mpitutorial | MPI programming lessons in C and executable code examples | Learning library

TL;DR The main issue in your code (and likely) the reason data displays random values is the use of MPI_Irecv and MPI_Isend without the call of MPI_Wait (or MPI_Test).

MPI_Irecv and MPI_Isend are nonblocking communication routines, therefore one needs to use the MPI_Wait (or use MPI_Test to test for the completion of the request) to ensure that the message is completed, and that the data in the send/receive buffer can be again safely manipulated.

Let us imagine that you send an array of ints using MPI_Isend without calling MPI_Wait; in such case you are not really sure when you can safely modify (or deallocate the memory of) that array. The same applies to MPI_Irecv. Nonetheless, calling MPI_Wait ensures that from that point onwards one case read/write (or deallocate the memory of) the buffer without risks of undefined behavior or inconsistent data.

During the MPI_Isend the content of the buffer (e.g., the array of ints) has to be read and sent; like-wise during the MPI_Irecv the content of the receiving buffer has to arrive. In the meantime, one can overlap some computation with the ongoing process, however this computation cannot change (or read) the contend of the send/recv buffer. Then one calls the MPI_Wait to ensure that from that point onwards the data send/recv can be safely read/modified without any issues.

In your code, however you call: