perf_data__seek
off_t offset = perf_data__seek(&inject->output, 0, SEEK_CUR);
data->file.size = perf_data__seek(data, 0, SEEK_CUR);
if (perf_data__seek(data, pos, SEEK_SET) == (off_t)-1) {
off_t perf_data__seek(struct perf_data *data, off_t offset, int whence);