The io_uring_sqe_set_data(3) function stores a user_data pointer with the submission queue entry sqe.
The io_uring_sqe_set_data64(3) function stores a 64-bit data value with the submission queue entry sqe.
After the caller has requested a submission queue entry (SQE) with io_uring_get_sqe(3), they can
associate a data pointer or value with the SQE. Once the completion arrives, the function
io_uring_cqe_get_data(3) or io_uring_cqe_get_data64(3) can be called to retrieve the data pointer or
value associated with the submitted request.
Note that if neither of these functions are called, or the user_data field in the sqe isn't set manually
either, then the field may contain a value from a previous use of this sqe. If an application relies on
always having a valid user_data value present, it must always assign one to each sqe.