Trying to receive a vector with MPI_Recv - c

Question

I'm implementing the Chan and Dehne sorting algorithm using MPI and the CGM realistic parallel model. So far each process receives N/p numbers from the original vector, each process then order their numbers sequentially using quick sort, each process then creates a sample from it's local vector (the sample has size p), each process then sends their sample over to P0; P0 should receive all samples in a bigger vector of size p*p so it can accommodate data from all processors. This is where I'm stuck, it seems to be working but for some reason after P0 receives all the data it exits with Signal: Segmentation fault (11). Thank you.
Here is the relevant part of the code:
// Step 2. Each process calculates it's local sample with size comm_sz
local_sample = create_local_sample(sub_vec, n_over_p, comm_sz);
// Step 3. Each process sends it's local sample to P0
if (my_rank == 0) {
global_sample_receiver = (int*)malloc(pow(comm_sz,2)*sizeof(int));
global_sample_receiver = local_sample;
for (i = 1; i < comm_sz; i++) {
MPI_Recv(global_sample_receiver+(i*comm_sz), comm_sz, MPI_INT,
i, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
}
} else {
MPI_Send(local_sample, comm_sz, MPI_INT, 0, 0, MPI_COMM_WORLD);
}
printf("P%d got here\n", my_rank);
MPI_Finalize();
What is funny is that every process reachs the command printf("P%d got here\n", my_rank); and therefor prints to the terminal. Also global_sample_receiver does contain the data it is supposed to contain at the end, but the program still finished with a segmentation fault.
Here is the output:
P2 got here
P0 got here
P3 got here
P1 got here
[Krabbe-Ubuntu:05969] *** Process received signal ***
[Krabbe-Ubuntu:05969] Signal: Segmentation fault (11)
[Krabbe-Ubuntu:05969] Signal code: Address not mapped (1)
[Krabbe-Ubuntu:05969] Failing at address: 0x18000003e7
--------------------------------------------------------------------------
mpiexec noticed that process rank 0 with PID 5969 on node Krabbe-Ubuntu
exited on signal 11 (Segmentation fault).
--------------------------------------------------------------------------
Edit: I found the problem, turns out local_sample also needed a malloc.

Answer 1

The issue is you overwrite global_sample_receiver (which is a pointer) with local_sample (which is an other pointer) on rank zero.
If you want to set the first comm_sz elements of global_sample_receiver with the first comm_sz elements from local_sample, then you have to copy the data (e.g. not the pointer) manually.
memcpy(global_sample_receiver, local_sample, comm_sz * sizeof(int));
That being said, the natural MPI way of doing this is via MPI_Gather().
Here is what step 3 would look like :
// Step 3. Each process sends it's local sample to P0
if (my_rank == 0) {
global_sample_receiver = (int*)malloc(pow(comm_sz,2)*sizeof(int));
}
MPI_Gather(global_sample_receiver,comm_sz, MPI_INT, local_sample, comm_sz, MPI_INT, 0, MPI_COMM_WORLD);