mongoc_index_opt_geo_t(3) on Linux
The text 847f76f8c84b, as Debian 9 (Stretch) shipped it.
#include <mongoc.h> typedef struct { uint8_t twod_sphere_version; uint8_t twod_bits_precision; double twod_location_min; double twod_location_max; double haystack_bucket_size; uint8_t *padding[32]; } mongoc_index_opt_geo_t;