ch21_04.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

strings to the <tt class="literal">NULL</tt>-terminated <tt class="literal">args</tt> list passed to <tt class="literal">call_argv</tt>.
For other data types, or to examine return values, you'll need to
manipulate the Perl stack.  That's touched on lightly later; for the
down and dirty, read the <em class="emphasis">perlcall</em> manpage bundled with
Perl.</p>






<h3 class="sect2">21.4.4. Evaluating a Perl Statement from C</h3>

<p>
<a name="INDEX-3768"></a><a name="INDEX-3769"></a><a name="INDEX-3770"></a><a name="INDEX-3771"></a><a name="INDEX-3772"></a><a name="INDEX-3773"></a>
Perl provides two functions for evaluating snippets of Perl code:
<tt class="literal">eval_sv</tt> and <tt class="literal">eval_pv</tt>, described in the <em class="emphasis">perlapi</em>
manpage.  Arguably, these are the only routines you'll ever need
to execute Perl code from within your C program.  The code executed
can be as long as you wish, contain multiple statements, and employ
<tt class="literal">use</tt>, <tt class="literal">require</tt>, or <tt class="literal">do</tt> to include other Perl files.</p>

<p>
<tt class="literal">eval_pv</tt> lets you evaluate individual Perl strings and then
extract variables for coercion into C types.  The following program,
<em class="emphasis">string.c</em>, executes three Perl strings, extracting an <tt class="literal">int</tt> from
the first, a <tt class="literal">float</tt> from the second, and a <tt class="literal">char *</tt> from the third:
<blockquote>
<pre class="programlisting">#include &lt;EXTERN.h&gt;
#include &lt;perl.h&gt;

static PerlInterpreter *my_perl;

main (int argc, char **argv, char **env)
{
   STRLEN n_a;
   char *embedding[] = { "", "-e", "0" };

   my_perl = perl_alloc();
   perl_construct( my_perl );

   perl_parse(my_perl, NULL, 3, embedding, NULL);
   perl_run(my_perl);

   /** Treat $a as an integer **/
   eval_pv("$a = 3; $a **= 2", TRUE);
   printf("a = %d\n", SvIV(get_sv("a", FALSE)));

   /** Treat $a as a float **/
   eval_pv("$a = 3.14; $a **= 2", TRUE);
   printf("a = %f\n", SvNV(get_sv("a", FALSE)));

   /** Treat $a as a string **/
   eval_pv("$a = 'relreP kcaH rehtonA tsuJ'; $a = reverse($a);", TRUE);
   printf("a = %s\n", SvPV(get_sv("a", FALSE), n_a));

   perl_destruct(my_perl);
   perl_free(my_perl);
}</pre>
</blockquote>
</p>

<p>
<a name="INDEX-3774"></a>
All of the functions with <tt class="literal">Sv</tt> in their names convert Perl scalars to
C types.  They're described in the <em class="emphasis">perlguts</em> and <em class="emphasis">perlapi</em> manpages.  If
you compile and run this program, you'll see the results of using
<tt class="literal">SvIV</tt> to create an <tt class="literal">int</tt>, <tt class="literal">SvNV</tt> to create a <tt class="literal">float</tt>, and
<tt class="literal">SvPV</tt> to create a C string:
<blockquote>
<pre class="programlisting">a = 9
a = 9.859600
a = Just Another Hack Perler</pre>
</blockquote>

In the previous example, we've created a global variable to temporarily
store the computed value of our evaluated expression.  It is also
possible (and in most cases better form) to use the return value
of <tt class="literal">eval_pv</tt> instead of throwing it away:
<blockquote>
<pre class="programlisting">SV *val = eval_pv("reverse 'relreP kcaH rehtonA tsuJ'", TRUE);
printf("%s\n", SvPV(val,n_a));</pre>
</blockquote>
<a name="INDEX-3775"></a>
The <em class="emphasis">perlembed</em> manpage bundled with Perl includes
a demonstration of <tt class="literal">eval_sv</tt> that lets you make use of
Perl's regular expression capabilities from your C 

program.
</p>






<h3 class="sect2">21.4.5. Fiddling with the Perl Stack from C</h3>

<p>
<a name="INDEX-3776"></a><a name="INDEX-3777"></a><a name="INDEX-3778"></a><a name="INDEX-3779"></a>
When trying to explain stacks, most computer science
textbooks<a href="#FOOTNOTE-2">[2]</a> mumble
something about spring-loaded columns of cafeteria plates: the last
thing you pushed on the stack is the first thing you pop off.  That'll
do for our purposes: your C program will push some arguments onto "the

