ch29_02.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

contains the wait status value (see its interpretation under <tt class="literal">system</tt>)
of the command associated with the other end of the pipe. For example:
<blockquote>
<pre class="programlisting">open(OUTPUT, '| sort -rn | lpr -p') # pipe to sort and lpr
    or die "Can't start sortlpr pipe: $!";
print OUTPUT @lines;                # print stuff to output
close OUTPUT                        # wait for sort to finish
    or warn $! ? "Syserr closing sortlpr pipe: $!"
               : "Wait status $? from sortlpr pipe";</pre>
</blockquote>

A filehandle produced by <em class="emphasis">dup</em>(2)ing a pipe is treated as an ordinary
filehandle, so <tt class="literal">close</tt> will not wait for the child on that filehandle.
You have to wait for the child by closing the original filehandle.
For example:
<blockquote>
<pre class="programlisting">open(NETSTAT, "netstat -rn |")
    or die "can't run netstat: $!";
open(STDIN, "&lt;&amp;NETSTAT")
    or die "can't dup to stdin: $!";</pre>
</blockquote>

If you close <tt class="literal">STDIN</tt> above, there is no wait, but if you
close <tt class="literal">NETSTAT</tt>, there is.</p>

<p>If you somehow manage to reap an exited pipe child on your own, the
close will fail.  This could happen if you had a <tt class="literal">$SIG{CHLD}</tt>
handler of your own that got triggered when the pipe child exited, or if
you intentionally called <tt class="literal">waitpid</tt> on the process ID returned from the
<tt class="literal">open</tt> call.</p>






<h3 class="sect2">29.2.17. closedir&nbsp;&nbsp;&nbsp;&nbsp; <img src="figs/dollarbang.gif"> <img src="figs/xarg.gif"> <img src="figs/xu.gif"></h3>

<p>
<blockquote>
<pre class="programlisting">closedir <em class="replaceable">DIRHANDLE</em>
</pre>
</blockquote>
<a name="INDEX-4701"></a><a name="INDEX-4702"></a>
This function closes a directory opened by <tt class="literal">opendir</tt>
and returns the success of that operation.  See the examples under
<tt class="literal">readdir</tt>.  <em class="replaceable">DIRHANDLE</em>
may be an expression whose value can be used as an indirect
dirhandle, usually the real dirhandle name.</p>






<h3 class="sect2">29.2.18. connect&nbsp;&nbsp;&nbsp;&nbsp; <img src="figs/dollarbang.gif"> <img src="figs/xarg.gif"> <img src="figs/xt.gif"> <img src="figs/xu.gif"></h3>

<p>
<blockquote>
<pre class="programlisting">connect <em class="replaceable">SOCKET</em>, <em class="replaceable">NAME</em>
</pre>
</blockquote>
<a name="INDEX-4703"></a><a name="INDEX-4704"></a>
This function initiates a connection with another process that is
waiting at an <tt class="literal">accept</tt>. The function returns true if it succeeded,
false otherwise.  <em class="replaceable">NAME</em>
should be a packed network address of the proper type for the socket.
For example, assuming <tt class="literal">SOCK</tt> is a previously created socket:
<blockquote>
<pre class="programlisting">use Socket;

my ($remote, $port) = ("www.perl.com", 80);
my $destaddr = sockaddr_in($port, inet_aton($remote));
connect SOCK, $destaddr
    or die "Can't connect to $remote at port $port: $!";</pre>
</blockquote>

To disconnect a socket, use either <tt class="literal">close</tt> or <tt class="literal">shutdown</tt>.  See also the
examples in the section "Sockets" in <a href="ch16_01.htm">Chapter 16, "Interprocess Communication"</a>.  See <em class="emphasis">connect</em>(2).</p>






<h3 class="sect2">29.2.19. cos &nbsp;&nbsp;&nbsp;&nbsp; <img src="figs/dollarunderscore.gif"></h3>

<p>
<blockquote>
<pre class="programlisting">cos <em class="replaceable">EXPR</em>
cos</pre>
</blockquote>
<a name="INDEX-4705"></a>
This function returns the cosine of <em class="replaceable">EXPR</em> (expressed in radians).
For example, the following script will print a cosine table of angles
measured in degrees:
<blockquote>
<pre class="programlisting"># Here's the lazy way of getting degrees-to-radians.

