利用mpi求解微分方程時，經常會遇到不同進程的通訊，特別是如下形式的通訊：
進程0->進程1->進程2->進程3...->進程n->進程0
這時，若單純的利用MPI_Send, MPI_Recv函數進行通訊的話，容易造成死鎖，下面介紹MPI_Sendrecv的來解決這個問題。顧名思義，MPI_Sendrecv表示的作用是將本進程的信息發送出去，並接收其他進程的信息，其調用方式如下：
MPI_Sendrecv( void *sendbuf //initial address of send buffer
int sendcount //number of entries to send
MPI_Datatype sendtype //type of entries in send buffer
int dest //rank of destination
int sendtag //send tag
void *recvbuf //initial address of receive buffer
int recvcount //max number of entries to receive
MPI_Datatype recvtype //type of entries in receive buffer (這里數目是按實數的數目，若數據類型為MPI_COMPLEX時，傳遞的數目要乘以2)
int source //rank of source
int recvtag //receive tag
MPI_Comm comm //group communicator
MPI_Status status //return status;
下面給出一個實例：
#include<stdio.h>
#include "mpi.h"
#include <math.h>
#define n 4
int
main(int argc, char* argv[]){
int nProcs, Rank, i;
double A0[n],A1[n];
MPI_Status status;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &nProcs);
MPI_Comm_rank(MPI_COMM_WORLD, &Rank);
for(int i=0; i<n; i++){
A0[i] = Rank;
A1[i] = Rank;
}
printf("\nBefore exchange A0 A1:\n");
for(i=0;i<n;i++){
printf("rank:%d\t%f\t%f\n",Rank, A0[i], A1[i]);
}
int rightrank = (Rank + 1) % nProcs;
int leftrank = (Rank + nProcs-1)%nProcs;
MPI_Barrier(MPI_COMM_WORLD);
MPI_Sendrecv(A0, n, MPI_DOUBLE, rightrank,990,
A1, n, MPI_DOUBLE, leftrank,990, MPI_COMM_WORLD,&status);
MPI_Finalize();
printf("After exchange A0 A1\n");
for(i=0;i<n;i++){
printf("rank:%d %f\t%f\n",Rank, A0[i], A1[i]);
}
}
下面這條語句表示：將進程為Rank的A0發送到rightrank進程，並接收來自leftrank的A1。
MPI_Sendrecv(A0, n, MPI_DOUBLE, rightrank,990,
A1, n, MPI_DOUBLE, leftrank,990, MPI_COMM_WORLD,&status);
得到的數值結果如下：
Before exchange A0 A1 rank:0 0.000000 0.000000 rank:0 0.000000 0.000000 rank:0 0.000000 0.000000 rank:0 0.000000 0.000000 Before exchange A0 A1 rank:1 1.000000 1.000000 rank:1 1.000000 1.000000 rank:1 1.000000 1.000000 rank:1 1.000000 1.000000 Before exchange A0 A1 rank:2 2.000000 2.000000 rank:2 2.000000 2.000000 rank:2 2.000000 2.000000 rank:2 2.000000 2.000000 Before exchange A0 A1 rank:3 3.000000 3.000000 rank:3 3.000000 3.000000 rank:3 3.000000 3.000000 rank:3 3.000000 3.000000 After exchange A0 A1 rank:1 1.000000 0.000000 rank:1 1.000000 0.000000 rank:1 1.000000 0.000000 rank:1 1.000000 0.000000 After exchange A0 A1 rank:2 2.000000 1.000000 rank:2 2.000000 1.000000 rank:2 2.000000 1.000000 rank:2 2.000000 1.000000 After exchange A0 A1 rank:3 3.000000 2.000000 rank:3 3.000000 2.000000 rank:3 3.000000 2.000000 rank:3 3.000000 2.000000 After exchange A0 A1 rank:0 0.000000 3.000000 rank:0 0.000000 3.000000 rank:0 0.000000 3.000000 rank:0 0.000000 3.000000