| .\" Copyright (C) 2022 Jens Axboe <axboe@kernel.dk> |
| .\" |
| .\" SPDX-License-Identifier: LGPL-2.0-or-later |
| .\" |
| .TH io_uring_prep_openat 3 "March 13, 2022" "liburing-2.2" "liburing Manual" |
| .SH NAME |
| io_uring_prep_openat \- prepare an openat request |
| .SH SYNOPSIS |
| .nf |
| .B #include <sys/types.h> |
| .B #include <sys/stat.h> |
| .B #include <fcntl.h> |
| .B #include <liburing.h> |
| .PP |
| .BI "void io_uring_prep_openat(struct io_uring_sqe *" sqe "," |
| .BI " int " dfd "," |
| .BI " const char *" path "," |
| .BI " int " flags "," |
| .BI " mode_t " mode ");" |
| .PP |
| .BI "void io_uring_prep_openat_direct(struct io_uring_sqe *" sqe "," |
| .BI " int " dfd "," |
| .BI " const char *" path "," |
| .BI " int " flags "," |
| .BI " mode_t " mode "," |
| .BI " unsigned " file_index ");" |
| .fi |
| .SH DESCRIPTION |
| .PP |
| The |
| .BR io_uring_prep_openat (3) |
| function prepares an openat request. The submission queue entry |
| .I sqe |
| is setup to use the directory file descriptor |
| .I dfd |
| to start opening a file described by |
| .I path |
| and using the open flags in |
| .I flags |
| and using the file mode bits specified in |
| .IR mode . |
| |
| For a direct descriptor open request, the offset is specified by the |
| .I 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, |
| .B IOSQE_FIXED_FILE |
| must be set in the SQE |
| .I flags |
| member, and the SQE |
| .I 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 |
| .BR 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 |
| .BR io_uring_register_files_update (3). |
| Note that old kernels don't check the SQE |
| .I 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 |
| .B IORING_FILE_INDEX_ALLOC |
| is used as the |
| .I 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 |
| .I res |
| field just like it would be for a non-direct open request. If no more entries |
| are available in the direct descriptor table, |
| .B -ENFILE |
| is returned instead. |
| |
| These functions prepare an async |
| .BR openat (2) |
| request. See that man page for details. |
| |
| .SH RETURN VALUE |
| None |
| .SH ERRORS |
| The CQE |
| .I 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 |
| .B -1 |
| on failure and set |
| .I errno |
| to the actual error value, io_uring never uses |
| .IR errno . |
| Instead it returns the negated |
| .I errno |
| directly in the CQE |
| .I res |
| field. |
| .SH 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 |
| .B IORING_FEAT_SUBMIT_STABLE |
| flag passed back from |
| .BR io_uring_queue_init_params (3). |
| .SH SEE ALSO |
| .BR io_uring_get_sqe (3), |
| .BR io_uring_submit (3), |
| .BR io_uring_register (2), |
| .BR openat (2) |