$pi = atan2(1,1) * 4;
$piover180 = $pi/180;

# Print table.
for ($deg = 0; $deg &lt;= 90; $deg++) {
    printf "%3d %7.5f\n", $deg, cos($deg * $piover180);
}</pre>
</blockquote>

For the inverse cosine operation, you may use the <tt class="literal">acos()</tt> function
from the <tt class="literal">Math::Trig</tt> or <tt class="literal">POSIX</tt> modules, or use this relation:
<blockquote>
<pre class="programlisting">sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }</pre>
</blockquote>
</p>






<h3 class="sect2">29.2.20. crypt&nbsp;&nbsp;&nbsp;&nbsp; <img src="figs/xu.gif"></h3>

<p>
<blockquote>
<pre class="programlisting">crypt <em class="replaceable">PLAINTEXT</em>, <em class="replaceable">SALT</em>
</pre>
</blockquote>
<a name="INDEX-4706"></a><a name="INDEX-4707"></a><a name="INDEX-4708"></a><a name="INDEX-4709"></a>
This function computes a one-way hash of a string exactly in the
manner of <em class="emphasis">crypt</em>(3).  This is somewhat
useful for checking the password file for lousy
passwords,<a href="#FOOTNOTE-3">[3]</a> although what you really want to
do is prevent people from adding the bad passwords in the first place.</p>
<blockquote class="footnote">

<a name="FOOTNOTE-3"></a>
<p>[3] Only people with honorable intentions are
allowed to do this.</p>

</blockquote>

<p>
<tt class="literal">crypt</tt> is intended to be a one-way function, much
like breaking eggs to make an omelette.  There is no (known) way to
decrypt an encrypted password apart from exhaustive, brute-force
guessing.</p>

<p>When verifying an existing encrypted string, you should use the
encrypted text as the <em class="replaceable">SALT</em> (like <tt class="literal">crypt($plain, $crypted) eq $crypted</tt>).  This allows your code to work with the standard
<tt class="literal">crypt</tt>, and with more exotic implementations, too.</p>

<p>When choosing a new <em class="replaceable">SALT</em>, you minimally need to create a random two character string whose
characters come from the set <tt class="literal">[./0-9A-Za-z]</tt>
(like <tt class="literal">join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]</tt>).
Older implementations of <tt class="literal">crypt</tt> only needed the first two characters
of the <em class="replaceable">SALT</em>, but code that only gives the first two characters
is now considered nonportable.  See your local <em class="emphasis">crypt</em>(3) manpage
for interesting details.</p>

<p>Here's an example that makes sure that whoever runs this program knows
their own password:
<blockquote>
<pre class="programlisting">$pwd = (getpwuid ($&lt;))[1];    # Assumes we're on Unix.

system "stty -echo";   # or look into Term::ReadKey on CPAN
print "Password: ";
chomp($word = &lt;STDIN&gt;);
print "\n";
system "stty echo";

if (crypt($word, $pwd) ne $pwd) {
    die "Sorry...\n";
} else {
    print "ok\n";
}</pre>
</blockquote>

Of course, typing in your own password to whoever asks for it is
unwise.</p>

<p>Shadow password files are slightly more secure than traditional
password files, and you might have to be a superuser to access them.
Because few programs should run under such powerful privileges, you
might have the program maintain its own independent authentication

system by storing the <tt class="literal">crypt</tt> strings in a different
file than <em class="emphasis">/etc/passwd</em> or
<em class="emphasis">/etc/shadow</em>.</p>

<p>The <tt class="literal">crypt</tt> function is unsuitable for encrypting large quantities
of data, not least of all because you can't get the information
back.  Look at the <em class="emphasis">by-module/Crypt</em> and <em class="emphasis">by-module/PGP</em> directories
on your favorite CPAN mirror for a slew of potentially useful
modules.</p>






<h3 class="sect2">29.2.21. dbmclose&nbsp;&nbsp;&nbsp;&nbsp; <img src="figs/dollarbang.gif"> <img src="figs/xu.gif"></h3>

<p>
<blockquote>
<pre class="programlisting">dbmclose <em class="replaceable">HASH</em>
</pre>
</blockquote>
<a name="INDEX-4710"></a>
This function breaks the binding between a DBM (database management)
file and a hash.  
<a name="INDEX-4711"></a><tt class="literal">dbmclose</tt> is really just a call to
<tt class="literal">untie</tt> with the proper 
arguments, but is provided for backward compatibility with ancient
versions of Perl.</p>






<h3 class="sect2">29.2.22. dbmopen&nbsp;&nbsp;&nbsp;&nbsp; <img src="figs/dollarbang.gif"> <img src="figs/xu.gif"></h3>

