perlfaq7.1

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 "PERLFAQ7 1"
.TH PERLFAQ7 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"
perlfaq7 \- General Perl Language Issues ($Revision: 10100 $)
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This section deals with general Perl language issues that don't
clearly fit into any of the other sections.
.Sh "Can I get a BNF/yacc/RE for the Perl language?"
.IX Subsection "Can I get a BNF/yacc/RE for the Perl language?"
There is no \s-1BNF\s0, but you can paw your way through the yacc grammar in
perly.y in the source distribution if you're particularly brave.  The
grammar relies on very smart tokenizing code, so be prepared to
venture into toke.c as well.
.PP
In the words of Chaim Frenkel: \*(L"Perl's grammar can not be reduced to \s-1BNF\s0.
The work of parsing perl is distributed between yacc, the lexer, smoke
and mirrors.\*(R"
.Sh "What are all these $@%&* punctuation signs, and how do I know when to use them?"
.IX Subsection "What are all these $@%&* punctuation signs, and how do I know when to use them?"
They are type specifiers, as detailed in perldata:
.PP
.Vb 6
\&        $ for scalar values (number, string or reference)
\&        @ for arrays
\&        % for hashes (associative arrays)
\&        & for subroutines (aka functions, procedures, methods)
\&        * for all types of that symbol name.  In version 4 you used them like
\&          pointers, but in modern perls you can just use references.
.Ve
.PP
There are couple of other symbols that you're likely to encounter that aren't
really type specifiers:
.PP
.Vb 2
\&        <> are used for inputting a record from a filehandle.
\&        \e  takes a reference to something.
.Ve
.PP
Note that <\s-1FILE\s0> is \fIneither\fR the type specifier for files
nor the name of the handle.  It is the \f(CW\*(C`<>\*(C'\fR operator applied
to the handle \s-1FILE\s0.  It reads one line (well, record\*(--see
\&\*(L"$/\*(R" in perlvar) from the handle \s-1FILE\s0 in scalar context, or \fIall\fR lines
in list context.  When performing open, close, or any other operation
besides \f(CW\*(C`<>\*(C'\fR on files, or even when talking about the handle, do
\&\fInot\fR use the brackets.  These are correct: \f(CW\*(C`eof(FH)\*(C'\fR, \f(CW\*(C`seek(FH, 0,
2)\*(C'\fR and \*(L"copying from \s-1STDIN\s0 to \s-1FILE\s0\*(R".
.Sh "Do I always/never have to quote my strings or use semicolons and commas?"
.IX Subsection "Do I always/never have to quote my strings or use semicolons and commas?"
Normally, a bareword doesn't need to be quoted, but in most cases
probably should be (and must be under \f(CW\*(C`use strict\*(C'\fR).  But a hash key
consisting of a simple word (that isn't the name of a defined
subroutine) and the left-hand operand to the \f(CW\*(C`=>\*(C'\fR operator both
count as though they were quoted:
.PP
.Vb 4
\&        This                    is like this
\&        \-\-\-\-\-\-\-\-\-\-\-\-            \-\-\-\-\-\-\-\-\-\-\-\-\-\-\-
\&        $foo{line}              $foo{\*(Aqline\*(Aq}
\&        bar => stuff            \*(Aqbar\*(Aq => stuff
.Ve
.PP
The final semicolon in a block is optional, as is the final comma in a
list.  Good style (see perlstyle) says to put them in except for
one-liners:
.PP
.Vb 2
\&        if ($whoops) { exit 1 }
\&        @nums = (1, 2, 3);
\&        
\&        if ($whoops) {
\&                exit 1;
\&        }
\&
\&        @lines = (
\&        "There Beren came from mountains cold",
\&        "And lost he wandered under leaves",
\&        );
.Ve
.Sh "How do I skip some return values?"
.IX Subsection "How do I skip some return values?"
One way is to treat the return values as a list and index into it:
.PP
.Vb 1
\&        $dir = (getpwnam($user))[7];
.Ve
.PP
Another way is to use undef as an element on the left-hand-side:
.PP
.Vb 1
\&        ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
.Ve
.PP
You can also use a list slice to select only the elements that
you need:
.PP
.Vb 1
\&        ($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5];
.Ve
.Sh "How do I temporarily block warnings?"
.IX Subsection "How do I temporarily block warnings?"
If you are running Perl 5.6.0 or better, the \f(CW\*(C`use warnings\*(C'\fR pragma
allows fine control of what warning are produced.
See perllexwarn for more details.
.PP
.Vb 4
\&        {
\&        no warnings;          # temporarily turn off warnings
\&        $a = $b + $c;         # I know these might be undef
\&        }
.Ve
.PP
Additionally, you can enable and disable categories of warnings.
You turn off the categories you want to ignore and you can still
get other categories of warnings.  See perllexwarn for the
complete details, including the category names and hierarchy.
.PP
.Vb 4
\&        {
\&        no warnings \*(Aquninitialized\*(Aq;
\&        $a = $b + $c;
\&        }
.Ve
.PP
If you have an older version of Perl, the \f(CW$^W\fR variable (documented
in perlvar) controls runtime warnings for a block:
.PP
.Vb 4
\&        {
\&        local $^W = 0;        # temporarily turn off warnings
\&        $a = $b + $c;         # I know these might be undef
\&        }
.Ve
.PP
Note that like all the punctuation variables, you cannot currently
use \fImy()\fR on \f(CW$^W\fR, only \fIlocal()\fR.
.Sh "What's an extension?"
.IX Subsection "What's an extension?"
An extension is a way of calling compiled C code from Perl.  Reading
perlxstut is a good place to learn more about extensions.
.Sh "Why do Perl operators have different precedence than C operators?"
.IX Subsection "Why do Perl operators have different precedence than C operators?"
Actually, they don't.  All C operators that Perl copies have the same
precedence in Perl as they do in C.  The problem is with operators that C
doesn't have, especially functions that give a list context to everything
on their right, eg. print, chmod, exec, and so on.  Such functions are
called \*(L"list operators\*(R" and appear as such in the precedence table in
perlop.
.PP
A common mistake is to write:
.PP
.Vb 1
\&        unlink $file || die "snafu";
.Ve
.PP
This gets interpreted as:
.PP
.Vb 1
\&        unlink ($file || die "snafu");
.Ve
.PP
To avoid this problem, either put in extra parentheses or use the
super low precedence \f(CW\*(C`or\*(C'\fR operator:
.PP
.Vb 2
\&        (unlink $file) || die "snafu";
\&        unlink $file or die "snafu";
.Ve
.PP
The \*(L"English\*(R" operators (\f(CW\*(C`and\*(C'\fR, \f(CW\*(C`or\*(C'\fR, \f(CW\*(C`xor\*(C'\fR, and \f(CW\*(C`not\*(C'\fR)
deliberately have precedence lower than that of list operators for