The latter 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.
 
-=head1 How do I get one key from the terminal at a time, under POSIX?
-
-    #!/usr/bin/perl -w
-    use strict;
-    $| = 1;
-    for (1..4) {
-        my $got;
-        print "gimme: ";
-        $got = getone();
-        print "--> $got\n";
-    }
-    exit;
-
-    BEGIN {
-        use POSIX qw(:termios_h);
-
-        my ($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);
-            $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 = '';
-            cbreak();
-            sysread(STDIN, $key, 1);
-            cooked();
-            return $key;
-        }
-
-    }
-    END { cooked() }
-
 =head1 AUTHOR AND COPYRIGHT
 
 Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington.