perlfaq5.pod

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.

or a scalar variable containing one:

	($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
	print $ofh "Type it: ";
	$got = <$ifh>
	print $efh "What was that: $got";

If you're passing a filehandle to a function, you can write
the function in two ways:

	sub accept_fh {
		my $fh = shift;
		print $fh "Sending to indirect filehandle\n";
	}

Or it can localize a typeglob and use the filehandle directly:

	sub accept_fh {
		local *FH = shift;
		print  FH "Sending to localized filehandle\n";
	}

Both styles work with either objects or typeglobs of real filehandles.
(They might also work with strings under some circumstances, but this
is risky.)

	accept_fh(*STDOUT);
	accept_fh($handle);

In the examples above, we assigned the filehandle to a scalar variable
before using it.  That is because only simple scalar variables, not
expressions or subscripts of hashes or arrays, can be used with
built-ins like C<print>, C<printf>, or the diamond operator.  Using
something other than a simple scalar variable as a filehandle is
illegal and won't even compile:

	@fd = (*STDIN, *STDOUT, *STDERR);
	print $fd[1] "Type it: ";                           # WRONG
	$got = <$fd[0]>                                     # WRONG
	print $fd[2] "What was that: $got";                 # WRONG

With C<print> and C<printf>, you get around this by using a block and
an expression where you would place the filehandle:

	print  { $fd[1] } "funny stuff\n";
	printf { $fd[1] } "Pity the poor %x.\n", 3_735_928_559;
	# Pity the poor deadbeef.

That block is a proper block like any other, so you can put more
complicated code there.  This sends the message out to one of two places:

	$ok = -x "/bin/cat";
	print { $ok ? $fd[1] : $fd[2] } "cat stat $ok\n";
	print { $fd[ 1+ ($ok || 0) ]  } "cat stat $ok\n";

This approach of treating C<print> and C<printf> like object methods
calls doesn't work for the diamond operator.  That's because it's a
real operator, not just a function with a comma-less argument.  Assuming
you've been storing typeglobs in your structure as we did above, you
can use the built-in function named C<readline> to read a record just
as C<< <> >> does.  Given the initialization shown above for @fd, this
would work, but only because readline() requires a typeglob.  It doesn't
work with objects or strings, which might be a bug we haven't fixed yet.

	$got = readline($fd[0]);

Let it be noted that the flakiness of indirect filehandles is not
related to whether they're strings, typeglobs, objects, or anything else.
It's the syntax of the fundamental operators.  Playing the object
game doesn't help you at all here.

=head2 How can I set up a footer format to be used with write()?
X<footer>

There's no builtin way to do this, but L<perlform> has a couple of
techniques to make it possible for the intrepid hacker.

=head2 How can I write() into a string?
X<write, into a string>

See L<perlform/"Accessing Formatting Internals"> for an C<swrite()> function.

=head2 How can I open a filehandle to a string?
X<string>, X<open>, X<IO::Scalar>, X<filehandle>

(contributed by Peter J. Holzer, [e-mail])

Since Perl 5.8.0, you can pass a reference to a scalar instead of the
filename to create a file handle which you can used to read from or write to
a string:

	open(my $fh, '>', \$string) or die "Could not open string for writing";
	print $fh "foo\n";
	print $fh "bar\n";	# $string now contains "foo\nbar\n"

	open(my $fh, '<', \$string) or die "Could not open string for reading";
	my $x = <$fh>;	# $x now contains "foo\n"

With older versions of Perl, the C<IO::String> module provides similar
functionality.
    
=head2 How can I output my numbers with commas added?
X<number, commify>

(contributed by brian d foy and Benjamin Goldberg)

You can use L<Number::Format> to separate places in a number.
It handles locale information for those of you who want to insert
full stops instead (or anything else that they want to use,
really).

This subroutine will add commas to your number:

	sub commify {
		local $_  = shift;
		1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
		return $_;
		}

This regex from Benjamin Goldberg will add commas to numbers:

	s/(^[-+]?\d+?(?=(?>(?:\d{3})+)(?!\d))|\G\d{3}(?=\d))/$1,/g;

It is easier to see with comments:

	s/(
		^[-+]?             # beginning of number.
		\d+?               # first digits before first comma
		(?=                # followed by, (but not included in the match) :
			(?>(?:\d{3})+) # some positive multiple of three digits.
			(?!\d)         # an *exact* multiple, not x * 3 + 1 or whatever.
		)
		|                  # or:
		\G\d{3}            # after the last group, get three digits
		(?=\d)             # but they have to have more digits after them.
	)/$1,/xg;

=head2 How can I translate tildes (~) in a filename?
X<tilde> X<tilde expansion>

Use the <> (glob()) operator, documented in L<perlfunc>.  Older
versions of Perl require that you have a shell installed that groks
tildes.  Recent perl versions have this feature built in. The
File::KGlob module (available from CPAN) gives more portable glob
functionality.

