io_uring_prep_fixed_fd_install(3) liburing Manual io_uring_prep_fixed_fd_install(3)

io_uring_prep_fixed_fd_install - prepare fixed file fd installation request

#include <liburing.h>
void io_uring_prep_fixed_fd_install(struct io_uring_sqe *sqe,
                                    int fd,
                                    unsigned int file_flags,
                                    unsigned int flags);

The io_uring_prep_fixed_fd_install(3) helper prepares a fixed file descriptor installation. The submission queue entry sqe is setup to install the direct/fixed file descriptor fd with the specified file_flags file installation flags, and the flags request specific flags into the normal process file table.

One use case of direct/fixed file descriptors is to turn a regular file descriptor into a direct one, reducing the overhead of any request that needs to access this file. This helper provides a way to go the other way, turning a direct descriptor into a regular file descriptor that can then subsequently be used by regular system calls that take a normal file descriptor. This can be handy if no regular file descriptor exists for this direct descriptor. Either because it was instantiated directly as a fixed descriptor, or because the regular file was closed with close(2) after being turned into a direct descriptor.

Upon successful return of this request, both a normal and fixed file descriptor exists for the same file. Either one of them may be used to access the file. Either one of them may be closed without affecting the other one.

file_flags may be either zero, or set to O_CLOEXEC to indicate that the new regular file descriptor should be closed during exec. Setting this field to anything but those two values will result in the request being failed with -EINVAL in the CQE res field.

flags is as-of yet unused and must be set to zero.

None

The CQE res field will contain the result of the operation, which in this case will be the value of the new regular file descriptor. In case of failure, a negative value is returned.

io_uring_get_sqe(3), io_uring_submit(3), io_uring_register_files(3), io_uring_unregister_files(3), io_uring_prep_close_direct(3), io_uring_prep_openat_direct(3)

December 8, 2023 liburing-2.6