perlfunc.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.

.PD 0
.IP "abs" 8
.IX Item "abs"
.PD
Returns the absolute value of its argument.
If \s-1VALUE\s0 is omitted, uses \f(CW$_\fR.
.IP "accept \s-1NEWSOCKET\s0,GENERICSOCKET" 8
.IX Xref "accept"
.IX Item "accept NEWSOCKET,GENERICSOCKET"
Accepts an incoming socket connect, just as the \fIaccept\fR\|(2) system call
does.  Returns the packed address if it succeeded, false otherwise.
See the example in \*(L"Sockets: Client/Server Communication\*(R" in perlipc.
.Sp
On systems that support a close-on-exec flag on files, the flag will
be set for the newly opened file descriptor, as determined by the
value of $^F.  See \*(L"$^F\*(R" in perlvar.
.IP "alarm \s-1SECONDS\s0" 8
.IX Xref "alarm SIGALRM timer"
.IX Item "alarm SECONDS"
.PD 0
.IP "alarm" 8
.IX Item "alarm"
.PD
Arranges to have a \s-1SIGALRM\s0 delivered to this process after the
specified number of wallclock seconds has elapsed.  If \s-1SECONDS\s0 is not
specified, the value stored in \f(CW$_\fR is used. (On some machines,
unfortunately, the elapsed time may be up to one second less or more
than you specified because of how seconds are counted, and process
scheduling may delay the delivery of the signal even further.)
.Sp
Only one timer may be counting at once.  Each call disables the
previous timer, and an argument of \f(CW0\fR may be supplied to cancel the
previous timer without starting a new one.  The returned value is the
amount of time remaining on the previous timer.
.Sp
For delays of finer granularity than one second, the Time::HiRes module
(from \s-1CPAN\s0, and starting from Perl 5.8 part of the standard
distribution) provides \fIualarm()\fR.  You may also use Perl's four-argument
version of \fIselect()\fR leaving the first three arguments undefined, or you
might be able to use the \f(CW\*(C`syscall\*(C'\fR interface to access \fIsetitimer\fR\|(2) if
your system supports it. See perlfaq8 for details.
.Sp
It is usually a mistake to intermix \f(CW\*(C`alarm\*(C'\fR and \f(CW\*(C`sleep\*(C'\fR calls.
(\f(CW\*(C`sleep\*(C'\fR may be internally implemented in your system with \f(CW\*(C`alarm\*(C'\fR)
.Sp
If you want to use \f(CW\*(C`alarm\*(C'\fR to time out a system call you need to use an
\&\f(CW\*(C`eval\*(C'\fR/\f(CW\*(C`die\*(C'\fR pair.  You can't rely on the alarm causing the system call to
fail with \f(CW$!\fR set to \f(CW\*(C`EINTR\*(C'\fR because Perl sets up signal handlers to
restart system calls on some systems.  Using \f(CW\*(C`eval\*(C'\fR/\f(CW\*(C`die\*(C'\fR always works,
modulo the caveats given in \*(L"Signals\*(R" in perlipc.
.Sp
.Vb 10
\&    eval {
\&        local $SIG{ALRM} = sub { die "alarm\en" }; # NB: \en required
\&        alarm $timeout;
\&        $nread = sysread SOCKET, $buffer, $size;
\&        alarm 0;
\&    };
\&    if ($@) {
\&        die unless $@ eq "alarm\en";   # propagate unexpected errors
\&        # timed out
\&    }
\&    else {
\&        # didn\*(Aqt
\&    }
.Ve
.Sp
For more information see perlipc.
.IP "atan2 Y,X" 8
.IX Xref "atan2 arctangent tan tangent"
.IX Item "atan2 Y,X"
Returns the arctangent of Y/X in the range \-PI to \s-1PI\s0.
.Sp
For the tangent operation, you may use the \f(CW\*(C`Math::Trig::tan\*(C'\fR
function, or use the familiar relation:
.Sp
.Vb 1
\&    sub tan { sin($_[0]) / cos($_[0])  }
.Ve
.Sp
Note that atan2(0, 0) is not well-defined.