Perl stack", shut its eyes while some magic happens, and then pop the
results--the return value of your Perl subroutine--off the stack.</p>
<blockquote class="footnote">

<a name="FOOTNOTE-2"></a>
<p>[2] Plus the occasional Perl book.</p>

</blockquote>

<p>
<a name="INDEX-3780"></a><a name="INDEX-3781"></a><a name="INDEX-3782"></a>
We'll present an example here without much explanation.  To really
understand what's going on, you'll need to know how to convert between
C types and Perl types, with <tt class="literal">newSViv</tt> and <tt class="literal">sv_setnv</tt> and <tt class="literal">newAV</tt>
and all their friends described in the <em class="emphasis">perlguts</em> and <em class="emphasis">perlapi</em>
manpages.  Then you'll need to read <em class="emphasis">perlcall</em> to learn how to
manipulate the Perl stack.</p>

<p>
<a name="INDEX-3783"></a><a name="INDEX-3784"></a>
Because C has no built-in function for integer exponentiation, let's
make Perl's <tt class="literal">**</tt> operator available to it.  (This is less useful than
it sounds, since Perl implements <tt class="literal">**</tt> with C's <em class="emphasis">pow</em>(3) function.)
First we'll create an exponentiation function in a library file called
<em class="emphasis">power.pl</em>:
<blockquote>
<pre class="programlisting">sub expo {
    my ($a, $b) = @_;
    return $a ** $b;
}</pre>
</blockquote>

Now we'll create a C program, <em class="emphasis">power.c</em>, with a function called
<tt class="literal">PerlPower</tt> that pushes the two arguments onto the stack, invokes
<tt class="literal">expo</tt>, and pops the return value out:
<blockquote>
<pre class="programlisting">#include &lt;EXTERN.h&gt;
#include &lt;perl.h&gt;

static PerlInterpreter *my_perl;

/* "Real programmers can write assembly code in any language." */

static void
PerlPower(int a, int b)
{
  dSP;                            /* initialize stack pointer         */
  ENTER;                          /* everything created after here    */
  SAVETMPS;                       /* ...is a temporary variable.      */
  PUSHMARK(SP);                   /* remember the stack pointer       */
  XPUSHs(sv_2mortal(newSViv(a))); /* push the base onto the stack     */
  XPUSHs(sv_2mortal(newSViv(b))); /* push the exponent onto stack     */
  PUTBACK;                        /* make local stack pointer global  */
  call_pv("expo", G_SCALAR);      /* call the function                */
  SPAGAIN;                        /* refresh stack pointer            */
                                  /* pop the return value from stack  */
  printf ("%d to the %dth power is %d.\n", a, b, POPi);
  PUTBACK;
  FREETMPS;                       /* free that return value           */
  LEAVE;                          /* ...and the XPUSHed "mortal" args */
}

int main (int argc, char **argv, char **env)
{
  char *my_argv[] = { "", "power.pl" };

  my_perl = perl_alloc();
  perl_construct( my_perl );

  perl_parse(my_perl, NULL, 2, my_argv, (char **)NULL);
  perl_run(my_perl);

  PerlPower(3, 4);                      /*** Compute 3 ** 4 ***/

  perl_destruct(my_perl);
  perl_free(my_perl);
}</pre>
</blockquote>

You can compile <em class="emphasis">power.c</em> into <em class="emphasis">power</em> like so:
<blockquote>
<pre class="programlisting">% <tt class="userinput"><b>cc -o power power.c `perl -MExtUtils::Embed -e ccopts -e ldopts`</b></tt>
% <tt class="userinput"><b>power</b></tt>
3 to the 4th power is 81.</pre>
</blockquote>

Now your <em class="emphasis">power</em> program can sit around being different too.</p>



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