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This is because I have found that xarray doesn't > handle well the scenario where you allocate a NULL entry and then exchange it > with a real pointer. It appears xarray still considers that entry a "zero" > entry. This is unfortunate because DRM works that way (first allocates a NULL > entry and then replaces the entry with a real pointer). This is what XA_ZERO_ENTRY is for. Some APIs, like xa_alloc automatically promote NULL to XA_ZERO_ENTRY, others require it to be explicit. If you use the usual pattern of xa_alloc(NULL), xa_store(!NULL) then you should be fine, as far as I know. So long as the xarray was tagged as allocating. Jason