typedef struct _bson_mem_vtable_t {
   void *(*malloc) (size_t num_bytes);
   void *(*calloc) (size_t n_members, size_t num_bytes);
   void *(*realloc) (void *mem, size_t num_bytes);
   void (*free) (void *mem);
   void *padding[4];
} bson_mem_vtable_t;
void
bson_mem_set_vtable (const bson_mem_vtable_t *vtable);

This function MUST be called at the beginning of the process. Failure to do so will result in memory being freed by the wrong allocator.