MPI own code to find the minimum value in a vector - mpi

Question

I am trying to write my own MPI function that would compute the smallest number in a vector and broadcast that to all processes. I treat the processes as a binary tree, and find the minimum as I move from leaves to the root. Then I send message from the root to the leaves through its children. But I get a segmentation fault when I trying to receive the minimum value from the left child (process rank 3) of process rank 1 in an execution with just 4 processes ranked from 0 to 3.
void Communication::ReduceMin(double &partialMin, double &totalMin)
{
MPI_Barrier(MPI_COMM_WORLD);
double *leftChild, *rightChild;
leftChild = (double *)malloc(sizeof(double));
rightChild = (double *)malloc(sizeof(double));
leftChild[0]=rightChild[0]=1e10;
cout<<"COMM REDMIN: "<<myRank<<" "<<partialMin<<" "<<nProcs<<endl;
MPI_Status *status;
//MPI_Recv from 2*i+1 amd 2*i+2
if(nProcs > 2*myRank+1)
{
cout<<myRank<<" waiting from "<<2*myRank+1<<" for "<<leftChild[0]<<endl;
MPI_Recv((void *)&leftChild[0], 1, MPI_DOUBLE, 2*myRank+1, 2*myRank+1, MPI_COMM_WORLD, status); //SEG FAULT HERE
cout<<myRank<<" got from "<<2*myRank+1<<endl;
}
if(nProcs > 2*myRank+2)
{
cout<<myRank<<" waiting from "<<2*myRank+2<<endl;
MPI_Recv((void *)rightChild, 1, MPI_DOUBLE, 2*myRank+2, 2*myRank+2, MPI_COMM_WORLD, status);
cout<<myRank<<" got from "<<2*myRank+1<<endl;
}
//sum it up
cout<<myRank<<" finding the min"<<endl;
double myMin = min(min(leftChild[0], rightChild[0]), partialMin);
//MPI_Send to (i+1)/2-1
if(myRank!=0)
{
cout<<myRank<<" sending "<<myMin<<" to "<<(myRank+1)/2 -1 <<endl;
MPI_Send((void *)&myMin, 1, MPI_DOUBLE, (myRank+1)/2 - 1, myRank, MPI_COMM_WORLD);
}
double min;
//MPI_Recv from (i+1)/2-1
if(myRank!=0)
{
cout<<myRank<<" waiting from "<<(myRank+1)/2-1<<endl;
MPI_Recv((void *)&min, 1, MPI_DOUBLE, (myRank+1)/2 - 1, (myRank+1)/2 - 1, MPI_COMM_WORLD, status);
cout<<myRank<<" got from "<<(myRank+1)/2-1<<endl;
}
totalMin = min;
//MPI_send to 2*i+1 and 2*i+2
if(nProcs > 2*myRank+1)
{
cout<<myRank<<" sending to "<<2*myRank+1<<endl;
MPI_Send((void *)&min, 1, MPI_DOUBLE, 2*myRank+1, myRank, MPI_COMM_WORLD);
}
if(nProcs > 2*myRank+2)
{
cout<<myRank<<" sending to "<<2*myRank+1<<endl;
MPI_Send((void *)&min, 1, MPI_DOUBLE, 2*myRank+2, myRank, MPI_COMM_WORLD);
}
}
PS: I know I can use
MPI_Barrier(MPI_COMM_WORLD);
MPI_Reduce((void *)&partialMin, (void *)&totalMin, 1, MPI_DOUBLE, MPI_MIN, 0, MPI_COMM_WORLD);
MPI_Bcast((void *)&totalMin, 1, MPI_DOUBLE, 0, MPI_COMM_WORLD);
But I want to write my own code for fun.

Answer 1

The error is in the way you use the status argument in the receive calls. Instead of passing the address of an MPI_Status instance, you simply pass an uninitialised pointer and that leads to the crash:
MPI_Status *status; // status declared as a pointer and never initialised
...
MPI_Recv((void *)&leftChild[0], 1, MPI_DOUBLE, 2*myRank+1, 2*myRank+1,
MPI_COMM_WORLD, status); // status is an invalid pointer here
You should change your code to:
MPI_Status status;
...
MPI_Recv((void *)&leftChild[0], 1, MPI_DOUBLE, 2*myRank+1, 2*myRank+1,
MPI_COMM_WORLD, &status);
Since you do not examine at all the status in your code, you can simply pass MPI_STATUS_IGNORE in all calls:
MPI_Recv((void *)&leftChild[0], 1, MPI_DOUBLE, 2*myRank+1, 2*myRank+1,
MPI_COMM_WORLD, MPI_STATUS_IGNORE);