====== MPI Send and Receive ======

MPI program using **MPI_Send** and **MPI_Recv** for communication between processes. \\ This program will involve two processes: the master process (rank 0) will send a message to a worker process (rank 1), which will then receive the message and print it.

<code c mpi_send_receive.c>
#include <mpi.h>
#include <stdio.h>
#include <string.h>

int main(int argc, char** argv) {
    // Initialize the MPI environment
    MPI_Init(&argc, &argv);

    // Find out rank, size
    int world_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
    int world_size;
    MPI_Comm_size(MPI_COMM_WORLD, &world_size);

    // We are assuming at least 2 processes for this task
    if (world_size < 2) {
        fprintf(stderr, "World size must be greater than 1 for %s\n", argv[0]);
        MPI_Abort(MPI_COMM_WORLD, 1);
    }

    int number;
    if (world_rank == 0) {
        // If we are rank 0, set the number to 7 and send it to process 1
        number = 7;
        MPI_Send(&number, 1, MPI_INT, 1, 0, MPI_COMM_WORLD);
        printf("Process 0 sent number %d to process 1\n", number);
    } else if (world_rank == 1) {
        // If we are rank 1, receive the number from process 0
        MPI_Recv(&number, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
        printf("Process 1 received number %d from process 0\n", number);
    }

    // Finalize the MPI environment
    MPI_Finalize();
}
</code>

**MPI Initialization**: **MPI_Init** initializes the MPI environment. This must be called before any other MPI functions.

**Rank and Size**: **MPI_Comm_rank** gets the rank of the current process, and **MPI_Comm_size** gets the total number of processes.

**Check Number of Processes**: We check if the total number of processes is at least 2, as our example requires two processes.

**Sending and Receiving**:

**Master Process (Rank 0)**: Sets an integer (number) to 7 and sends it to process 1 using MPI_Send. The parameters of MPI_Send include the data to be sent, the count of elements, the datatype, the destination rank, a tag to identify the message, and the communicator.

**Worker Process (Rank 1)**: Receives the integer from process 0 using MPI_Recv. The parameters of MPI_Recv include the buffer to receive the data, the count of elements, the datatype, the source rank, a tag that matches the sent message, the communicator, and a status object (here we use MPI_STATUS_IGNORE as we do not need the status).

**Finalization**: MPI_Finalize finalizes the MPI environment and should be called when the MPI operations are complete.

To run this program, compile it with an MPI compiler like **mpicc** and execute it with an MPI execution command like **mpirun** or **mpiexec**, specifying at least 2 processes.