AttributeReference.rst

    struct my_pair *buffer;
    MPI_Send(buffer, 1, MPI_DOUBLE_INT /*, ...  */); // no warning

    struct my_int_pair { int a; int b; }
    struct my_int_pair *buffer2;
    MPI_Send(buffer2, 1, MPI_DOUBLE_INT /*, ...  */); // warning: actual buffer element
                                                      // type 'struct my_int_pair'
                                                      // doesn't match specified MPI_Datatype

* ``must_be_null`` specifies that the expression should be a null pointer
  constant, for example:

  .. code-block:: c++

    /* In mpi.h */
    extern struct mpi_datatype mpi_datatype_null
        __attribute__(( type_tag_for_datatype(mpi, void, must_be_null) ));

    #define MPI_DATATYPE_NULL ((MPI_Datatype) &mpi_datatype_null)

    /* In user code */
    MPI_Send(buffer, 1, MPI_DATATYPE_NULL /*, ...  */); // warning: MPI_DATATYPE_NULL
                                                        // was specified but buffer
                                                        // is not a null pointer