.IP "bind \s-1SOCKET\s0,NAME" 8
.IX Xref "bind"
.IX Item "bind SOCKET,NAME"
Binds a network address to a socket, just as the bind system call
does.  Returns true if it succeeded, false otherwise.  \s-1NAME\s0 should be a
packed address of the appropriate type for the socket.  See the examples in
\&\*(L"Sockets: Client/Server Communication\*(R" in perlipc.
.IP "binmode \s-1FILEHANDLE\s0, \s-1LAYER\s0" 8
.IX Xref "binmode binary text DOS Windows"
.IX Item "binmode FILEHANDLE, LAYER"
.PD 0
.IP "binmode \s-1FILEHANDLE\s0" 8
.IX Item "binmode FILEHANDLE"
.PD
Arranges for \s-1FILEHANDLE\s0 to be read or written in \*(L"binary\*(R" or \*(L"text\*(R"
mode on systems where the run-time libraries distinguish between
binary and text files.  If \s-1FILEHANDLE\s0 is an expression, the value is
taken as the name of the filehandle.  Returns true on success,
otherwise it returns \f(CW\*(C`undef\*(C'\fR and sets \f(CW$!\fR (errno).
.Sp
On some systems (in general, \s-1DOS\s0 and Windows-based systems) \fIbinmode()\fR
is necessary when you're not working with a text file.  For the sake
of portability it is a good idea to always use it when appropriate,
and to never use it when it isn't appropriate.  Also, people can
set their I/O to be by default \s-1UTF\-8\s0 encoded Unicode, not bytes.
.Sp
In other words: regardless of platform, use \fIbinmode()\fR on binary data,
like for example images.
.Sp
If \s-1LAYER\s0 is present it is a single string, but may contain multiple
directives. The directives alter the behaviour of the file handle.
When \s-1LAYER\s0 is present using binmode on text file makes sense.
.Sp
If \s-1LAYER\s0 is omitted or specified as \f(CW\*(C`:raw\*(C'\fR the filehandle is made
suitable for passing binary data. This includes turning off possible \s-1CRLF\s0
translation and marking it as bytes (as opposed to Unicode characters).
Note that, despite what may be implied in \fI\*(L"Programming Perl\*(R"\fR (the
Camel) or elsewhere, \f(CW\*(C`:raw\*(C'\fR is \fInot\fR simply the inverse of \f(CW\*(C`:crlf\*(C'\fR
\&\*(-- other layers which would affect the binary nature of the stream are
\&\fIalso\fR disabled. See PerlIO, perlrun and the discussion about the
\&\s-1PERLIO\s0 environment variable.
.Sp
The \f(CW\*(C`:bytes\*(C'\fR, \f(CW\*(C`:crlf\*(C'\fR, and \f(CW\*(C`:utf8\*(C'\fR, and any other directives of the
form \f(CW\*(C`:...\*(C'\fR, are called I/O \fIlayers\fR.  The \f(CW\*(C`open\*(C'\fR pragma can be used to
establish default I/O layers.  See open.
.Sp
\&\fIThe \s-1LAYER\s0 parameter of the \fIbinmode()\fI function is described as \*(L"\s-1DISCIPLINE\s0\*(R"
in \*(L"Programming Perl, 3rd Edition\*(R".  However, since the publishing of this
book, by many known as \*(L"Camel \s-1III\s0\*(R", the consensus of the naming of this
functionality has moved from \*(L"discipline\*(R" to \*(L"layer\*(R".  All documentation
of this version of Perl therefore refers to \*(L"layers\*(R" rather than to
\&\*(L"disciplines\*(R".  Now back to the regularly scheduled documentation...\fR
.Sp
To mark \s-1FILEHANDLE\s0 as \s-1UTF\-8\s0, use \f(CW\*(C`:utf8\*(C'\fR or \f(CW\*(C`:encoding(utf8)\*(C'\fR.
\&\f(CW\*(C`:utf8\*(C'\fR just marks the data as \s-1UTF\-8\s0 without further checking,
while \f(CW\*(C`:encoding(utf8)\*(C'\fR checks the data for actually being valid
\&\s-1UTF\-8\s0. More details can be found in PerlIO::encoding.
