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.

.IP "continue \s-1BLOCK\s0" 8
.IX Xref "continue"
.IX Item "continue BLOCK"
.PD 0
.IP "continue" 8
.IX Item "continue"
.PD
\&\f(CW\*(C`continue\*(C'\fR is actually a flow control statement rather than a function.  If
there is a \f(CW\*(C`continue\*(C'\fR \s-1BLOCK\s0 attached to a \s-1BLOCK\s0 (typically in a \f(CW\*(C`while\*(C'\fR or
\&\f(CW\*(C`foreach\*(C'\fR), it is always executed just before the conditional is about to
be evaluated again, just like the third part of a \f(CW\*(C`for\*(C'\fR loop in C.  Thus
it can be used to increment a loop variable, even when the loop has been
continued via the \f(CW\*(C`next\*(C'\fR statement (which is similar to the C \f(CW\*(C`continue\*(C'\fR
statement).
.Sp
\&\f(CW\*(C`last\*(C'\fR, \f(CW\*(C`next\*(C'\fR, or \f(CW\*(C`redo\*(C'\fR may appear within a \f(CW\*(C`continue\*(C'\fR
block.  \f(CW\*(C`last\*(C'\fR and \f(CW\*(C`redo\*(C'\fR will behave as if they had been executed within
the main block.  So will \f(CW\*(C`next\*(C'\fR, but since it will execute a \f(CW\*(C`continue\*(C'\fR
block, it may be more entertaining.
.Sp
.Vb 9
\&    while (EXPR) {
\&        ### redo always comes here
\&        do_something;
\&    } continue {
\&        ### next always comes here
\&        do_something_else;
\&        # then back the top to re\-check EXPR
\&    }
\&    ### last always comes here
.Ve
.Sp
Omitting the \f(CW\*(C`continue\*(C'\fR section is semantically equivalent to using an
empty one, logically enough.  In that case, \f(CW\*(C`next\*(C'\fR goes directly back
to check the condition at the top of the loop.
.Sp
If the \*(L"switch\*(R" feature is enabled, \f(CW\*(C`continue\*(C'\fR is also a
function that will break out of the current \f(CW\*(C`when\*(C'\fR or \f(CW\*(C`default\*(C'\fR
block, and fall through to the next case. See feature and
\&\*(L"Switch statements\*(R" in perlsyn for more information.
.IP "cos \s-1EXPR\s0" 8
.IX Xref "cos cosine acos arccosine"
.IX Item "cos EXPR"
.PD 0
.IP "cos" 8
.IX Item "cos"
.PD
Returns the cosine of \s-1EXPR\s0 (expressed in radians).  If \s-1EXPR\s0 is omitted,
takes cosine of \f(CW$_\fR.
.Sp
For the inverse cosine operation, you may use the \f(CW\*(C`Math::Trig::acos()\*(C'\fR
function, or use this relation:
.Sp
.Vb 1
\&    sub acos { atan2( sqrt(1 \- $_[0] * $_[0]), $_[0] ) }
.Ve
.IP "crypt \s-1PLAINTEXT\s0,SALT" 8
.IX Xref "crypt digest hash salt plaintext password decrypt cryptography passwd encrypt"
.IX Item "crypt PLAINTEXT,SALT"
Creates a digest string exactly like the \fIcrypt\fR\|(3) function in the C
library (assuming that you actually have a version there that has not
been extirpated as a potential munitions).
.Sp
\&\fIcrypt()\fR is a one-way hash function.  The \s-1PLAINTEXT\s0 and \s-1SALT\s0 is turned
into a short string, called a digest, which is returned.  The same
\&\s-1PLAINTEXT\s0 and \s-1SALT\s0 will always return the same string, but there is no
(known) way to get the original \s-1PLAINTEXT\s0 from the hash.  Small
changes in the \s-1PLAINTEXT\s0 or \s-1SALT\s0 will result in large changes in the
digest.
.Sp
There is no decrypt function.  This function isn't all that useful for
cryptography (for that, look for \fICrypt\fR modules on your nearby \s-1CPAN\s0
mirror) and the name \*(L"crypt\*(R" is a bit of a misnomer.  Instead it is
primarily used to check if two pieces of text are the same without
having to transmit or store the text itself.  An example is checking
if a correct password is given.  The digest of the password is stored,
not the password itself.  The user types in a password that is
\&\fIcrypt()\fR'd with the same salt as the stored digest.  If the two digests
match the password is correct.
.Sp
When verifying an existing digest string you should use the digest as
the salt (like \f(CW\*(C`crypt($plain, $digest) eq $digest\*(C'\fR).  The \s-1SALT\s0 used
to create the digest is visible as part of the digest.  This ensures
\&\fIcrypt()\fR will hash the new string with the same salt as the digest.
