Re: How big is an array?

["Kyle R . Burton" <mortis@voicenet.com>]

> mjd-perl-pm@plover.com writes:
>  [...]
>  > 
>  > So for example the string "BLARF" probably takes about 30 bytes.  (24
>  > overhead plus 6 for "BLARF\0")
> 
> Perl uses null-terminated strings internally?   How odd.  How, then,
> is a string with embedded NUL characters represented?

I beleive the null terminator is maintained by Perl so the string can easily
be passed to C funcitons that expect it.  Perl also tracks the length of the
scalar seperatly from the char* data (see SvCUR and SvCUR_set for more about
that).  In that example, SvCUR would be 5 even though there is 6 bytes
of storage used by "BLARF\0".

You can toy with the difference between C and Perl easily enough with Inline:

use strict;
use warnings;

my $a = "BLARF";
c_testFunction($a);
print "Perl: $a\n";
c_testFunction2($a);

use Inline C => <<'EOC';

void
c_testFunction( SV* var )
{
  char* p;
  STRLEN len;
  p = SvPV(var,len);
  printf("C1: string: %d:%s\n",len,p);
  SvCUR_set(var,3);
}


void
c_testFunction2( SV* var )
{
  char* p;
  STRLEN len;
  p = SvPV(var,len);
  printf("C2: string: %d:%s\n",len,p);
}

EOC

You should get ouptut like:

  C1: string: 5:BLARF
  Perl: BLA
  C2: string: 3:BLARF

The remainder of BLARF is still there even though Perl sees it as only
being 3 characters.  To be more correct, we should have put a null
at *(p+3) as to not confuse subsequent C functions.  Or we could have used
the safer format of %s for printf:

  printf("C2: string: %d:%.*%s\n",len,len,p);


Kyle

-- 

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mortis@voicenet.com                            http://www.voicenet.com/~mortis
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