/usr/include/stxxl/bits/io/ is in libstxxl-dev 1.4.1-2+b1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/stxxl/bits/io/boostfd_file.h
/usr/include/stxxl/bits/io/completion_handler.h
/usr/include/stxxl/bits/io/create_file.h
/usr/include/stxxl/bits/io/disk_queued_file.h
/usr/include/stxxl/bits/io/disk_queues.h
/usr/include/stxxl/bits/io/file.h
/usr/include/stxxl/bits/io/fileperblock_file.h
/usr/include/stxxl/bits/io/io.h
/usr/include/stxxl/bits/io/iostats.h
/usr/include/stxxl/bits/io/linuxaio_file.h
/usr/include/stxxl/bits/io/linuxaio_queue.h
/usr/include/stxxl/bits/io/linuxaio_request.h
/usr/include/stxxl/bits/io/mem_file.h
/usr/include/stxxl/bits/io/mmap_file.h
/usr/include/stxxl/bits/io/request.h
/usr/include/stxxl/bits/io/request_interface.h
/usr/include/stxxl/bits/io/request_operations.h
/usr/include/stxxl/bits/io/request_queue.h
/usr/include/stxxl/bits/io/request_queue_impl_1q.h
/usr/include/stxxl/bits/io/request_queue_impl_qwqr.h
/usr/include/stxxl/bits/io/request_queue_impl_worker.h
/usr/include/stxxl/bits/io/request_with_state.h
/usr/include/stxxl/bits/io/request_with_waiters.h
/usr/include/stxxl/bits/io/serving_request.h
/usr/include/stxxl/bits/io/simdisk_file.h
/usr/include/stxxl/bits/io/syscall_file.h
/usr/include/stxxl/bits/io/ufs_file_base.h
/usr/include/stxxl/bits/io/wbtl_file.h
/usr/include/stxxl/bits/io/wfs_file_base.h
/usr/include/stxxl/bits/io/wincall_file.h