This allows your code to work with the standard crypt and
with more exotic implementations.  In other words, do not assume
anything about the returned string itself, or how many bytes in the
digest matter.
.Sp
Traditionally the result is a string of 13 bytes: two first bytes of
the salt, followed by 11 bytes from the set \f(CW\*(C`[./0\-9A\-Za\-z]\*(C'\fR, and only
the first eight bytes of the digest string mattered, but alternative
hashing schemes (like \s-1MD5\s0), higher level security schemes (like C2),
and implementations on non-UNIX platforms may produce different
strings.
.Sp
When choosing a new salt create a random two character string whose
characters come from the set \f(CW\*(C`[./0\-9A\-Za\-z]\*(C'\fR (like \f(CW\*(C`join \*(Aq\*(Aq, (\*(Aq.\*(Aq,
\&\*(Aq/\*(Aq, 0..9, \*(AqA\*(Aq..\*(AqZ\*(Aq, \*(Aqa\*(Aq..\*(Aqz\*(Aq)[rand 64, rand 64]\*(C'\fR).  This set of
characters is just a recommendation; the characters allowed in
the salt depend solely on your system's crypt library, and Perl can't
restrict what salts \f(CW\*(C`crypt()\*(C'\fR accepts.
.Sp
Here's an example that makes sure that whoever runs this program knows
their password:
.Sp
.Vb 1
\&    $pwd = (getpwuid($<))[1];
\&
\&    system "stty \-echo";
\&    print "Password: ";
\&    chomp($word = <STDIN>);
\&    print "\en";
\&    system "stty echo";
\&
\&    if (crypt($word, $pwd) ne $pwd) {
\&        die "Sorry...\en";
\&    } else {
\&        print "ok\en";
\&    }
.Ve
.Sp
Of course, typing in your own password to whoever asks you
for it is unwise.
.Sp
The crypt function is unsuitable for hashing large quantities
of data, not least of all because you can't get the information
back.  Look at the Digest module for more robust algorithms.
.Sp
If using \fIcrypt()\fR on a Unicode string (which \fIpotentially\fR has
characters with codepoints above 255), Perl tries to make sense
of the situation by trying to downgrade (a copy of the string)
the string back to an eight-bit byte string before calling \fIcrypt()\fR
(on that copy).  If that works, good.  If not, \fIcrypt()\fR dies with
\&\f(CW\*(C`Wide character in crypt\*(C'\fR.
.IP "dbmclose \s-1HASH\s0" 8
.IX Xref "dbmclose"
.IX Item "dbmclose HASH"
[This function has been largely superseded by the \f(CW\*(C`untie\*(C'\fR function.]
.Sp
Breaks the binding between a \s-1DBM\s0 file and a hash.
.IP "dbmopen \s-1HASH\s0,DBNAME,MASK" 8
.IX Xref "dbmopen dbm ndbm sdbm gdbm"
.IX Item "dbmopen HASH,DBNAME,MASK"
[This function has been largely superseded by the \f(CW\*(C`tie\*(C'\fR function.]
.Sp
This binds a \fIdbm\fR\|(3), \fIndbm\fR\|(3), \fIsdbm\fR\|(3), \fIgdbm\fR\|(3), or Berkeley \s-1DB\s0 file to a
hash.  \s-1HASH\s0 is the name of the hash.  (Unlike normal \f(CW\*(C`open\*(C'\fR, the first
argument is \fInot\fR a filehandle, even though it looks like one).  \s-1DBNAME\s0
is the name of the database (without the \fI.dir\fR or \fI.pag\fR extension if
any).  If the database does not exist, it is created with protection
specified by \s-1MASK\s0 (as modified by the \f(CW\*(C`umask\*(C'\fR).  If your system supports
only the older \s-1DBM\s0 functions, you may perform only one \f(CW\*(C`dbmopen\*(C'\fR in your
program.  In older versions of Perl, if your system had neither \s-1DBM\s0 nor
ndbm, calling \f(CW\*(C`dbmopen\*(C'\fR produced a fatal error; it now falls back to
\&\fIsdbm\fR\|(3).
.Sp
If you don't have write access to the \s-1DBM\s0 file, you can only read hash
variables, not set them.  If you want to test whether you can write,
either use file tests or try setting a dummy hash entry inside an \f(CW\*(C`eval\*(C'\fR,
which will trap the error.
