How to store null in Perl hash

Question

I want to use Perl hash in my C code (XS) as a set, so I only need to keep keys in hash. Is it possible to store something like null or another constant value to avoid creating unnecessary value?

Something like this:

int add_value(HV *hash, SV *value)
{
    // just an example of key
    char key[64];
    sprintf(key, "%p", value);
    if (hv_exists(hash, key, strlen(key)) return 0;

    // here I need something instead of ?
    return hv_stores(hash, key, ?) != NULL;
}

One of possible solution could be to store value itself, but maybe there is special constant for undef or null.

Answer 1

&PL_sv_undef is the undefined value, but you can't, unfortunately, use it naively in hashes and arrays. Quoting perlguts:

Generally, if you want to store an undefined value in an AV or HV, you should not use &PL_sv_undef , but rather create a new undefined value using the newSV function, for example:

av_store( av, 42, newSV(0) );
hv_store( hv, "foo", 3, newSV(0), 0 );