.Sp
In general, \fIbinmode()\fR should be called after \fIopen()\fR but before any I/O
is done on the filehandle.  Calling \fIbinmode()\fR will normally flush any
pending buffered output data (and perhaps pending input data) on the
handle.  An exception to this is the \f(CW\*(C`:encoding\*(C'\fR layer that
changes the default character encoding of the handle, see open.
The \f(CW\*(C`:encoding\*(C'\fR layer sometimes needs to be called in
mid-stream, and it doesn't flush the stream.  The \f(CW\*(C`:encoding\*(C'\fR
also implicitly pushes on top of itself the \f(CW\*(C`:utf8\*(C'\fR layer because
internally Perl will operate on \s-1UTF\-8\s0 encoded Unicode characters.
.Sp
The operating system, device drivers, C libraries, and Perl run-time
system all work together to let the programmer treat a single
character (\f(CW\*(C`\en\*(C'\fR) as the line terminator, irrespective of the external
representation.  On many operating systems, the native text file
representation matches the internal representation, but on some
platforms the external representation of \f(CW\*(C`\en\*(C'\fR is made up of more than
one character.
.Sp
Mac \s-1OS\s0, all variants of Unix, and Stream_LF files on \s-1VMS\s0 use a single
character to end each line in the external representation of text (even
though that single character is \s-1CARRIAGE\s0 \s-1RETURN\s0 on Mac \s-1OS\s0 and \s-1LINE\s0 \s-1FEED\s0
on Unix and most \s-1VMS\s0 files). In other systems like \s-1OS/2\s0, \s-1DOS\s0 and the
various flavors of MS-Windows your program sees a \f(CW\*(C`\en\*(C'\fR as a simple \f(CW\*(C`\ecJ\*(C'\fR,
but what's stored in text files are the two characters \f(CW\*(C`\ecM\ecJ\*(C'\fR.  That
means that, if you don't use \fIbinmode()\fR on these systems, \f(CW\*(C`\ecM\ecJ\*(C'\fR
sequences on disk will be converted to \f(CW\*(C`\en\*(C'\fR on input, and any \f(CW\*(C`\en\*(C'\fR in
your program will be converted back to \f(CW\*(C`\ecM\ecJ\*(C'\fR on output.  This is what
you want for text files, but it can be disastrous for binary files.
.Sp
Another consequence of using \fIbinmode()\fR (on some systems) is that
special end-of-file markers will be seen as part of the data stream.
For systems from the Microsoft family this means that if your binary
data contains \f(CW\*(C`\ecZ\*(C'\fR, the I/O subsystem will regard it as the end of
the file, unless you use \fIbinmode()\fR.
.Sp
\&\fIbinmode()\fR is not only important for \fIreadline()\fR and \fIprint()\fR operations,
but also when using \fIread()\fR, \fIseek()\fR, \fIsysread()\fR, \fIsyswrite()\fR and \fItell()\fR
(see perlport for more details).  See the \f(CW$/\fR and \f(CW\*(C`$\e\*(C'\fR variables
in perlvar for how to manually set your input and output
line-termination sequences.
.IP "bless \s-1REF\s0,CLASSNAME" 8
.IX Xref "bless"
.IX Item "bless REF,CLASSNAME"
.PD 0
.IP "bless \s-1REF\s0" 8
.IX Item "bless REF"
.PD
This function tells the thingy referenced by \s-1REF\s0 that it is now an object
in the \s-1CLASSNAME\s0 package.  If \s-1CLASSNAME\s0 is omitted, the current package
is used.  Because a \f(CW\*(C`bless\*(C'\fR is often the last thing in a constructor,
it returns the reference for convenience.  Always use the two-argument
version if a derived class might inherit the function doing the blessing.
See perltoot and perlobj for more about the blessing (and blessings)
of objects.
.Sp
Consider always blessing objects in CLASSNAMEs that are mixed case.
Namespaces with all lowercase names are considered reserved for
Perl pragmata.  Builtin types have all uppercase names. To prevent
confusion, you may wish to avoid such package names as well.  Make sure
that \s-1CLASSNAME\s0 is a true value.
