Nonblocking collective file operations are defined only for data access routines with explicit offsets and individual file pointers but not with shared file pointers.
Nonblocking collective file operations are subject to the same restrictions as blocking collective I/O operations. All processes belonging to the group of the communicator that was used to open the file must call collective I/O operations (blocking and nonblocking) in the same order. This is consistent with the ordering rules for collective operations in threaded environments. For a complete discussion, please refer to the semantics set forth in Section Correctness.
Nonblocking collective I/O operations do not match with blocking collective I/O operations. Multiple nonblocking collective I/O operations can be outstanding on a single file handle. High quality MPI implementations should be able to support a large number of pending nonblocking I/O operations.
All nonblocking collective I/O calls are local and return immediately, irrespective of the status of other processes. The call initiates the operation that may progress independently of any communication, computation, or I/O. The call returns a request handle, which must be passed to a completion call. Input buffers should not be modified and output buffers should not be accessed before the completion call returns. The same progress rules described for nonblocking collective operations apply for nonblocking collective I/O operations. For a complete discussion, please refer to the semantics set forth in Section Nonblocking Collective Operations.