.Sp
Note that functions such as \f(CW\*(C`keys\*(C'\fR and \f(CW\*(C`values\*(C'\fR may return huge lists
when used on large \s-1DBM\s0 files.  You may prefer to use the \f(CW\*(C`each\*(C'\fR
function to iterate over large \s-1DBM\s0 files.  Example:
.Sp
.Vb 6
\&    # print out history file offsets
\&    dbmopen(%HIST,\*(Aq/usr/lib/news/history\*(Aq,0666);
\&    while (($key,$val) = each %HIST) {
\&        print $key, \*(Aq = \*(Aq, unpack(\*(AqL\*(Aq,$val), "\en";
\&    }
\&    dbmclose(%HIST);
.Ve
.Sp
See also AnyDBM_File for a more general description of the pros and
cons of the various dbm approaches, as well as DB_File for a particularly
rich implementation.
.Sp
You can control which \s-1DBM\s0 library you use by loading that library
before you call \fIdbmopen()\fR:
.Sp
.Vb 3
\&    use DB_File;
\&    dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
\&        or die "Can\*(Aqt open netscape history file: $!";
.Ve
.IP "defined \s-1EXPR\s0" 8
.IX Xref "defined undef undefined"
.IX Item "defined EXPR"
.PD 0
.IP "defined" 8
.IX Item "defined"
.PD
Returns a Boolean value telling whether \s-1EXPR\s0 has a value other than
the undefined value \f(CW\*(C`undef\*(C'\fR.  If \s-1EXPR\s0 is not present, \f(CW$_\fR will be
checked.
.Sp
Many operations return \f(CW\*(C`undef\*(C'\fR to indicate failure, end of file,
system error, uninitialized variable, and other exceptional
conditions.  This function allows you to distinguish \f(CW\*(C`undef\*(C'\fR from
other values.  (A simple Boolean test will not distinguish among
\&\f(CW\*(C`undef\*(C'\fR, zero, the empty string, and \f(CW"0"\fR, which are all equally
false.)  Note that since \f(CW\*(C`undef\*(C'\fR is a valid scalar, its presence
doesn't \fInecessarily\fR indicate an exceptional condition: \f(CW\*(C`pop\*(C'\fR
returns \f(CW\*(C`undef\*(C'\fR when its argument is an empty array, \fIor\fR when the
element to return happens to be \f(CW\*(C`undef\*(C'\fR.
.Sp
You may also use \f(CW\*(C`defined(&func)\*(C'\fR to check whether subroutine \f(CW&func\fR
has ever been defined.  The return value is unaffected by any forward
declarations of \f(CW&func\fR.  Note that a subroutine which is not defined
may still be callable: its package may have an \f(CW\*(C`AUTOLOAD\*(C'\fR method that
makes it spring into existence the first time that it is called \*(-- see
perlsub.
.Sp
Use of \f(CW\*(C`defined\*(C'\fR on aggregates (hashes and arrays) is deprecated.  It
used to report whether memory for that aggregate has ever been
allocated.  This behavior may disappear in future versions of Perl.
You should instead use a simple test for size:
.Sp
.Vb 2
\&    if (@an_array) { print "has array elements\en" }
\&    if (%a_hash)   { print "has hash members\en"   }
.Ve
.Sp
When used on a hash element, it tells you whether the value is defined,
not whether the key exists in the hash.  Use \*(L"exists\*(R" for the latter
purpose.
.Sp
Examples:
.Sp
.Vb 6
\&    print if defined $switch{\*(AqD\*(Aq};
\&    print "$val\en" while defined($val = pop(@ary));
\&    die "Can\*(Aqt readlink $sym: $!"
\&        unless defined($value = readlink $sym);
\&    sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
\&    $debugging = 0 unless defined $debugging;
.Ve
.Sp
Note:  Many folks tend to overuse \f(CW\*(C`defined\*(C'\fR, and then are surprised to
discover that the number \f(CW0\fR and \f(CW""\fR (the zero-length string) are, in fact,
defined values.  For example, if you say
.Sp
.Vb 1
\&    "ab" =~ /a(.*)b/;
.Ve
.Sp
The pattern match succeeds, and \f(CW$1\fR is defined, despite the fact that it
matched \*(L"nothing\*(R".  It didn't really fail to match anything.  Rather, it
matched something that happened to be zero characters long.  This is all
very above-board and honest.  When a function returns an undefined value,
it's an admission that it couldn't give you an honest answer.  So you
should use \f(CW\*(C`defined\*(C'\fR only when you're questioning the integrity of what
you're trying to do.  At other times, a simple comparison to \f(CW0\fR or \f(CW""\fR is
what you want.
.Sp
See also \*(L"undef\*(R", \*(L"exists\*(R", \*(L"ref\*(R".
.IP "delete \s-1EXPR\s0" 8
.IX Xref "delete"
.IX Item "delete EXPR"
Given an expression that specifies a hash element, array element, hash slice,
or array slice, deletes the specified element(s) from the hash or array.
In the case of an array, if the array elements happen to be at the end,
the