How do I send a dynamic array from slave to the ma

Question

I'm finishing off a simple MPI program and I'm struggling on the last part of the project. I send 2 ints containing a start point and end point to the slave node. And using these I need to create an array and populate it. I need to send this back to the Master node. Slave code below:

printf("Client waiting for start point and endpoint array\n");fflush(stdout);

int startEnd [2];

MPI_Recv(startEnd, 2, MPI_INT, 0, 100, MPI_COMM_WORLD, &status);
int end = startEnd[1];
int start = startEnd[0];
printf("Recieved Start End of %d \t %d\n", startEnd[0], startEnd[1]);fflush(stdout);

unsigned char TargetHash[MAX_HASH_LEN];
MPI_Recv(TargetHash, MAX_HASH_LEN, MPI_CHAR, 0, 100, MPI_COMM_WORLD, &status);

int sizeToCompute = (end - start);
uint64* pStartPosIndexE = new uint64[sizeToCompute];
int iterator = 0;
for (int nPos = end; nPos >= start; nPos--)
{
    cwc.SetHash(TargetHash);
    cwc.HashToIndex(nPos);
    int i;
    for (i = nPos + 1; i <= cwc.GetRainbowChainLength() - 2; i++)
    {
        cwc.IndexToPlain();
        cwc.PlainToHash();
        cwc.HashToIndex(i);
    }

    pStartPosIndexE[iterator] = cwc.GetIndex();                 
}

Is this the correct way to create the array of dynamic length and how would I send this array back to the master node?

Answer 1

Sending dynamically allocated arrays is no different than sending static arrays. When the array size varies, the receive code gets a bit more complicated, but not that much more complicated:

// ---------- Sender code ----------

MPI_Send(pStartPosIndexE, sizeToCompute, MPI_UINT64, 99, ...);

// --------- Receiver code ---------

// Wait for a message with tag 99
MPI_Status status;
MPI_Probe(MPI_ANY_SOURCE, 99, MPI_COMM_WORLD, &status);

// Get the number of elements in the message
int nElems;
MPI_Get_elements(&status, MPI_UINT64_T, &nElems);

// Allocate buffer of appropriate size
uint64 *result = new uint64[nElems];

// Receive the message
MPI_Recv(result, nElems, MPI_UINT64_T, status.MPI_SOURCE, 99, ...);

Using MPI_Probe with source rank of MPI_ANY_SOURCE is what is usually done in master/worker applications where workers are processed on a first-come-first-served basis.