Nested Data_Wrap_Struct

pmapi.c

static VALUE rb_pmLookupDesc(VALUE self, VALUE pmid) {
    pmDesc *pm_desc;

    pm_desc = ALLOC(pmDesc);

    pmLookupDesc(NUM2UINT(pmid), pm_desc))

    return rb_pmapi_pmdesc_new(pm_desc);
}

pmapi.h

typedef struct pmUnits {
    unsigned int	pad : 8;
    signed int		scaleCount : 4;	/* one of PM_COUNT_* below */
    unsigned int	scaleTime : 4;	/* one of PM_TIME_* below */
    unsigned int	scaleSpace : 4;	/* one of PM_SPACE_* below */
    signed int		dimCount : 4;	/* event dimension */
    signed int		dimTime : 4;	/* time dimension */
    signed int		dimSpace : 4;	/* space dimension */
} pmUnits;	

typedef struct pmDesc {
    pmID	pmid;		/* unique identifier */
    int		type;		/* base data type (see below) */
    pmInDom	indom;		/* instance domain */
    int		sem;		/* semantics of value (see below) */
    pmUnits	units;		/* dimension and units */
} pmDesc;

pmapi_pmdesc.c

static VALUE rb_pmapi_pmdesc_pmunits(VALUE self) {
    pmUnits *pm_units = &rb_pmapi_pmdesc_ptr(self)->units;
    return rb_pmapi_pmunits_new(pm_units);
}

/* This is only ever called when pmDesc is created via native Ruby. 
Because we are only memcpy()ing the data, back, it is possible for the 
pmUnits class to get out of sync with the underlying pmDesc struct */
static VALUE rb_pmapi_pmdesc_pmunits_set(VALUE self, VALUE pm_units) {
    pmUnits *pm_units_from_pmUnits_class = rb_pmapi_pmunits_get(pm_units);

    memcpy(&rb_pmapi_pmdesc_ptr(self)->units, pm_units_from_pmUnits_class, sizeof(pmUnits));

    return Qnil;
}

VALUE rb_pmapi_pmdesc_new(pmDesc *pm_desc) {
    return Data_Wrap_Struct(pcp_pmapi_pmdesc_class, 0, rb_pmapi_pmdesc_free, pm_desc);
}

void init_rb_pmapi_pmdesc(VALUE pmapi_class) {
    ...
    rb_define_method(pcp_pmapi_pmdesc_class, "units", rb_pmapi_pmdesc_pmunits, 0);
    rb_define_private_method(pcp_pmapi_pmdesc_class, "units=", rb_pmapi_pmdesc_pmunits_set, 1);
    ...
}

pmapi_pmunits.c

/* Allocation via this function requires that the callee is the one that frees pmUnits.
 * When this object is created in Ruby code, it allocates and cleans up appropriately.
 * When constructed here, its done so by the pmDesc class and its manages the freeing */
VALUE rb_pmapi_pmunits_new(pmUnits *pm_units) {
    return Data_Wrap_Struct(pcp_pmapi_pmunits_class, 0, 0, pm_units);
}

static pmUnits* rb_pmapi_pmunits_ptr(VALUE self) {
    pmUnits *pm_units;

    Data_Get_Struct(self, pmUnits, pm_units);

    return pm_units;
}

pmUnits *rb_pmapi_pmunits_get(VALUE self) {
    return rb_pmapi_pmunits_ptr(self);
}

pmdesc.rb

require 'pcp/pmapi'

module PCP
  class PMAPI
    class PmDesc
      def initialize(pmid, type, indom, sem, units)
        self.pmid = pmid
        self.type = type
        self.indom = indom
        self.sem = sem
        raise ::TypeError, "#{units.class} are invalid units. Must be of type PCP::PMAPI::PmUnits" unless units.is_a?(PCP::PMAPI::PmUnits)
        self.units = units
      end
    end
  end
end