Within Perl, you may use this directly:

	$filename =~ s{
	  ^ ~             # find a leading tilde
	  (               # save this in $1
	      [^/]        # a non-slash character
	            *     # repeated 0 or more times (0 means me)
	  )
	}{
	  $1
	      ? (getpwnam($1))[7]
	      : ( $ENV{HOME} || $ENV{LOGDIR} )
	}ex;

=head2 How come when I open a file read-write it wipes it out?
X<clobber> X<read-write> X<clobbering> X<truncate> X<truncating>

Because you're using something like this, which truncates the file and
I<then> gives you read-write access:

	open(FH, "+> /path/name");		# WRONG (almost always)

Whoops.  You should instead use this, which will fail if the file
doesn't exist.

	open(FH, "+< /path/name");  	# open for update

Using ">" always clobbers or creates.  Using "<" never does
either.  The "+" doesn't change this.

Here are examples of many kinds of file opens.  Those using sysopen()
all assume

	use Fcntl;

To open file for reading:

	open(FH, "< $path")                                 || die $!;
	sysopen(FH, $path, O_RDONLY)                        || die $!;

To open file for writing, create new file if needed or else truncate old file:

	open(FH, "> $path") || die $!;
	sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT)        || die $!;
	sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666)  || die $!;

To open file for writing, create new file, file must not exist:

	sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT)         || die $!;
	sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666)   || die $!;

To open file for appending, create if necessary:

	open(FH, ">> $path") || die $!;
	sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT)       || die $!;
	sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) || die $!;

To open file for appending, file must exist:

	sysopen(FH, $path, O_WRONLY|O_APPEND)               || die $!;

To open file for update, file must exist:

	open(FH, "+< $path")                                || die $!;
	sysopen(FH, $path, O_RDWR)                          || die $!;

To open file for update, create file if necessary:

	sysopen(FH, $path, O_RDWR|O_CREAT)                  || die $!;
	sysopen(FH, $path, O_RDWR|O_CREAT, 0666)            || die $!;

To open file for update, file must not exist:

	sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT)           || die $!;
	sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666)     || die $!;

To open a file without blocking, creating if necessary:

	sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
	    or die "can't open /foo/somefile: $!":

Be warned that neither creation nor deletion of files is guaranteed to
be an atomic operation over NFS.  That is, two processes might both
successfully create or unlink the same file!  Therefore O_EXCL
isn't as exclusive as you might wish.

See also the new L<perlopentut> if you have it (new for 5.6).

=head2 Why do I sometimes get an "Argument list too long" when I use E<lt>*E<gt>?
X<argument list too long>

The C<< <> >> operator performs a globbing operation (see above).
In Perl versions earlier than v5.6.0, the internal glob() operator forks
csh(1) to do the actual glob expansion, but
csh can't handle more than 127 items and so gives the error message
C<Argument list too long>.  People who installed tcsh as csh won't
have this problem, but their users may be surprised by it.

To get around this, either upgrade to Perl v5.6.0 or later, do the glob
yourself with readdir() and patterns, or use a module like File::KGlob,
one that doesn't use the shell to do globbing.

=head2 Is there a leak/bug in glob()?
X<glob>

Due to the current implementation on some operating systems, when you
use the glob() function or its angle-bracket alias in a scalar
context, you may cause a memory leak and/or unpredictable behavior.  It's
best therefore to use glob() only in list context.

=head2 How can I open a file with a leading ">" or trailing blanks?
X<filename, special characters>

(contributed by Brian McCauley)

The special two argument form of Perl's open() function ignores
trailing blanks in filenames and infers the mode from certain leading
characters (or a trailing "|"). In older versions of Perl this was the
only version of open() and so it is prevalent in old code and books.

Unless you have a particular reason to use the two argument form you
should use the three argument form of open() which does not treat any
characters in the filename as special.

	open FILE, "<", "  file  ";  # filename is "   file   "
	open FILE, ">", ">file";     # filename is ">file"

=head2 How can I reliably rename a file?
X<rename> X<mv> X<move> X<file, rename> X<ren>

If your operating system supports a proper mv(1) utility or its
functional equivalent, this works:

	rename($old, $new) or system("mv", $old, $new);

It may be more portable to use the File::Copy module instead.
You just copy to the new file to the new name (checking return
values), then delete the old one.  This isn't really the same
semantically as a rename(), which preserves meta-information like
permissions, timestamps, inode info, etc.

Newer versions of File::Copy export a move() function.

=head2 How can I lock a file?
X<lock> X<file, lock> X<flock>

Perl's builtin flock() function (see L<perlfunc> for details) will call
flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and
later), and lockf(3) if neither of the two previous system calls exists.
On some systems, it may even use a different form of native locking.
Here are some gotchas with Perl's flock():

=over 4

=item 1

Produces a fatal error if none of the three system calls (or their
close equivalent) exists.

=item 2

lockf(3) does not provide shared locking, and requires that the
filehandle be open for writing (or appending, or read/writing).

=item 3

Some versions of flock() can't lock files over a network (e.g. on NFS file
systems), so you'd need to force the use of fcntl(2) when you build Perl.
But even this is dubious at best.  See the flock entry of L<perlfunc>
and the F<INSTALL> file in the source distribution for information on
building Perl to do this.

