#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.
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)
The man page io_uring_prep_openat_direct(3) is an alias of io_uring_prep_openat(3).