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io_uring can currently open/close regular files or fixed/direct descriptors. Or you can instantiate a fixed descriptor from a regular one, and then close the regular descriptor. But you currently can't turn a purely fixed/direct descriptor into a regular file descriptor. IORING_OP_FIXED_FD_INSTALL adds support for installing a direct descriptor into the normal file table, just like receiving a file descriptor or opening a new file would do. This is all nicely abstracted into receive_fd(), and hence adding support for this is truly trivial. Since direct descriptors are only usable within io_uring itself, it can be useful to turn them into real file descriptors if they ever need to be accessed via normal syscalls. This can either be a transitory thing, or just a permanent transition for a given direct descriptor. By default, new fds are installed with O_CLOEXEC set. The application can disable O_CLOEXEC by setting IORING_FIXED_FD_NO_CLOEXEC in the sqe->install_fd_flags member. Suggested-by: Christian Brauner <brauner@kernel.org> Reviewed-by: Christian Brauner <brauner@kernel.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.