Seg fault while using MPI_Scatter - c++

Question

I have problem with MPI_Scatter. Dont know hot to use it and my current program crashes with seg fault when I launch.
I guess that the problem in parameters of MPI_Scatter, particularly in calling it with right operator (& or * or void), but I've tried almost every combination and nothing actually helped.
#include <iostream>
#include <stdio.h>
#include <mpi.h>
// k = 3, N = 12, 1,2,3, 4,5,6, 7,8,9, 10,11,12
int main(int argc, char **argv) {
int N, size, myrank;
int k;
std::cin >> N;
std::cin >> k;
int *mass = new int[N];
int *recv = new int[k];
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
if (myrank == 0) {
std::cout << "get k and n \n";
for (int i = 0; i < N; ++i) {
mass[i] = i;
std::cout << i << " written\n";
}
}
MPI_Scatter(mass, k, MPI_INT, recv, k, MPI_INT, 0, MPI_COMM_WORLD);
int sum = 0;
std::cout << "myrank" << myrank << '\n';
for (int i = 0; i < k; ++i) {
std::cout << recv[i] << '\n';
}
MPI_Finalize();
return 0;
}
When I launch this code, it prints this:
N = 12
k = 3
get k and n
0 written
1 written
2 written
3 written
4 written
5 written
6 written
7 written
8 written
9 written
10 written
11 written
myrank0
0
1
2
myrank1
myrank3
myrank2
[1570583203.522390] [calc:32739:0] mpool.c:38 UCX WARN object 0x7fe1f08b2f60 was not returned to mpool mm_recv_desc
[1570583203.523214] [calc:32740:0] mpool.c:38 UCX WARN object 0x7f4643986f60 was not returned to mpool mm_recv_desc
[1570583203.524205] [calc:32741:0] mpool.c:38 UCX WARN object 0x7f22535d4f60 was not returned to mpool mm_recv_desc

Answer 1

MPI typically redirects stdout to rank 0, so N and k are not correctly set on the other ranks.
Here is a working version of your program
#include <iostream>
#include <cassert>
#include <stdio.h>
#include <mpi.h>
// k = 3, N = 12, 1,2,3, 4,5,6, 7,8,9, 10,11,12
int main(int argc, char **argv) {
int k, N, size, myrank;
int *mass;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
if (myrank == 0) {
std::cout << "get k and n \n";
std::cin >> N;
std::cin >> k;
assert (N >= k*size);
mass = new int[N];
for (int i = 0; i < N; ++i) {
mass[i] = i;
std::cout << i << " written\n";
}
}
MPI_Bcast(&k, 1, MPI_INT, 0, MPI_COMM_WORLD);
int *recv = new int[k];
MPI_Scatter(mass, k, MPI_INT, recv, k, MPI_INT, 0, MPI_COMM_WORLD);
int sum = 0;
std::cout << "myrank" << myrank << '\n';
for (int i = 0; i < k; ++i) {
std::cout << recv[i] << '\n';
}
MPI_Finalize();
return 0;
}