.Sp
See \*(L"Perl Modules\*(R" in perlmod.
.IP "break" 8
.IX Item "break"
Break out of a \f(CW\*(C`given()\*(C'\fR block.
.Sp
This keyword is enabled by the \*(L"switch\*(R" feature: see feature
for more information.
.IP "caller \s-1EXPR\s0" 8
.IX Xref "caller call stack stack stack trace"
.IX Item "caller EXPR"
.PD 0
.IP "caller" 8
.IX Item "caller"
.PD
Returns the context of the current subroutine call.  In scalar context,
returns the caller's package name if there is a caller, that is, if
we're in a subroutine or \f(CW\*(C`eval\*(C'\fR or \f(CW\*(C`require\*(C'\fR, and the undefined value
otherwise.  In list context, returns
.Sp
.Vb 2
\&    # 0         1          2
\&    ($package, $filename, $line) = caller;
.Ve
.Sp
With \s-1EXPR\s0, it returns some extra information that the debugger uses to
print a stack trace.  The value of \s-1EXPR\s0 indicates how many call frames
to go back before the current one.
.Sp
.Vb 2
\&    #  0         1          2      3            4
\&    ($package, $filename, $line, $subroutine, $hasargs,
\&
\&    #  5          6          7            8       9         10
\&    $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
\&     = caller($i);
.Ve
.Sp
Here \f(CW$subroutine\fR may be \f(CW\*(C`(eval)\*(C'\fR if the frame is not a subroutine
call, but an \f(CW\*(C`eval\*(C'\fR.  In such a case additional elements \f(CW$evaltext\fR and
\&\f(CW$is_require\fR are set: \f(CW$is_require\fR is true if the frame is created by a
\&\f(CW\*(C`require\*(C'\fR or \f(CW\*(C`use\*(C'\fR statement, \f(CW$evaltext\fR contains the text of the
\&\f(CW\*(C`eval EXPR\*(C'\fR statement.  In particular, for an \f(CW\*(C`eval BLOCK\*(C'\fR statement,
\&\f(CW$subroutine\fR is \f(CW\*(C`(eval)\*(C'\fR, but \f(CW$evaltext\fR is undefined.  (Note also that
each \f(CW\*(C`use\*(C'\fR statement creates a \f(CW\*(C`require\*(C'\fR frame inside an \f(CW\*(C`eval EXPR\*(C'\fR
frame.)  \f(CW$subroutine\fR may also be \f(CW\*(C`(unknown)\*(C'\fR if this particular
subroutine happens to have been deleted from the symbol table.
\&\f(CW$hasargs\fR is true if a new instance of \f(CW@_\fR was set up for the frame.
\&\f(CW$hints\fR and \f(CW$bitmask\fR contain pragmatic hints that the caller was
compiled with.  The \f(CW$hints\fR and \f(CW$bitmask\fR values are subject to change
between versions of Perl, and are not meant for external use.
.Sp
\&\f(CW$hinthash\fR is a reference to a hash containing the value of \f(CW\*(C`%^H\*(C'\fR when the
caller was compiled, or \f(CW\*(C`undef\*(C'\fR if \f(CW\*(C`%^H\*(C'\fR was empty. Do not modify the values
of this hash, as they are the actual values stored in the optree.
.Sp
Furthermore, when called from within the \s-1DB\s0 package, caller returns more
detailed information: it sets the list variable \f(CW@DB::args\fR to be the
arguments with which the subroutine was invoked.
.Sp
Be aware that the optimizer might have optimized call frames away before
\&\f(CW\*(C`caller\*(C'\fR had a chance to get the information.  That means that \f(CWcaller(N)\fR
might not return information about the call frame you expect it do, for
\&\f(CW\*(C`N > 1\*(C'\fR.  In particular, \f(CW@DB::args\fR might have information from the
previous time \f(CW\*(C`caller\*(C'\fR was called.
.IP "chdir \s-1EXPR\s0" 8
.IX Xref "chdir cd directory, change"
.IX Item "chdir EXPR"