I believe (having been dabbling in XS stuff recently for PDL and PDL::OpenCV) that your best bet here would be to return an SV* from your XS function. Then your CODE section would do: 
char *tmp = proc_desc(a,b);
sv_setpv(RETVAL, tmp, 0);
free(tmp);

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In reply to Re: Perl, C and deallocation responsibility by etj
in thread Perl, C and deallocation responsibility by SheWolf

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