In MPI How to broadcast c++ vector?

Question

I want to broadcast C++ vector using MPI.

I am not allowed to use boost.mpi

Right now I use most upvoted answer from Vector Usage in MPI(C++) but it doesn't works..

Ok, here is the code:

// declaration of variables (ParsedData object will contain these variables)
int generators_count, intervals_count;
std::vector<float> mean_arr, variance_arr, interval_begins_arr, interval_ends_arr;
std::vector<int> amount_of_numbers_to_generate_arr;

// load data into parsed_data object
if (process_id == 0) {
  ParsedData parsed_data = load_input_data(filename);
  intervals_count = parsed_data.intervals_count;
  generators_count = parsed_data.generators_count;
  mean_arr = parsed_data.mean_arr;
  variance_arr = parsed_data.variance_arr;
  interval_begins_arr = parsed_data.interval_begins_arr;
  interval_ends_arr = parsed_data.interval_ends_arr;
  amount_of_numbers_to_generate_arr = parsed_data.amount_of_numbers_to_generate_arr;
}

// send size of vectors to all processes - that MPI code works
MPI_Bcast(&intervals_count, 1, MPI_INT, 0, MPI_COMM_WORLD);
MPI_Bcast(&generators_count, 1, MPI_INT, 0, MPI_COMM_WORLD);

// reserve memory for vectors
if (process_id != 0) {
  mean_arr.reserve(generators_count);
  variance_arr.reserve(generators_count);
  amount_of_numbers_to_generate_arr.reserve(generators_count);

  interval_begins_arr.reserve(intervals_count);
  interval_ends_arr.reserve(intervals_count);
}

// stolen from https://stackoverflow.com/questions/2546298/vector-usage-in-mpic
// broadcast each vector, following code compiles but not works properly
MPI_Bcast(&mean_arr[0], mean_arr.size(), MPI_FLOAT, 0, MPI_COMM_WORLD);
MPI_Bcast(&variance_arr[0], variance_arr.size(), MPI_FLOAT, 0, MPI_COMM_WORLD);
MPI_Bcast(&interval_begins_arr[0], interval_begins_arr.size(), MPI_FLOAT, 0, MPI_COMM_WORLD);
MPI_Bcast(&interval_ends_arr[0], interval_ends_arr.size(), MPI_FLOAT, 0, MPI_COMM_WORLD);
MPI_Bcast(&amount_of_numbers_to_generate_arr[0], amount_of_numbers_to_generate_arr.size(), MPI_FLOAT, 0, MPI_COMM_WORLD);

When I just simply puts results on different process, I got wrong answer:

  cout << "process ID " << process_id << endl << "amount_of_numbers_to_generate_arr[0] " << amount_of_numbers_to_generate_arr[0] << endl;

Result:

using process 0
process ID 0
amount_of_numbers_to_generate_arr[0] 100000000
using process 1
process ID 1
amount_of_numbers_to_generate_arr[0] -423989576
using process 2
process ID 2
amount_of_numbers_to_generate_arr[0] 1741864632

And it should be:

using process 0
process ID 0
amount_of_numbers_to_generate_arr[0] 100000000
using process 1
process ID 1
amount_of_numbers_to_generate_arr[0] 100000000
using process 2
process ID 2
amount_of_numbers_to_generate_arr[0] 100000000

Anyway how to fix that?

Or maybe there is a another approach to do that?

Answer 1

When you are "reserving memory for the vectors", you are not actually resizing the vectors. In the MPI_Bcast calls that follow, the sizes of the vectors are still 0, so no values are received.

To actually resize the vectors, call resize instead of reserve.