3.20. The functions MPI Pack and MPI Unpack provide an alternative to derived
datatypes for grouping data. MPI Pack copies the data to be sent, one block at
a time, into a user-provided buffer. The buffer can then be sent and received.
After the data is received, MPI Unpack can be used to unpack it from the
receive buffer. The syntax of MPI Pack is
We could therefore pack the input data to the trapezoidal rule program with
the following code:
The key is the position argument. When MPI Pack is called, position should refer to the first available slot in pack buf. When MPI Pack returns, it refers to the first available slot after the data that was just packed, so after process 0 executes this code, all the processes can call MPI Bcast:
MPI Bcast(pack buf, 100, MPI PACKED, 0, comm);
Note that the MPI datatype for a packed buffer is MPI PACKED. Now the other processes can unpack the data using: MPI Unpack:
This can be used by "reversing" the steps in MPI Pack, that is, the data is unpacked one block at a time starting with position = 0.
Write another Get input function for the trapezoidal rule program. This one should use MPI Pack on process 0 and MPI Unpack on the other processes.