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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | NOTES | SEE ALSO | COLOPHON |
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io_uring_prep_openat(3) liburing Manual io_uring_prep_openat(3)
io_uring_prep_openat - prepare an openat request
#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);
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.
None
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.
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).
io_uring_get_sqe(3), io_uring_submit(3), io_uring_register(2),
openat(2)
This page is part of the liburing (A library for io_uring)
project. Information about the project can be found at
⟨https://github.com/axboe/liburing⟩. If you have a bug report for
this manual page, send it to [email protected]. This page
was obtained from the project's upstream Git repository
⟨https://github.com/axboe/liburing⟩ on 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-03.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
liburing-2.2 March 13, 2022 io_uring_prep_openat(3)
Pages that refer to this page: io_uring_prep_fixed_fd_install(3), io_uring_register_file_alloc_range(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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