Why does MPI give segmentation fault when I allocate memory for a second time for a second array?

Question

So here is the code that works with // before the lines that cause the segmentation fault:

#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>

int main (int argc, char *argv[]) {
  float *delta;
  int size,rank,*arr;

  MPI_Init (&argc, &argv); //initialize MPI library
  MPI_Comm_size(MPI_COMM_WORLD, &size); //get number of processes
  MPI_Comm_rank(MPI_COMM_WORLD, &rank); //get my process id
  
    if (rank==0){
        //MPI_Alloc_mem(2*sizeof(int),MPI_INFO_NULL,arr);
        MPI_Alloc_mem(3* sizeof(float),MPI_INFO_NULL,delta);
        delta[0]=1;
        delta[1]=2;
        //arr[0]=1;
        for (int i=1; i<size; i++){
            MPI_Send(delta,3,MPI_FLOAT,i,4,MPI_COMM_WORLD);
            //MPI_Send(arr,2,MPI_INT,i,4,MPI_COMM_WORLD);
        }
    }
    else{
        MPI_Recv(delta, 3, MPI_FLOAT, 0, 4, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
        //MPI_Recv(arr, 2, MPI_INT, 0, 4, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
  }
  printf("delta:%f from process: %d\n",delta[0],rank);
  //printf("arr:%d from process: %d\n",arr[0],rank);
  MPI_Finalize(); //MPI cleanup
  return 0;
}

And here is the code that doesn't work:

#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>

int main (int argc, char *argv[]) {
  float *delta;
  int size,rank,*arr;

  MPI_Init (&argc, &argv); //initialize MPI library
  MPI_Comm_size(MPI_COMM_WORLD, &size); //get number of processes
  MPI_Comm_rank(MPI_COMM_WORLD, &rank); //get my process id
  
    if (rank==0){
        MPI_Alloc_mem(2*sizeof(int),MPI_INFO_NULL,arr);
        MPI_Alloc_mem(3* sizeof(float),MPI_INFO_NULL,delta);
        delta[0]=1;
        delta[1]=2;
        arr[0]=1;
        for (int i=1; i<size; i++){
            MPI_Send(delta,3,MPI_FLOAT,i,4,MPI_COMM_WORLD);
            MPI_Send(arr,2,MPI_INT,i,4,MPI_COMM_WORLD);
        }
    }
    else{
        MPI_Recv(delta, 3, MPI_FLOAT, 0, 4, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
        MPI_Recv(arr, 2, MPI_INT, 0, 4, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
  }
  printf("delta:%f from process: %d\n",delta[0],rank);
  printf("arr:%d from process: %d\n",arr[0],rank);
  MPI_Finalize(); //MPI cleanup
  return 0;
}

Why does allocating memory for a second time cause this? I also tried this with malloc instead of MPI_Alloc_mem just in case but didn't really change anything.

Answer 1

I am not familiar with MPI_Alloc_mem so I am using malloc. As @Victore Eijkhout mentioned, you need to allocate the same amount of memory for your array on every process. here is a working version of your code:

#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>

int main (int argc, char *argv[]) {
  float *delta;
  int *arr;
  int size,rank;
  int tag;
  MPI_Status status;
  MPI_Init (&argc, &argv); //initialize MPI library
  MPI_Comm_size(MPI_COMM_WORLD, &size); //get number of processes
  MPI_Comm_rank(MPI_COMM_WORLD, &rank); //get my process id
  
    if (rank==0){
        arr = malloc(2 * sizeof(int));
        delta = malloc(3 * sizeof(float));
        //MPI_Alloc_mem(2*sizeof(int),MPI_INFO_NULL,arr);
        //MPI_Alloc_mem(3*sizeof(float),MPI_INFO_NULL,delta);

        delta[0]=1;
        delta[1]=2;
        arr[0]=10;
        arr[1] = 20;
        for (int i=1; i<size; i++){
            MPI_Send(delta,3,MPI_FLOAT,i, 4,MPI_COMM_WORLD);
            MPI_Send(arr,2,MPI_INT,i, 4, MPI_COMM_WORLD);
        }
    }
    else{
        arr = malloc(2 * sizeof(int));
        delta = malloc(3 * sizeof(float));
        MPI_Recv(delta, 3, MPI_FLOAT, 0, 4, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
        MPI_Recv(arr, 2, MPI_INT, 0, 4, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
  }
  printf("Process %d: arr:[%d, %d] | delta:[%f, %f]\n", rank, arr[0], arr[1], delta[0], delta[1]);

  MPI_Finalize(); //MPI cleanup
  return 0;
}

output:

Process 3: arr:[10, 20] | delta:[1.000000, 2.000000]
Process 0: arr:[10, 20] | delta:[1.000000, 2.000000]
Process 2: arr:[10, 20] | delta:[1.000000, 2.000000]
Process 1: arr:[10, 20] | delta:[1.000000, 2.000000]

Answer 2

You allocate your buffers only on process zero: on the other processes there is no buffer. Maybe you're confused how send/recv work: they don't send a buffer, but the contents. So the receiver needs to have space allocated to receive the data in. The receive call does not create the buffer, it only puts data in it.