Passed in for setup. Copied back with updated info on success.

The flags, sq_thread_cpu, and sq_thread_idle fields are used to configure the io_uring instance. If no flags are specified, the io_uring instance is setup for interrupt driven I/O. I/O may be submitted using enter and can be reaped by polling the completion queue.

The rest of the fields are filled in by the kernel, and provide the information necessary to memory map the submission queue, completion queue, and the array of submission queue entries.

sq_off describes the offsets of various ring buffer fields. Taken together, sq_entries and sq_off provide all of the information necessary for accessing the submission queue ring buffer and the submission queue entry array. The submission queue can be mapped with a call like:

ptr = mmap(0, sq_off.array + sq_entries * sizeof(__u32),
           PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE,
           ring_fd, IORING_OFF_SQ_RING);

where sq_off is the io_sqring_offsets structure, and ring_fd is the file descriptor returned from setup. The addition of sq_off.array to the length of the region accounts for the fact that the ring located at the end of the data structure. As an example, the ring buffer head pointer can be accessed by adding sq_off.head to the address returned from mmap(2):

head = ptr + sq_off.head;

The array of submission queue entries is mapped with:

sqentries = mmap(0, sq_entries * sizeof(struct io_uring_sqe),
                 PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE,
                 ring_fd, IORING_OFF_SQES);

The completion queue is described by cq_entries and cq_off. The completion queue is simpler, since the entries are not separated from the queue itself, and can be mapped with:

ptr = mmap(0, cq_off.cqes + cq_entries * sizeof(struct io_uring_cqe),        
           PROT_READ|PROT_WRITE, MAP_SHARED|MAP_POPULATE, ring_fd,
           IORING_OFF_CQ_RING);

Layout

struct io_uring_params {
    __u32 sq_entries;
    __u32 cq_entries;
    __u32 flags;
    __u32 sq_thread_cpu;
    __u32 sq_thread_idle;
    __u32 features;
    __u32 wq_fd;
    __u32 resv[3];
    struct io_sqring_offsets sq_off;
    struct io_cqring_offsets cq_off;
}