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函式原型

MPI_Comm_group(
	MPI_Comm comm,
	MPI_Group* group)

group 和communicator類似，但是缺少上下文所以無法實現程序間通訊，只能獲取組間的rank以及size。
MPI_Comm_create_group是與group相關的最常用函式之一，它可以利用一個group物件建立新的通訊域。
函式原型：

MPI_Comm_create_group(
	MPI_Comm comm,
	MPI_Group group,
	int tag,
	MPI_Comm* newcomm)
)

MPI_Group_incl函式：挑選組內特定rank程序構建新的group
函式原型：

MPI_Group_incl(
	MPI_Group group,
	int n,
	const int ranks[],
	MPI_Group* newgroup)

下面展示一個例項：

// Get the rank and size in the original communicator
int world_rank, world_size;
MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
MPI_Comm_size(MPI_COMM_WORLD, &world_size);

// Get the group of processes in MPI_COMM_WORLD
MPI_Group world_group;
MPI_Comm_group(MPI_COMM_WORLD, &world_group);

int n = 7;
const int ranks[7] = {1, 2, 3, 5, 7, 11, 13};

// Construct a group containing all of the prime ranks in world_group
MPI_Group prime_group;
MPI_Group_incl(world_group, 7, ranks, &prime_group);

// Create a new communicator based on the group
MPI_Comm prime_comm;
MPI_Comm_create_group(MPI_COMM_WORLD, prime_group, 0, &prime_comm);

int prime_rank = -1, prime_size = -1;
// If this rank isn't in the new communicator, it will be
// MPI_COMM_NULL. Using MPI_COMM_NULL for MPI_Comm_rank or
// MPI_Comm_size is erroneous
if (MPI_COMM_NULL != prime_comm) {
	MPI_Comm_rank(prime_comm, &prime_rank);
	MPI_Comm_size(prime_comm, &prime_size);
}

printf("WORLD RANK/SIZE: %d/%d \t PRIME RANK/SIZE: %d/%d\n",
	world_rank, world_size, prime_rank, prime_size);

MPI_Group_free(&world_group);
MPI_Group_free(&prime_group);
MPI_Comm_free(&prime_comm);