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_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.

       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

       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)