io_uring_prep_openat_direct(3) — Linux manual page


io_uring_prep_openat(3)      liburing Manual     io_uring_prep_openat(3)

NAME         top

       io_uring_prep_openat - prepare an openat request

SYNOPSIS         top

       #include <sys/types.h>
       #include <sys/stat.h>
       #include <fcntl.h>
       #include <liburing.h>

       void io_uring_prep_openat(struct io_uring_sqe *sqe,
                                 int dfd,
                                 const char *path,
                                 int flags,
                                 mode_t mode);

       void io_uring_prep_openat_direct(struct io_uring_sqe *sqe,
                                        int dfd,
                                        const char *path,
                                        int flags,
                                        mode_t mode,
                                        unsigned file_index);

DESCRIPTION         top

       The io_uring_prep_openat(3) function prepares an openat request.
       The submission queue entry sqe is setup to use the directory file
       descriptor dfd to start opening a file described by path and
       using the open flags in flags and using the file mode bits
       specified in mode.

       For a direct descriptor open request, the offset is specified by
       the file_index argument. Direct descriptors are io_uring private
       file descriptors. They avoid some of the overhead associated with
       thread shared file tables, and can be used in any io_uring
       request that takes a file descriptor. To do so, IOSQE_FIXED_FILE
       must be set in the SQE flags member, and the SQE fd field should
       use the direct descriptor value rather than the regular file
       descriptor. Direct descriptors are managed like registered files.

       If the direct variant is used, the application must first have
       registered a file table using io_uring_register_files(3) of the
       appropriate size. Once registered, a direct accept request may
       use any entry in that table, as long as it is within the size of
       the registered table.  If a specified entry already contains a
       file, the file will first be removed from the table and closed.
       It's consistent with the behavior of updating an existing file
       with io_uring_register_files_update(3).  Note that old kernels
       don't check the SQE file_index field, which is not a problem for
       liburing helpers, but users of the raw io_uring interface need to
       zero SQEs to avoid unexpected behavior.

       If IORING_FILE_INDEX_ALLOC is used as the file_index for a direct
       open, then io_uring will allocate a free direct descriptor in the
       existing table. The allocated descriptor is returned in the CQE
       res field just like it would be for a non-direct open request. If
       no more entries are available in the direct descriptor table,
       -ENFILE is returned instead.

       These functions prepare an async openat(2) request. See that man
       page for details.

RETURN VALUE         top


ERRORS         top

       The CQE res field will contain the result of the operation. See
       the related man page for details on possible values. Note that
       where synchronous system calls will return -1 on failure and set
       errno to the actual error value, io_uring never uses errno.
       Instead it returns the negated errno directly in the CQE res

NOTES         top

       As with any request that passes in data in a struct, that data
       must remain valid until the request has been successfully
       submitted. It need not remain valid until completion. Once a
       request has been submitted, the in-kernel state is stable. Very
       early kernels (5.4 and earlier) required state to be stable until
       the completion occurred. Applications can test for this behavior
       by inspecting the IORING_FEAT_SUBMIT_STABLE flag passed back from

SEE ALSO         top

       io_uring_get_sqe(3), io_uring_submit(3), io_uring_register(2),

COLOPHON         top

       This page is part of the liburing (A library for io_uring)
       project.  Information about the project can be found at 
       ⟨⟩.  If you have a bug report for
       this manual page, send it to  This page
       was obtained from the project's upstream Git repository
       ⟨⟩ on 2024-06-14.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2024-06-03.)  If you discover any rendering
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liburing-2.2                 March 13, 2022      io_uring_prep_openat(3)

Pages that refer to this page: io_uring_prep_fixed_fd_install(3)io_uring_register_file_alloc_range(3)