<p>
<blockquote>
<pre class="programlisting">dbmopen <em class="replaceable">HASH</em>, <em class="replaceable">DBNAME</em>, <em class="replaceable">MODE</em>
</pre>
</blockquote>
<a name="INDEX-4712"></a><a name="INDEX-4713"></a>
This binds a DBM file to a hash (that is, an associative array).
(DBM stands for database management, and consists of a set of C
library routines that allow random access to records via a hashing
algorithm.)  <em class="replaceable">HASH</em> is the name of the hash (including the <tt class="literal">%</tt>).
<em class="replaceable">DBNAME</em> is the name of the database (without any <em class="emphasis">.dir</em> or <em class="emphasis">.pag</em>
extension).  If the database does not exist and a valid <em class="replaceable">MODE</em>
is specified, the database is created with the protection specified
by <em class="replaceable">MODE</em>, as modified by the umask.  To prevent creation of the
database if it doesn't exist, you may specify a <em class="replaceable">MODE</em> of <tt class="literal">undef</tt>,
and the function will return false if it can't find an
existing database.  Values assigned to the hash prior to the
<tt class="literal">dbmopen</tt> are not accessible.</p>

<p>The <tt class="literal">dbmopen</tt> function is really just a call to
<tt class="literal">tie</tt> with the proper arguments, but is provided for
backward compatibility with ancient versions of Perl.  You can control
which DBM library you use by using the <tt class="literal">tie</tt>
interface directly or by loading the appropriate module before you
call <tt class="literal">dbmopen</tt>. Here's an example that works on some
systems for versions of <tt class="literal">DB_File</tt> similar to the
version in your Netscape browser:
<blockquote>
<pre class="programlisting">use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.dat", undef)
    or die "Can't open netscape history file: $!";

while (($url, $when) = each %NS_Hist) {
    next unless defined($when);
    chop ($url, $when);        # kill trailing null bytes
    printf "Visited %s at %s.\n", $url,
        scalar(localtime(unpack("V",$when)));
}</pre>
</blockquote>

If you don't have write access to the DBM file, you can only read
the hash variables, not set them. If you want to test whether you
can write, either use a file test like <tt class="literal">-w $file</tt>, or try setting a
dummy hash entry inside an <tt class="literal">eval {}</tt>, which will trap the exception.</p>

<p>Functions such as <tt class="literal">keys</tt> and <tt class="literal">values</tt> may return huge list values
when used on large DBM files.  You may prefer to use the <tt class="literal">each</tt>
function to iterate over large DBM files so that you don't load the
whole thing in memory at once.</p>

<p>Hashes bound to DBM files have the same limitations as the type of DBM
package you're using, including restrictions on how much data you can
put into a bucket.  If you stick to short keys and values, it's rarely
a problem.  See also the <tt class="literal">DB_File</tt> module in <a href="ch32_01.htm">Chapter 32, "Standard Modules"</a>.</p>

<p>Another thing you should bear in mind is that many existing DBM
databases contain null-terminated keys and values because they were
set up with C programs in mind.  The Netscape history file and the
old <em class="emphasis">sendmail</em> aliases file are examples.  Just use <tt class="literal">"$key\0"</tt>
when pulling out a value, and remove the null from the value.
<blockquote>
<pre class="programlisting">$alias = $aliases{"postmaster\0"};
chop $alias;   # kill the null</pre>
</blockquote>
<a name="INDEX-4714"></a><a name="INDEX-4715"></a><a name="INDEX-4716"></a>
There is currently no built-in way to lock a generic DBM file.  Some
would consider this a bug.  The <tt class="literal">GDBM_File</tt> module does attempt to provide
locking at the granularity of the entire file.  When in doubt,
your best bet is to use a separate lock file.</p>






<h3 class="sect2">29.2.23. defined&nbsp;&nbsp;&nbsp;&nbsp; <img src="figs/dollarunderscore.gif"></h3>

<p>
<blockquote>
<pre class="programlisting">defined <em class="replaceable">EXPR</em>
defined</pre>
</blockquote>
<a name="INDEX-4717"></a><a name="INDEX-4718"></a><a name="INDEX-4719"></a><a name="INDEX-4720"></a>
This function returns a Boolean value saying whether <em class="replaceable">EXPR</em> has a
defined value or not.  Most of the data you deal with is defined, but
a scalar that contains no valid string, numeric, or reference value is
said to contain the undefined value, or <tt class="literal">undef</tt> for short.
Initializing a scalar variable to a particular value will define it,
and it will stay defined until you assign an undefined value to it or
explicitly call the <tt class="literal">undef</tt> function on that variable.</p>

<p>Many operations return <tt class="literal">undef</tt> under exceptional
conditions, such as at end-of-file, when using an uninitialized