io_uring_prep_readv(3)

NAME
io_uring_prep_readv - prepare vector I/O read request

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

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

DESCRIPTION

The io_uring_prep_readv(3) prepares a vectored IO read request. The submission queue entry sqe is setup to use the file descriptor fd to start reading nr_vecs into the iovecs array at the specified offset. On files that support seeking, if the offset is set to -1, the read operation commences at the file offset, and the file offset is incremented by the number of bytes read. See read(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 must be 0 or -1. After the read 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 one iovec, it is more efficient to use io_uring_prep_read(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). This function accepts an array of iovec's with a size_t number of bytes each, but io_uring_cqe's result code is an __s32 value, so in theory a short read with a large enough iov_len value could generate an ambiguous return. But the number of bytes actually transferred has the same limit as read(2) so this cannot happen in practice.

SEE ALSO
io_uring_get_sqe(3), io_uring_prep_read(3), io_uring_prep_readv2(3), io_uring_submit(3)