MPI_Scatter segmentation fault with malloc - c

Question

Can you explain why this code works perfectly fine:
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
typedef struct Point
{
double x;
double y;
} point;
int main(int argc, char *argv[]) {
int rank, size;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Datatype mpi_point;
MPI_Type_contiguous(2, MPI_DOUBLE, &mpi_point);
MPI_Type_commit(&mpi_point);
point local[2];
if(rank==0)
{
point *buf;
buf = malloc(10*sizeof(point));
for(int i = 0; i<10; i++)
{
buf[i].x= (double)i;
buf[i].y= (double)i+i;
}
MPI_Scatter(buf, 2, mpi_point, &local, 2, mpi_point, 0, MPI_COMM_WORLD);
free(buf);
}
else
{
point *buf = NULL;
MPI_Scatter(buf, 2, mpi_point, &local, 2, mpi_point, 0, MPI_COMM_WORLD);
}
printf("Hello, process %d has points: 1-> %f %f 2-> %f %f\n", rank, local[0].x, local[0].y, local[1].x, local[1].y);
MPI_Finalize();
return 0;
}
and this one below doesn't work? A lot of error output ending with segmentation fault.
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
typedef struct Point
{
double x;
double y;
} point;
int main(int argc, char *argv[]) {
int rank, size;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Datatype mpi_point;
MPI_Type_contiguous(2, MPI_DOUBLE, &mpi_point);
MPI_Type_commit(&mpi_point);
point *local;
if(rank==0)
{
point *buf;
buf = malloc(10*sizeof(point));
for(int i = 0; i<10; i++)
{
buf[i].x= (double)i;
buf[i].y= (double)i+i;
}
MPI_Scatter(buf, 2, mpi_point, &local, 2, mpi_point, 0, MPI_COMM_WORLD);
free(buf);
}
else
{
point *buf = NULL;
local = malloc(2*sizeof(point));
MPI_Scatter(buf, 2, mpi_point, &local, 2, mpi_point, 0, MPI_COMM_WORLD);
}
printf("Hello, process %d has points: 1-> %f %f 2-> %f %f\n", rank, local[0].x, local[0].y, local[1].x, local[1].y);
MPI_Finalize();
return 0;
}
The only difference is that local in one case is defined as a vector "directly" and in the other one it's a pointer and then I do the malloc. Shouldn't it be the same thing? Both of them are allocated in the stack with the same dimension, so how is it possible that the one with malloc doesn't work?