I'm trying to solve a simple problem with MPI, my implementation is MPICH2 and my code is in fortran. I have used the blocking send and receive, the idea is so simple but when I run it it crashes!!! I have absolutely no idea what is wrong? can anyone make quote on this issue please? there is a piece of the code: integer,parameter::IM=100,JM=100 REAL,ALLOCATABLE ::T(:,:),TF(:,:) CALL MPI_COMM_RANK(MPI_COMM_WORLD,RNK,IERR) CALL MPI_COMM_SIZE(MPI_COMM_WORLD,SIZ,IERR) prv = rnk-1 nxt = rnk+1 LIM = INT(IM/SIZ) IF (rnk==0) THEN ALLOCATE(TF(IM,JM)) prv = MPI_PROC_NULL ELSEIF(rnk==siz-1) THEN NXT = MPI_PROC_NULL LIM = LIM+MOD(IM,SIZ) END IF IF (MOD(RNK,2)==0) THEN CALL MPI_SEND(T(2,:),JM+2,MPI_REAL,PRV,10,MPI_COMM_WORLD,IERR) CALL MPI_RECV(T(1,:),JM+2,MPI_REAL,PRV,20,MPI_COMM_WORLD,STAT,IERR) ELSE CALL MPI_RECV(T(LIM+2,:),JM+2,MPI_REAL,NXT,10,MPI_COMM_WORLD,STAT,IERR) CALL MPI_SEND(T(LIM+1,:),JM+2,MPI_REAL,NXT,20,MPI_COMM_WORLD,IERR) END IF as I understood even processes are not receiving anything while the odd ones finish sending successfully, in some cases when I added some print to observe what is going on I saw that the variable NXT is changing during the sending procedure!!! for example all the odd process was sending message to process 0 not their next one!