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)