NAME

       io_uring_prep_openat - prepare an openat request

SYNOPSIS

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

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

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

       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

       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.  Similarly io_uring_prep_open(3) prepares
       an open request.

       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 and is specified in file_index , 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).

       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.

       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  subsequent  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.

       The directory file descriptor dfd is always a regular file descriptor.

       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.

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

RETURN VALUE

       None

ERRORS

       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 field.

NOTES

       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 io_uring_queue_init_params(3).

SEE ALSO

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

liburing-2.2                                     March 13, 2022                          io_uring_prep_openat(3)