static ssize_t reader_callback_thunk(void *cls, uint64_t pos, char *buf, size_t max);
ssize_t reader_callback(uint64_t pos, char *buf, size_t max);
static ssize_t reader_callback_thunk(void *cls, uint64_t pos, char *buf, size_t max);
ssize_t reader_callback(uint64_t pos, char *buf, size_t max);