io_uring_prep_writev2 - Man Page

prepare vector I/O write request with flags

Synopsis

#include <sys/uio.h>
#include <liburing.h>

void io_uring_prep_writev2(struct io_uring_sqe *sqe,
                           int fd,
                           const struct iovec *iovecs,
                           unsigned nr_vecs,
                           __u64 offset,
                           int flags);

Description

The io_uring_prep_writev2(3) prepares a vectored IO write request. The submission queue entry sqe is setup to use the file descriptor fd to start writing nr_vecs from the iovecs array at the specified offset. The behavior of the function can be controlled with the flags parameter.

Supported values for flags are:

RWF_HIPRI

High priority request, poll if possible

RWF_DSYNC

per-IO O_DSYNC

RWF_SYNC

per-IO O_SYNC

RWF_NOWAIT

per-IO, return -EAGAIN if operation would block

RWF_APPEND

per-IO O_APPEND

On files that support seeking, if the offset is set to -1, the write operation commences at the file offset, and the file offset is incremented by the number of bytes written. See write(2) for more details. Note that for an async API, reading and updating the current file offset may result in unpredictable behavior, unless access to the file is serialized. It is not encouraged to use this feature if it's possible to provide the desired IO offset from the application or library.

On files that are not capable of seeking, the offset is ignored.

After the write has been prepared, it can be submitted with one of the submit functions.

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

Unless an application explicitly needs to pass in more than iovec, it is more efficient to use io_uring_prep_write(3) rather than this function, as no state has to be maintained for a non-vectored IO 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).

See Also

io_uring_get_sqe(3), io_uring_prep_write(3), io_uring_prep_writev(3), io_uring_submit(3)

Referenced By

io_uring_prep_writev(3), io_uring_prep_writev2(3).

November 15, 2021 liburing-2.1 liburing Manual