From:	Sisyphus	Date:	Thu Jul 31 06:41:55 2008
Subject:	Re: Allocating a char* and returning to Perl from a C function

----- Original Message ----- 
From: "Paulo Filipe Andrade" <pfca@mega.ist.utl.pt>
.
.
> However, an optimization could be done if I could avoid copying the 
> string I'm working on before returning it.

I found the following demo on one of my boxes.

-----------------------------------------
use warnings;
use Devel::Peek;

use Inline C => Config =>
    BUILD_NOISY => 1;

use Inline C => <<'EOC';

SV * foo_1(int len) {
     char * str;
     SV * outsv;
     int i;
     srand(time((time_t) NULL));
     New(1, str, len * sizeof(char), char);
     if(str == NULL)
       croak("Failed to allocate memory in foo_1()");
     for(i = 0; i < len; ++i)
        str[i] = rand() % 256;

     outsv = newSVpv(str, len);
     Safefree(str);
     return outsv;
}

SV * foo_2(int len) {
     SV * outsv;
     char c;
     int i;

     outsv = NEWSV(0, len);
     srand(time((time_t) NULL));

     for(i = 0; i < len; ++i) {
        c = rand() % 256;
        sv_insert(outsv, i, 1, &c, 1);
        }

     SvPOK_on(outsv);
     SvCUR_set(outsv, len);
     *SvEND(outsv) = 0;

     return outsv;
}

SV * foo_3(int len) {
     SV * outsv;
     char c;
     int i;

     outsv = NEWSV(0, len);
     srand(time((time_t) NULL));

     for(i = 0; i < len; ++i)
        SvPVX(outsv)[i] = rand() % 256;

     SvPOK_on(outsv);
     SvCUR_set(outsv, len);
     *SvEND(outsv) = 0;

     return outsv;
}

EOC

$len = 70;

$s1 = foo_1($len);
$s2 = foo_2($len);
$s3 = foo_3($len);

print "$s1\n$s2\n$s3\n";


Dump($s1);
print "\n11111111111111\n";

Dump($s2);
print "\n22222222222222\n";

Dump($s3);
print "\n33333333333333\n";
-----------------------------------------
It uses NEWSV(). I think newSV(), as already mentioned by Nicholas, is 
deemed preferable.

foo_1() is essentially the same as presented earlier.
foo_2() fills the sv with random chars using sv_insert().
foo_3() fills the sv with random chars using SvPVX().

I haven't used the sv_2pvbyte_nolen() function that you mentioned. (It's 
documented in perlapi, and therefore quite acceptable.)

In addition to SvCUR_set() and SvPOK_on(), which Nicholas also mentioned, 
note the use of SvEND() to add the terminating "\0". I'm not entirely sure 
whether that's necessary (or correct).

I also find that calling foo_2() produces a "Use of uninitialized value in 
subroutine entry..." warning, for reasons that I can't immediately spot 
(anyone ?), but foo_2() is probably a less desirable option than foo_3() 
anyway.

Always happy to add to the confusion :-)

Cheers,
Rob