MPI provides the following window initialization functions: MPI_WIN_CREATE, MPI_WIN_ALLOCATE, MPI_WIN_ALLOCATE_SHARED, and MPI_WIN_CREATE_DYNAMIC, which are collective over the group of an intracommunicator. MPI_WIN_CREATE allows each MPI process to specify a ``window'' in its memory that is made available for accesses by other MPI processes. The call returns an opaque object that represents the group of MPI processes that own and access the set of windows, and the attributes of each window, as specified by the initialization call. MPI_WIN_ALLOCATE and MPI_WIN_ALLOCATE_SHARED differ from MPI_WIN_CREATE in that the user does not pass allocated memory; instead MPI_WIN_ALLOCATE and MPI_WIN_ALLOCATE_SHARED return a pointer to memory allocated by the MPI implementation. MPI_WIN_ALLOCATE_SHARED differs from MPI_WIN_ALLOCATE in that the allocated memory is guaranteed to be accessible from all MPI processes in the window's group with direct load/store accesses. Some restrictions may apply to the specified communicator. MPI_WIN_CREATE_DYNAMIC creates a window that allows the user to dynamically control which memory is exposed by the window.