MPI发送/接收错误

我用c++写了一个测试程序，以确保我理解MPI发送/接收是如何工作的(显然我没有)。这个测试运行在4个处理器上。我感兴趣的具体情况是，处理器0将数组"send_n"发送给处理器2，处理器2在数组"recv_s"中接收它。在初始发送之后，数组是正确的(它应该都是5)，但是在我做了一些额外的发送/接收之后，数组以某种方式改变了。我哪里做错了?

#include <stdlib.h>
#include <iostream>
using namespace std;
# include "mpi.h"
void passBCs(double recv_n[],double recv_e[],double recv_s[],double recv_w[]);
int getNextProcID(int pID, int direction);
int procID,numProcs;
int gridx=2,gridy=2;
int procGridX=2, procGridY=2; 
int main(){
int i,j,k;
int cryptIDs[2]={0,3};
int villusIDs[2]={1,2};
double recv_n[gridx*5],recv_e[gridy*5],recv_s[gridx*5],recv_w[gridy*5];
MPI::Init();
procID=MPI::COMM_WORLD.Get_rank();
numProcs=MPI::COMM_WORLD.Get_size();
if(procID==0){cout<<"MPI Initializedn";}

passBCs(recv_n,recv_e,recv_s,recv_w);   
MPI::COMM_WORLD.Barrier();

if(procID==2){
for(i=0;i<10;i++){cout<<"Test Buffer="<<recv_s[i]<<"n";}}
MPI::Finalize();    
if(procID==0){cout<<"Test Run Exiting Normallyn";}
}   

 void passBCs(double recv_n[],double recv_e[],double recv_s[],double recv_w[]){
int i,j,k,nId,eId,sId,wId,n_rId,e_rId,s_rId,w_rId;
int ntag,etag,stag,wtag;
double send_n[gridx*5],send_e[gridy*5],send_s[gridx*5],send_w[gridy*5];

ntag=0;
etag=1;
stag=2;
wtag=3;
if(procID==0){
for(i=0;i<10;i++){
    send_n[i]=5;
    send_s[i]=1;
    send_e[i]=2;
    send_w[i]=3;
}}
else{   for(i=0;i<10;i++){
    send_n[i]=0;
    send_s[i]=0;
    send_e[i]=0;
    send_w[i]=0;
}}

nId=getNextProcID(procID,0);
eId=getNextProcID(procID,1);
sId=getNextProcID(procID,2);
wId=getNextProcID(procID,3);
n_rId=getNextProcID(procID,2);
e_rId=getNextProcID(procID,3);
s_rId=getNextProcID(procID,0);
w_rId=getNextProcID(procID,1);
if(procID==2){cout<<"South Recv ID="<<n_rId<<"n";}
if(procID==0){cout<<"Proc 0 sending North to "<<nId<<"n";}
MPI::COMM_WORLD.Send(&send_n[0],20,MPI::DOUBLE,nId,ntag);
MPI::COMM_WORLD.Recv(&recv_s[0],20,MPI::DOUBLE,n_rId,ntag);
if(procID==2){
for(i=0;i<10;i++){cout<<"Test Buffer0="<<recv_s[i]<<"n";}}
MPI::COMM_WORLD.Send(&send_e[0],20,MPI::DOUBLE,eId,etag);
MPI::COMM_WORLD.Recv(&recv_w[0],20,MPI::DOUBLE,e_rId,etag);
if(procID==2){
for(i=0;i<10;i++){cout<<"Test Buffer1="<<recv_s[i]<<"n";}}
MPI::COMM_WORLD.Send(&send_s[0],20,MPI::DOUBLE,sId,stag);
MPI::COMM_WORLD.Recv(&recv_n[0],20,MPI::DOUBLE,s_rId,stag);
MPI::COMM_WORLD.Send(&send_w[0],20,MPI::DOUBLE,wId,wtag);
MPI::COMM_WORLD.Recv(&recv_e[0],20,MPI::DOUBLE,w_rId,wtag);
}
int getNextProcID(int pID, int direction){
//Returns the ID number for the processor that is "direction" to the give proc id.
//0=north,1=east,2=south,3=west;
int x_pos,y_pos,nextID; 
x_pos=pID%procGridX;
y_pos=pID/procGridY;
if(direction==0){y_pos++;}
if(direction==1){x_pos++;}
if(direction==2){y_pos--;}
if(direction==3){x_pos--;}
if(x_pos<0){x_pos=procGridX-1;}
if(x_pos>=procGridX){x_pos=0;}
if(y_pos<0){y_pos=procGridY-1;}
if(y_pos>=procGridY){y_pos=0;}
nextID=y_pos*procGridY+x_pos;
return nextID;  
}

输出为:

MPI Initialized
South Recv ID=0
Proc 0 sending North to 2
Test Buffer0=5
Test Buffer0=5
Test Buffer0=5
Test Buffer0=5
Test Buffer0=5
Test Buffer0=5
Test Buffer0=5
Test Buffer0=5
Test Buffer0=5
Test Buffer0=5
Test Buffer1=0
Test Buffer1=0
Test Buffer1=0
Test Buffer1=0
Test Buffer1=0
Test Buffer1=0
Test Buffer1=0
Test Buffer1=0
Test Buffer1=5
Test Buffer1=5
Test Buffer=0
Test Buffer=0
Test Buffer=0
Test Buffer=0
Test Buffer=0
Test Buffer=0
Test Buffer=0
Test Buffer=0
Test Buffer=5
Test Buffer=5
Test Run Exiting Normally