MPI_Send to multiple POSIX threads running on the same process - c++

Question

I start n POSIX threads on process #0 to listen to incoming calls from processes #0...#n. However, my code is not working, instead I get a segfault. I think the problem might be because of overlapping buffers. I am new to C++. Can you suggest a solution?
void *listener(void *arg) {
...
int status_switch;
while (true) {
MPI_Recv(&status_switch, 1, MPI_INT, fd->id, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
...
}
}
int main(int argc, char * argv[])
{
MPI_Init_thread(&argc, &argv, MPI_THREAD_MULTIPLE, &provided);
int world_size;
MPI_Comm_size(MPI_COMM_WORLD, &world_size);
int world_rank;
MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
if (world_rank == root)
{
int nthreads = world_size;
threads=(pthread_t *)malloc(nthreads*sizeof(threads));
fd=(struct_t *)malloc(sizeof(struct_t)*nthreads);
//Start listeners for each node on node0
for (int i = 0; i < world_size; i++) {
fd[i].id=i;
pthread_create(&threads[i], NULL, listener, (void *)(fd+i) );
}
}
int status_switch;
status_switch = 0;
MPI_Send(&status_switch, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);
...
}

Answer 1

I'm not familiar with MPI, but you seem to be allocating the wrong amount of space here:
threads = (pthread_t *) malloc(nthreads*sizeof(threads));
It is very likely that your code segfaults because of this.
If threads is a pthread_t *, you want to allocate nthreads*sizeof(*threads) - enough space to hold nthreads instances of pthread_t.
Also, you shouldn't cast the result of malloc.