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)