Ubuntu Manpage: io_uring_prep_openat - prepare an openat request

name
synopsis
description
return value
errors
notes
see also

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).