If sqe->file_index is set to this for opcodes that instantiate a new direct descriptor (like openat/openat2/accept), then io_uring will allocate an available direct descriptor instead of having the application pass one in. The picked direct descriptor will be returned in cqe->res, or -ENFILE if the space is full. Available since Linux 5.19
pub const IORING_FILE_INDEX_ALLOC = maxInt(u32)
pub const IORING_FILE_INDEX_ALLOC = maxInt(u32)