io_uring_prep_openat2 - prepare an openat2 request

       The  io_uring_prep_openat2(3)  function  prepares  an  openat2 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
       instructions on how to open the file given in how.

       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 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 the 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 openat2(2) request. See that man page for details.

       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.

       io_uring_prep_openat2 - prepare an openat2 request

       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).

       None

io_uring_get_sqe(3), io_uring_submit(3), io_uring_register(2), openat2(2)

liburing-2.2                                     March 13, 2022                         io_uring_prep_openat2(3)

#include<sys/types.h>#include<sys/stat.h>#include<fcntl.h>#include<linux/openat2.h>#include<liburing.h>voidio_uring_prep_openat2(structio_uring_sqe*sqe,intdfd,constchar*path,structopen_how*how);voidio_uring_prep_openat2_direct(structio_uring_sqe*sqe,intdfd,constchar*path,structopen_how*how,unsignedfile_index);