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.

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.\" ========================================================================
.\"
.IX Title "PERLFAQ5 1"
.TH PERLFAQ5 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.SH "NAME"
perlfaq5 \- Files and Formats ($Revision: 10126 $)
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This section deals with I/O and the \*(L"f\*(R" issues: filehandles, flushing,
formats, and footers.
.Sh "How do I flush/unbuffer an output filehandle?  Why must I do this?"
.IX Xref "flush buffer unbuffer autoflush"
.IX Subsection "How do I flush/unbuffer an output filehandle?  Why must I do this?"
Perl does not support truly unbuffered output (except insofar as you
can \f(CW\*(C`syswrite(OUT, $char, 1)\*(C'\fR), although it does support is \*(L"command
buffering\*(R", in which a physical write is performed after every output
command.
.PP
The C standard I/O library (stdio) normally buffers characters sent to
devices so that there isn't a system call for each byte. In most stdio
implementations, the type of output buffering and the size of the
buffer varies according to the type of device. Perl's \f(CW\*(C`print()\*(C'\fR and
\&\f(CW\*(C`write()\*(C'\fR functions normally buffer output, while \f(CW\*(C`syswrite()\*(C'\fR
bypasses buffering all together.
.PP
If you want your output to be sent immediately when you execute
\&\f(CW\*(C`print()\*(C'\fR or \f(CW\*(C`write()\*(C'\fR (for instance, for some network protocols),
you must set the handle's autoflush flag. This flag is the Perl
variable \f(CW$|\fR and when it is set to a true value, Perl will flush the
handle's buffer after each \f(CW\*(C`print()\*(C'\fR or \f(CW\*(C`write()\*(C'\fR. Setting \f(CW$|\fR
affects buffering only for the currently selected default filehandle.
You choose this handle with the one argument \f(CW\*(C`select()\*(C'\fR call (see
\&\*(L"$|\*(R" in perlvar and \*(L"select\*(R" in perlfunc).
.PP
Use \f(CW\*(C`select()\*(C'\fR to choose the desired handle, then set its
per-filehandle variables.
.PP
.Vb 3
\&        $old_fh = select(OUTPUT_HANDLE);
\&        $| = 1;
\&        select($old_fh);
.Ve
.PP
Some modules offer object-oriented access to handles and their
variables, although they may be overkill if this is the only thing you
do with them.  You can use \f(CW\*(C`IO::Handle\*(C'\fR:
.PP
.Vb 3
\&        use IO::Handle;
\&        open my( $printer ), ">", "/dev/printer");   # but is this?
\&        $printer\->autoflush(1);
.Ve
.PP
or \f(CW\*(C`IO::Socket\*(C'\fR (which inherits from \f(CW\*(C`IO::Handle\*(C'\fR):
.PP
.Vb 2
\&        use IO::Socket;           # this one is kinda a pipe?
\&        my $sock = IO::Socket::INET\->new( \*(Aqwww.example.com:80\*(Aq );
\&
\&        $sock\->autoflush();
.Ve
.PP
You can also flush an \f(CW\*(C`IO::Handle\*(C'\fR object without setting
\&\f(CW\*(C`autoflush\*(C'\fR. Call the \f(CW\*(C`flush\*(C'\fR method to flush the buffer yourself:
.PP
.Vb 3
\&        use IO::Handle;
\&        open my( $printer ), ">", "/dev/printer"); 
\&        $printer\->flush; # one time flush
.Ve
.Sh "How do I change, delete, or insert a line in a file, or append to the beginning of a file?"
.IX Xref "file, editing"
.IX Subsection "How do I change, delete, or insert a line in a file, or append to the beginning of a file?"
(contributed by brian d foy)
.PP
The basic idea of inserting, changing, or deleting a line from a text
file involves reading and printing the file to the point you want to
make the change, making the change, then reading and printing the rest
of the file. Perl doesn't provide random access to lines (especially
since the record input separator, \f(CW$/\fR, is mutable), although modules
such as \f(CW\*(C`Tie::File\*(C'\fR can fake it.
