int SIZE, bsize;
SIZE = 6;
/*the space needed to send data with the number of SIZE and type of MPI_DOUBLE*/
MPI_Pack_size( SIZE, MPI_DOUBLE, MPI_COMM_WORLD, &bsize );
/*the space needed for MPI_Bsend*/
tmpbuffer = (double *) malloc( bsize + 2*MPI_BSEND_OVERHEAD );
MPI_Buffer_attach( tmpbuffer, bsize + MPI_BSEND_OVERHEAD );
我的问题是：
（1）为tmpbuffer分配的内存为bsize + 2*MPI_BSEND_OVERHEAD，当涉及MPI_Buffer_attach时，为什么其大小更改为bsize + MPI_BSEND_OVERHEAD？
（2）“ MPI_BSEND_OVERHEAD”是什么意思？为何要按2计时？