perf_data__read
ssz = perf_data__read(data, buf, min(size, (off_t)sizeof(buf)));
ssize_t perf_data__read(struct perf_data *data, void *buf, size_t size);
ret = perf_data__read(data, header, sizeof(*header));
err = perf_data__read(session->data, event,
err = perf_data__read(session->data, p,