.PP
A Perl program to do these tasks takes the basic form of opening a
file, printing its lines, then closing the file:
.PP
.Vb 2
\&        open my $in,  \*(Aq<\*(Aq,  $file      or die "Can\*(Aqt read old file: $!";
\&        open my $out, \*(Aq>\*(Aq, "$file.new" or die "Can\*(Aqt write new file: $!";
\&
\&        while( <$in> )
\&                {
\&                print $out $_;
\&                }
\&
\&   close $out;
.Ve
.PP
Within that basic form, add the parts that you need to insert, change,
or delete lines.
.PP
To prepend lines to the beginning, print those lines before you enter
the loop that prints the existing lines.
.PP
.Vb 2
\&        open my $in,  \*(Aq<\*(Aq,  $file      or die "Can\*(Aqt read old file: $!";
\&        open my $out, \*(Aq>\*(Aq, "$file.new" or die "Can\*(Aqt write new file: $!";
\&
\&        print "# Add this line to the top\en"; # <\-\-\- HERE\*(AqS THE MAGIC
\&
\&        while( <$in> )
\&                {
\&                print $out $_;
\&                }
\&
\&   close $out;
.Ve
.PP
To change existing lines, insert the code to modify the lines inside
the \f(CW\*(C`while\*(C'\fR loop. In this case, the code finds all lowercased
versions of \*(L"perl\*(R" and uppercases them. The happens for every line, so
be sure that you're supposed to do that on every line!
.PP
.Vb 2
\&        open my $in,  \*(Aq<\*(Aq,  $file      or die "Can\*(Aqt read old file: $!";
\&        open my $out, \*(Aq>\*(Aq, "$file.new" or die "Can\*(Aqt write new file: $!";
\&
\&        print "# Add this line to the top\en";
\&
\&        while( <$in> )
\&                {
\&                s/\eb(perl)\eb/Perl/g;
\&                print $out $_;
\&                }
\&
\&   close $out;
.Ve
.PP
To change only a particular line, the input line number, \f(CW$.\fR, is
useful. First read and print the lines up to the one you  want to
change. Next, read the single line you want to change, change it, and
print it. After that, read the rest of the lines and print those:
.PP
.Vb 5
\&        while( <$in> )   # print the lines before the change
\&                {
\&                print $out $_;
\&                last if $. == 4; # line number before change
\&                }
\&
\&        my $line = <$in>;
\&        $line =~ s/\eb(perl)\eb/Perl/g;
\&        print $out $line;
\&
\&        while( <$in> )   # print the rest of the lines
\&                {
\&                print $out $_;
\&                }
.Ve
.PP
To skip lines, use the looping controls. The \f(CW\*(C`next\*(C'\fR in this example
skips comment lines, and the \f(CW\*(C`last\*(C'\fR stops all processing once it
encounters either \f(CW\*(C`_\|_END_\|_\*(C'\fR or \f(CW\*(C`_\|_DATA_\|_\*(C'\fR.
.PP
.Vb 6
\&        while( <$in> )
\&                {
\&                next if /^\es+#/;             # skip comment lines
\&                last if /^_\|_(END|DATA)_\|_$/;  # stop at end of code marker
\&                print $out $_;
\&                }
.Ve
.PP
Do the same sort of thing to delete a particular line by using \f(CW\*(C`next\*(C'\fR
to skip the lines you don't want to show up in the output. This
example skips every fifth line:
.PP
.Vb 5
\&        while( <$in> )
\&                {
\&                next unless $. % 5;
\&                print $out $_;
\&                }
.Ve
.PP
If, for some odd reason, you really want to see the whole file at once
rather than processing line by line, you can slurp it in (as long as
you can fit the whole thing in memory!):