Hi Tye,

While it's true that SvPV_nolen is safer for reading, SvPVX and SvPV_nolen are equally dangerous with regards to the write operation that TheDauthi wants to deploy. In both cases, it is absolutely necessary to...

1. allocate space via newSV(STRLEN), SvGROW(SV*, STRLEN), etc.
2. make the SV* "POK" via SvPOK_on(SV*), so that it knows it contains a string.

The SvPV_nolen macro first checks the SV's private SVf_POK flag to see whether it contains a string. If it does, then it returns the pointer to the string, via SvPVX(SV*). If it doesn't, it calls sv_2pv_flags, which spits out a Use of uninitialized value warning, upgrades the sv, and returns (char*)"". That's safe to read from, but if you try to write to it... kaboom.

Here's a demo app...

#!/usr/bin/perl
use strict;
use warnings;

use Inline C => <<'END_C';

void
POKe() {
    SV   *good_sv,  *bad_sv;
    char *good_ptr, *bad_ptr;

    good_sv = newSV(83);
    bad_sv  = newSV(83);

    SvPOK_on(good_sv); /* !!!! */

    good_ptr = SvPV_nolen(good_sv);
    Copy("Joy!", good_ptr, 4, char);
    SvCUR_set(good_sv, 4);
    fprintf(stderr, "%s\n", SvPVX(good_sv));

    bad_ptr = SvPV_nolen(bad_sv);

    fprintf(stderr, "wait for it...\n");

    Copy("DEATH!", bad_ptr, 6, char); 
    
    fprintf(stderr, "in heaven, everything is fine...");
}

END_C

POKe();
[download]

... and here's the output on my system...

slothbear:~/perltest marvin$ perl sv_poke.plx 
Joy!
Use of uninitialized value in subroutine entry at sv_poke.plx line 33.
wait for it...
Bus error
slothbear:~/perltest marvin$
[download]