mpi dynamic receive (dynamic means buffer size)

https://mpitutorial.com/tutorials/dynamic-receiving-with-mpi-probe-and-mpi-status/

get received msg len

MPI_Status structure

The three primary pieces of information include:

1. The rank of the sender. .MPI_SOURCE
2. The tag of the message. .MPI_TAG
3. The length of the message. The length of the message does not have a predefined element in the status structure. Instead, we have to find out the length of the message with MPI_Get_count.

MPI_Get_count(
    MPI_Status* status,
    MPI_Datatype datatype,
    int* count)

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

int main(int argc, char** argv) {
  MPI_Init(NULL, NULL);

  int world_size;
  MPI_Comm_size(MPI_COMM_WORLD, &world_size);
  if (world_size != 2) {
    fprintf(stderr, "Must use two processes for this example\n");
    MPI_Abort(MPI_COMM_WORLD, 1);
  }
  int world_rank;
  MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);

  const int MAX_NUMBERS = 100;
  int numbers[MAX_NUMBERS];
  int number_amount;
  if (world_rank == 0) {
    // Pick a random amount of integers to send to process one
    srand(time(NULL));
    number_amount = (rand() / (float)RAND_MAX) * MAX_NUMBERS;
    // Send the amount of integers to process one
    MPI_Send(numbers, number_amount, MPI_INT, 1, 0, MPI_COMM_WORLD);
    printf("0 sent %d numbers to 1\n", number_amount);
  } else if (world_rank == 1) {
    MPI_Status status;
    // Receive at most MAX_NUMBERS from process zero
    MPI_Recv(numbers, MAX_NUMBERS, MPI_INT, 0, 0, MPI_COMM_WORLD, &status);
    // After receiving the message, check the status to determine how many
    // numbers were actually received
    MPI_Get_count(&status, MPI_INT, &number_amount);
    // Print off the amount of numbers, and also print additional information
    // in the status object
    printf("1 received %d numbers from 0. Message source = %d, tag = %d\n",
           number_amount, status.MPI_SOURCE, status.MPI_TAG);
  }
  MPI_Barrier(MPI_COMM_WORLD);
  MPI_Finalize();
}

MPI_Probe

you can use MPI_Probe to query the message size before actually receiving it.

MPI_Probe(
    int source,
    int tag,
    MPI_Comm comm,
    MPI_Status* status)

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

int main(int argc, char** argv) {
  MPI_Init(NULL, NULL);

  int world_size;
  MPI_Comm_size(MPI_COMM_WORLD, &world_size);
  if (world_size != 2) {
    fprintf(stderr, "Must use two processes for this example\n");
    MPI_Abort(MPI_COMM_WORLD, 1);
  }
  int world_rank;
  MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);

  int number_amount;
  if (world_rank == 0) {
    const int MAX_NUMBERS = 100;
    int numbers[MAX_NUMBERS];
    // Pick a random amont of integers to send to process one
    srand(time(NULL));
    number_amount = (rand() / (float)RAND_MAX) * MAX_NUMBERS;
    // Send the amount of integers to process one
    MPI_Send(numbers, number_amount, MPI_INT, 1, 0, MPI_COMM_WORLD);
    printf("0 sent %d numbers to 1\n", number_amount);
  } else if (world_rank == 1) {
    MPI_Status status;
    // Probe for an incoming message from process zero
    MPI_Probe(0, 0, MPI_COMM_WORLD, &status);
    // When probe returns, the status object has the size and other
    // attributes of the incoming message. Get the size of the message.
    MPI_Get_count(&status, MPI_INT, &number_amount);
    // Allocate a buffer just big enough to hold the incoming numbers
    int* number_buf = (int*)malloc(sizeof(int) * number_amount);
    // Now receive the message with the allocated buffer
    MPI_Recv(number_buf, number_amount, MPI_INT, 0, 0, MPI_COMM_WORLD,
             MPI_STATUS_IGNORE);
    printf("1 dynamically received %d numbers from 0.\n",
           number_amount);
    free(number_buf);
  }
  MPI_Finalize();
}