Two potentially non-obvious but traditional flock semantics are that
it waits indefinitely until the lock is granted, and that its locks are
I<merely advisory>.  Such discretionary locks are more flexible, but
offer fewer guarantees.  This means that files locked with flock() may
be modified by programs that do not also use flock().  Cars that stop
for red lights get on well with each other, but not with cars that don't
stop for red lights.  See the perlport manpage, your port's specific
documentation, or your system-specific local manpages for details.  It's
best to assume traditional behavior if you're writing portable programs.
(If you're not, you should as always feel perfectly free to write
for your own system's idiosyncrasies (sometimes called "features").
Slavish adherence to portability concerns shouldn't get in the way of
your getting your job done.)

For more information on file locking, see also
L<perlopentut/"File Locking"> if you have it (new for 5.6).

=back

=head2 Why can't I just open(FH, "E<gt>file.lock")?
X<lock, lockfile race condition>

A common bit of code B<NOT TO USE> is this:

	sleep(3) while -e "file.lock";	# PLEASE DO NOT USE
	open(LCK, "> file.lock");		# THIS BROKEN CODE

This is a classic race condition: you take two steps to do something
which must be done in one.  That's why computer hardware provides an
atomic test-and-set instruction.   In theory, this "ought" to work:

	sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
		or die "can't open  file.lock: $!";

except that lamentably, file creation (and deletion) is not atomic
over NFS, so this won't work (at least, not every time) over the net.
Various schemes involving link() have been suggested, but
these tend to involve busy-wait, which is also less than desirable.

=head2 I still don't get locking.  I just want to increment the number in the file.  How can I do this?
X<counter> X<file, counter>

Didn't anyone ever tell you web-page hit counters were useless?
They don't count number of hits, they're a waste of time, and they serve
only to stroke the writer's vanity.  It's better to pick a random number;
they're more realistic.

Anyway, this is what you can do if you can't help yourself.

	use Fcntl qw(:DEFAULT :flock);
	sysopen(FH, "numfile", O_RDWR|O_CREAT) 	 or die "can't open numfile: $!";
	flock(FH, LOCK_EX) 				 or die "can't flock numfile: $!";
	$num = <FH> || 0;
	seek(FH, 0, 0) 				 or die "can't rewind numfile: $!";
	truncate(FH, 0) 				 or die "can't truncate numfile: $!";
	(print FH $num+1, "\n")			 or die "can't write numfile: $!";
	close FH 					 or die "can't close numfile: $!";

Here's a much better web-page hit counter:

	$hits = int( (time() - 850_000_000) / rand(1_000) );

If the count doesn't impress your friends, then the code might.  :-)

=head2 All I want to do is append a small amount of text to the end of a file.  Do I still have to use locking?
X<append> X<file, append>

If you are on a system that correctly implements flock() and you use the
example appending code from "perldoc -f flock" everything will be OK
even if the OS you are on doesn't implement append mode correctly (if
such a system exists.) So if you are happy to restrict yourself to OSs
that implement flock() (and that's not really much of a restriction)
then that is what you should do.

If you know you are only going to use a system that does correctly
implement appending (i.e. not Win32) then you can omit the seek() from
the code in the previous answer.

If you know you are only writing code to run on an OS and filesystem that
does implement append mode correctly (a local filesystem on a modern
Unix for example), and you keep the file in block-buffered mode and you
write less than one buffer-full of output between each manual flushing
of the buffer then each bufferload is almost guaranteed to be written to
the end of the file in one chunk without getting intermingled with
anyone else's output. You can also use the syswrite() function which is
simply a wrapper around your systems write(2) system call.

There is still a small theoretical chance that a signal will interrupt
the system level write() operation before completion.  There is also a
possibility that some STDIO implementations may call multiple system
level write()s even if the buffer was empty to start.  There may be some
systems where this probability is reduced to zero.

=head2 How do I randomly update a binary file?
X<file, binary patch>

If you're just trying to patch a binary, in many cases something as
simple as this works:

	perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs

However, if you have fixed sized records, then you might do something more
like this:

	$RECSIZE = 220; # size of record, in bytes
	$recno   = 37;  # which record to update
	open(FH, "+<somewhere") || die "can't update somewhere: $!";
	seek(FH, $recno * $RECSIZE, 0);
	read(FH, $record, $RECSIZE) == $RECSIZE || die "can't read record $recno: $!";
	# munge the record
	seek(FH, -$RECSIZE, 1);
	print FH $record;
	close FH;

Locking and error checking are left as an exercise for the reader.
Don't forget them or you'll be quite sorry.

=head2 How do I get a file's timestamp in perl?
X<timestamp> X<file, timestamp>

If you want to retrieve the time at which the file was last
read, written, or had its meta-data (owner, etc) changed,
you use the B<-A>, B<-M>, or B<-C> file test operations as
documented in L<perlfunc>.  These retrieve the age of the
file (measured against the start-time of your program) in