MPI_ERR_TRUNCATE in MPI_Unpack

mpitest.cpp

#include <vector>
#include <mpi.h>

int main(int argc, char *argv[])
{
  MPI_Init(&argc, &argv);
  {
    MPI_Request req1, req2;
    std::vector<int> vec = {1, 2, 3};
    int packSize = sizeof(int) * vec.size();
    int position = 0;
    std::vector<char> packSendBuf(packSize);
    int vecSize = vec.size();
    MPI_Pack(vec.data(), vec.size(), MPI_INT, packSendBuf.data(), packSize, &position, MPI_COMM_WORLD);
    
    int estimatedPackSize = 0;
    MPI_Pack_size(vec.size(), MPI_INT, MPI_COMM_WORLD, &estimatedPackSize);
    std::cout << "packSize = " << packSize << ", estimatedPackSize = " << estimatedPackSize << std::endl;
    std::cout << "position after pack = " << position << std::endl;
    
    MPI_Isend(&vecSize, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, &req1);
    MPI_Isend(packSendBuf.data(), position, MPI_PACKED, 0, 0, MPI_COMM_WORLD, &req2);
  }
  {
    int vecSize, msgSize;
    int packSize = 0, position = 0;
        
    MPI_Recv(&vecSize, 1, MPI_INT, 0, MPI_ANY_TAG, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
    
    MPI_Status status;
    MPI_Probe(0, MPI_ANY_TAG, MPI_COMM_WORLD, &status);
    MPI_Get_count(&status, MPI_PACKED, &packSize);
    char packBuffer[packSize];
    std::cout << "packSize from get_count = " << packSize << std::endl;
    
    std::vector<int> vec(vecSize);
    MPI_Recv(packBuffer, packSize, MPI_PACKED, 0, MPI_ANY_TAG, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
    MPI_Unpack(packBuffer, vecSize, &position, vec.data(), vecSize, MPI_INT, MPI_COMM_WORLD);
  }
  MPI_Finalize();
}