MPI

Note that all processes are executing the same program. This is called single program, multiple data or SPMD in the parallel programming literature.

The format of the functions MPI_Send and MPI_Recv are:

#include <stdio.h>
#include <string.h>
#include <mpi.h> 

const int MAX_STRING = 100;

int main(void) 
{
  char       greeting[MAX_STRING];
  int        comm_sz;
  int        my_rank;
  int q;

  MPI_Init(NULL, NULL); 
  MPI_Comm_size(MPI_COMM_WORLD, &comm_sz); 
  MPI_Comm_rank(MPI_COMM_WORLD, &my_rank); 

  if (my_rank != 0) 
  { 
    sprintf (greeting, "Greetings from process %d of %d!", my_rank, comm_sz);
    MPI_Send(greeting, strlen(greeting)+1, MPI_CHAR, 0, 0, MPI_COMM_WORLD); 
  } 
  else 
  {  
    printf("From myself: Greetings process %d of %d!\n", my_rank, comm_sz);
    for (q = 1; q < comm_sz; q++) 
    {
      MPI_Recv(greeting, MAX_STRING, MPI_CHAR, q, 0,MPI_COMM_WORLD, MPI_STATUS_IGNORE);
      printf("Receiving from others: %s\n", greeting);
    } 
  }
  MPI_Finalize(); 

  return 0;
}

Demo.c
mpicc a.c : To compile this program enter
mpiexec -n 5 ./a.out: To Execute this program for 5 processes enter
mpicc -g -Wall a.c : To compile a C program more accurately and get all the possible errors.
mpicc -g -Wall a.c -o aa : Have a different name for the executable file.

MPI_Init(NULL, NULL)

This is sililar to the function: initialize() for the Sequent Simulater.
The prototype(signature, first line of the definition of a funciton) is the format of :
        int MPI_Init(int *argc, char ***argv)
argc accepts the address of an integer location and argv accepts the address of an array of strings.

(int argc, char\*\* argv) argc accepts an integer and argv accepts an array of strings. But in: (int\* argc, char\*\*\* argv) argc accepts the address of an integer location and argv accepts the address of an array of strings.

MPI_Init does all the necessary setups such as creating a message buffer (i.e. array of characters), giving ranks(0,1,2...) to the processes.

MPI_Comm_size(MPI_COMM_WORLD, &comm_sz)

This is similar to the function: m_set_procs(...) in the Sequent Simulator. comm_sz is the number of the processes we like to create. For the above execution: mpiexec -n 5 ./a.out the value of the variable is : 5.

Note: Do not give any number to the variables: myRank, and comm_sz in the programm. It is at run time that you give a number.

int MPI_Comm_size(MPI_Comm comm, int* comm_sz_p)

The first parameter is of type MPI_Comm which is a predefined data type and the second parameter accepts the address of an integer location.

MPI_Comm_rank(MPI_COMM_WORLD, &my_rank)

This is similar to the function: m_get_myid() in the Sequent Simulator.This funcion obtains the rank of the running process which would be the content of the parameter: my_rank.

//if-part
  sprintf(greeting, "Greetings from process %d of %d!", my_rank, comm_sz);
  MPI_Send(greeting, strlen(greeting)+1, MPI_CHAR, 0, 0, MPI_COMM_WORLD);

The build-in function: sprintf gives a string to the array(greeting).
MPI_Send(size,content(all characters), ranks(0), tag(0),MPI_COMM_WORLD(determines to which group of processes this message should be sent)
MPI_COMM_WORLD 决定信息发送给进程的哪组（这里只有一组）。Tag(0) 例如3个进程想发送字符串去打印，发送给0 参数，但是第4个进程发送字符串并让接受者显示字符串的长度而发送1参数。

NOTE: In the call: MPI_Send we have: strlen(greeting) + 1
"+1" is there to make a string in an array of characters by adding an extra(escape) character:'\0'. This character terminates the string.

  char x[] = {'a', 'b', 'c'};
  printf("%d\n", strlen(x));

//We may not get 3 for the length of x. But if we have:

  char x[] = {'a', 'b', 'c', '\0'};
  printf("%d\n", strlen(x));
// we get 3 for the length if x as a string.

The additional "+1" is in the above program to include this extra character.

	 printf("From myself: Greetings process %d of %d!\n", my_rank, comm_sz);
  for (q = 1; q < comm_sz; q++) 
  {
     MPI_Recv(greeting, MAX_STRING, MPI_CHAR, q, 0, MPI_COMM_WORLD,MPI_STATUS_IGNORE);
     printf("Receiving from others: %s\n", greeting);
  }

For example when the value of q is 1 we have:

MPI_Recv(greeting, MAX_STRING, MPI_CHAR, 1, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);

Recving message from rank 1
Saving the message into the greeting array.
Informing the length of the string is maximum 100

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