From: Doug MacEachern <dougm@opengroup.org>
Date: Sat, 5 Apr 1997 15:24:43 +0000 (-0500)
Subject: Document sample function perl_eval()
X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=8f1832628ef5654922d1b8e5959e65894d09ac5d;p=p5sagit%2Fp5-mst-13.2.git

Document sample function perl_eval()

Tim, your comments have changed perl_eval() from a quick & dirty
example to something I'd like to see part of the Perl API, maybe
called perl_eval_pv though.

p5p-msgid: 199704051524.KAA06090@postman.osf.org
---

diff --git a/pod/perlcall.pod b/pod/perlcall.pod
index 0bfd142..85e0237 100644
--- a/pod/perlcall.pod
+++ b/pod/perlcall.pod
@@ -1915,6 +1915,44 @@ refers to the last.
 
 =back
 
+=head2 Creating and calling an anonymous subroutine in C
+
+As we've already shown, L<perl_call_sv> can be used to invoke an
+anonymous subroutine.  However, our example showed how Perl script
+invoking an XSUB to preform this operation.  Let's see how it can be
+done inside our C code:
+
+   SV *perl_eval(char *string, int croak_on_error)
+   {
+       dSP;
+       SV *sv = newSVpv(string,0);
+
+       PUSHMARK(sp);
+       perl_eval_sv(sv, G_SCALAR);
+       SvREFCNT_dec(sv);
+
+       SPAGAIN;
+       sv = POPs;
+       PUTBACK;
+
+       if (croak_on_error && SvTRUE(GvSV(errgv)))
+	     croak(SvPV(GvSV(errgv),na));
+
+       return sv;
+   }
+
+ ...
+
+ SV *cvrv = perl_eval("sub { print 'You will not find me cluttering any namespace!' }", TRUE);
+
+ ...
+
+ perl_call_sv(cvrv, G_VOID|G_NOARGS);
+
+L<perl_eval_sv> is used to compile the anonymous subroutine, which can
+then be POPed off the stack.  Once this code reference is in hand, it
+can be mixed in with all the previous examples we've shown.
+
 =head1 SEE ALSO
 
 L<perlxs>, L<perlguts>, L<perlembed>
diff --git a/pod/perlembed.pod b/pod/perlembed.pod
index 9111be1..9e3fb52 100644
--- a/pod/perlembed.pod
+++ b/pod/perlembed.pod
@@ -326,6 +326,36 @@ I<SvPV()> to create a string:
    a = 9.859600
    a = Just Another Perl Hacker
 
+In the example above, we've created a global variable to temporarily
+store the computed value of our eval'd expression.  It is also
+possible and in most cases a better strategy to fetch the return value
+from L<perl_eval_sv> instead.  Example:
+
+   SV *perl_eval(char *string, int croak_on_error)
+   {
+       dSP;
+       SV *sv = newSVpv(string,0);
+
+       PUSHMARK(sp);
+       perl_eval_sv(sv, G_SCALAR);
+       SvREFCNT_dec(sv);
+
+       SPAGAIN;
+       sv = POPs;
+       PUTBACK;
+
+       if (croak_on_error && SvTRUE(GvSV(errgv)))
+	     croak(SvPV(GvSV(errgv),na));
+
+       return sv;
+   }
+   ...
+   SV *val = perl_eval("reverse 'rekcaH lreP rehtonA tsuJ'", TRUE);
+   printf("%s\n", SvPV(val,na));
+   ...
+
+This way, we avoid namespace pollution by not creating global
+variables and we've simplified our code as well.
 
 =head2 Performing Perl pattern matches and substitutions from your C program