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Point to Point Communication Routines: Blocking Message Passing Routines

The more commonly used MPI blocking message passing routines are described below.

MPI_Send

Basic blocking send operation. Routine returns only after the application buffer in the sending task is free for reuse. Note that this routine may be implemented differently on different systems. The MPI standard permits the use of a system buffer but does not require it. Some implementations may actually use a synchronous send (discussed below) to implement the basic blocking send. MPI_Send (&buf,count,datatype,dest,tag,comm) MPI_SEND (buf,count,datatype,dest,tag,comm,ierr)

MPI_Recv

Receive a message and block until the requested data is available in the application buffer in the receiving task. MPI_Recv (&buf,count,datatype,source,tag,comm,&status) MPI_RECV (buf,count,datatype,source,tag,comm,status,ierr)

MPI_Ssend

Synchronous blocking send: Send a message and block until the application buffer in the sending task is free for reuse and the destination process has started to receive the message. MPI_Ssend (&buf,count,datatype,dest,tag,comm) MPI_SSEND (buf,count,datatype,dest,tag,comm,ierr)

MPI_Sendrecv

Send a message and post a receive before blocking. Will block until the sending application buffer is free for reuse and until the receiving application buffer contains the received message.

MPI_Sendrecv (&sendbuf,sendcount,sendtype,dest,sendtag,
...... &recvbuf,recvcount,recvtype,source,recvtag,
...... comm,&status)
MPI_SENDRECV (sendbuf,sendcount,sendtype,dest,sendtag,
...... recvbuf,recvcount,recvtype,source,recvtag,
...... comm,status,ierr)

MPI_Wait

MPI_Waitany

MPI_Waitall

MPI_Waitsome

MPI_Wait blocks until a specified non-blocking send or receive operation has completed. For multiple non-blocking operations, the programmer can specify any, all or some completions.

MPI_Wait (&request,&status)
MPI_Waitany (count,&array_of_requests,&index,&status)
MPI_Waitall (count,&array_of_requests,&array_of_statuses)
MPI_Waitsome (incount,&array_of_requests,&outcount,
...... &array_of_offsets, &array_of_statuses)
MPI_WAIT (request,status,ierr)
MPI_WAITANY (count,array_of_requests,index,status,ierr)
MPI_WAITALL (count,array_of_requests,array_of_statuses,
...... ierr)
MPI_WAITSOME (incount,array_of_requests,outcount,
...... array_of_offsets, array_of_statuses,ierr)

MPI_Probe

Performs a blocking test for a message. The “wildcards” MPI_ANY_SOURCE and MPI_ANY_TAG may be used to test for a message from any source or with any tag. For the C routine, the actual source and tag will be returned in the status structure as status.MPI_SOURCE and status.MPI_TAG. For the Fortran routine, they will be returned in the integer array status(MPI_SOURCE) and status(MPI_TAG).

MPI_Probe (source,tag,comm,&status)
MPI_PROBE (source,tag,comm,status,ierr)

MPI_Get_count

Returns the source, tag and number of elements of datatype received. Can be used with both blocking and non-blocking receive operations. For the C routine, the actual source and tag will be returned in the status structure as status.MPI_SOURCE and status.MPI_TAG. For the Fortran routine, they will be returned in the integer array status(MPI_SOURCE) and status(MPI_TAG).

MPI_Get_count (&status,datatype,&count)
MPI_GET_COUNT (status,datatype,count,ierr)

Examples: Blocking Message Passing Routines

Task 0 pings task 1 and awaits return ping

C Language - Blocking Message Passing Example

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

main(int argc, char *argv[])  {
int numtasks, rank, dest, source, rc, count, tag=1;
char inmsg, outmsg='x';
MPI_Status Stat;   // required variable for receive routines

MPI_Init(&argc,&argv);
MPI_Comm_size(MPI_COMM_WORLD, &numtasks);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);

// task 0 sends to task 1 and waits to receive a return message
if (rank == 0) {
    dest = 1;
    source = 1;
    MPI_Send(&outmsg, 1, MPI_CHAR, dest, tag, MPI_COMM_WORLD);
    MPI_Recv(&inmsg, 1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &Stat);
    }

// task 1 waits for task 0 message then returns a message
else if (rank == 1) {
    dest = 0;
    source = 0;
    MPI_Recv(&inmsg, 1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &Stat);
    MPI_Send(&outmsg, 1, MPI_CHAR, dest, tag, MPI_COMM_WORLD);
    }

// query receive Stat variable and print message details
MPI_Get_count(&Stat, MPI_CHAR, &count);
printf("Task %d: Received %d char(s) from task %d with tag %d \n",
        rank, count, Stat.MPI_SOURCE, Stat.MPI_TAG);

MPI_Finalize();
}

Fortran - Blocking Message Passing Example

program ping
include 'mpif.h'

integer numtasks, rank, dest, source, count, tag, ierr
integer stat(MPI_STATUS_SIZE)   ! required variable for receive routines
character inmsg, outmsg
outmsg = 'x'
tag = 1

call MPI_INIT(ierr)
call MPI_COMM_RANK(MPI_COMM_WORLD, rank, ierr)
call MPI_COMM_SIZE(MPI_COMM_WORLD, numtasks, ierr)

! task 0 sends to task 1 and waits to receive a return message
if (rank .eq. 0) then
    dest = 1
    source = 1
    call MPI_SEND(outmsg, 1, MPI_CHARACTER, dest, tag, MPI_COMM_WORLD, ierr)
    call MPI_RECV(inmsg, 1, MPI_CHARACTER, source, tag, MPI_COMM_WORLD, stat, ierr)

! task 1 waits for task 0 message then returns a message
else if (rank .eq. 1) then
    dest = 0
    source = 0
    call MPI_RECV(inmsg, 1, MPI_CHARACTER, source, tag, MPI_COMM_WORLD, stat, err)
    call MPI_SEND(outmsg, 1, MPI_CHARACTER, dest, tag, MPI_COMM_WORLD, err)
endif

! query receive Stat variable and print message details
call MPI_GET_COUNT(stat, MPI_CHARACTER, count, ierr)
print *, 'Task ',rank,': Received', count, 'char(s) from task', &
        stat(MPI_SOURCE), 'with tag',stat(MPI_TAG)

call MPI_FINALIZE(ierr)

end