This declaration in cctki_GroupsOnGH.h is wrong; it should not have the pure attribute:

void *CCTKi_VarDataPtrI(const cGH *GH, int timelevel, int varindex) CCTK_ATTRIBUTE_PURE;

This function checks whether the caller has access to that grid variable and returns NULL if not. It does not always return the same pointer.

Also, the variable pointer changes over time if the driver reallocates storage.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2511/cctki_vardataptri-should-not-be-declared