Build with Homebrew or MacPorts MPICH 3.4.2 on macOS Monterey. Run with >1 processes. The code will segfault in the 5th call to `MPI_Reduce`. Setting `root = 0` avoids this.

test_reduce.c

#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv) {
  MPI_Init(&argc, &argv);
  MPI_Comm comm = MPI_COMM_WORLD;
  int size;
  MPI_Comm_size(comm, &size);
  int rank;
  MPI_Comm_rank(comm, &rank);
  int root = size - 1;
  int send = rank;
  int recv;
  for (int i = 0; i < 10; ++i) {
    if (rank == 0)
      fprintf(stderr, "MPI_Reduce i=%d\n", i);
    MPI_Reduce(&send, rank == root ? &recv : NULL, 1, MPI_INT, MPI_SUM, root,
               comm);
  }
  MPI_Finalize();
  return 0;
}