What is MPI_Start function?
MPI library
MPI (Message Passing Interface) is a library that allows you to write parallel programs in C or Fortran77. The library uses commonly available operating system services to create parallel processes and exchange information among these processes.
MPI_Start
This method starts a communication channel with a persistent request handle. The request handle is activated when the MPI_Start function returns.
Syntax
int MPI_Start(MPI_Request *request)
Parameters
requestis a communication request handle.
Return value
The function returns an error if it is unsuccessful. By default, the error aborts the MPI job.
In case of success, it returns MPI_SUCCESS- the value returned upon successful termination of any MPI routine.
Example
The following example illustrates how we can use MPI_Start with two processes where one is a sender and the other is a receiver:
#include <stdio.h>#include <stdlib.h>#include <mpi.h>int main(int argc, char* argv[]){MPI_Init(&argc, &argv);// Get the number of processes and check only 2 processes are usedint size;MPI_Comm_size(MPI_COMM_WORLD, &size);if(size != 2){printf("This application is meant to be run with 2 processes.\n");MPI_Abort(MPI_COMM_WORLD, EXIT_FAILURE);}// Get my rank and do the corresponding jobenum role_ranks { SENDER, RECEIVER };int my_rank;MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);switch(my_rank){case SENDER:{int data_sent;MPI_Request request;// Prepare the send request handleMPI_Send_init(&data_sent, 1, MPI_INT, RECEIVER, 0, MPI_COMM_WORLD, &request);data_sent = 12345;// Launch the sendMPI_Start(&request);printf("MPI process %d sends value %d.\n", my_rank, buffer_sent);break;}case RECEIVER:{int received;MPI_Recv(&received, 1, MPI_INT, SENDER, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);printf("MPI process %d received value %d.\n", my_rank, received);break;}}MPI_Finalize();return EXIT_SUCCESS;}
In the program above, we perform the following steps:
- Include the
mpi.hlibrary to use MPI functions. - Compute the number of processes using the
MPI_Comm_sizefunction and check that the application is run with 2 processes, i.e., a sender and a receiver (line 9). - For the sender, we create a request handle (line 24), prepare the send using the
MPI_Send_Initfunction (line 26), and launch the send using theMPI_Startfunction (line 29). - The receiver process receives the data sent by the sender in line 36 using the
MPI_Recvfunction.
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