mpi understanding isend and irecv -

i have 2 processes (0 , 1), 0 gets random number, if not mod(number,2)==0 , if bigger previous ones, sends (isend) process 1.

process 1 recives number (recv) , has calculate if prime number. if prime number, process 1 sends (isend) process 0. process 0 saves highest prime number (not final prime number, generates amount of random numbers have checked).

so in meanwhile process 1 checking if number received prime number, process 0 on random number. point process 0 may overwrite data in isend because process 1 on own work , not receiving. , want this.

program mpi_nonblocking use mpi implicit none  integer :: ierr, myid ,npe, istatus(mpi_status_size), num, num_copy, best_prime=1, resto, i, n, now(3), req11 integer,parameter :: seed = 86456, numbers=200  integer :: req1(numbers), req2(numbers) logical :: flag, res   call mpi_init(ierr)  call mpi_comm_rank(mpi_comm_world,myid,ierr) call mpi_comm_size(mpi_comm_world,npe,ierr)  if (npe/=2)     write(*,*)'avviare il programma con 2 processi (-np 2 ./mpi_nonblocking)'  else      if (myid == 0)          open(22,file='uomo_0.txt', status="unknown", form= "formatted")         open(24,file='uomo_00.txt', status="unknown", form= "formatted")         call itime(now)         write(*,*)         call srand(now(3))          n=1, numbers              req1(n)=0             req2(n)=0             num=irand()             write(24,*)'random number',num             if (num>best_prime)                 resto=mod(num,2)                 if (resto==0 .and. num/=2)                     write(24,*)'cycle'                     cycle                 end if             end if              write(22,*)'uomo 0 send',num,'non-blocking'             write(24,*)'uomo 0 send',num,'non-blocking'             call mpi_isend(num,1,mpi_integer,1,1,mpi_comm_world,req1(n),ierr)             write(24,*)num,req1(n)             call mpi_irecv(best_prime,1,mpi_integer,1,0,mpi_comm_world,req2(n),ierr)          end          close(22)           call mpi_send(-1,1,mpi_integer,1,1,mpi_comm_world,ierr)         print *,'best_prime is',best_prime          !do i=1, numbers             !write(24,*)'test isend',i,'flag value',req2(i)             !call mpi_test(req2(i),flag,istatus,ierr)             !if (flag .eqv. .false.)                 !write(24,*)'uomo 0 recv',i,'non-blocking fail'             !else if    (flag .eqv. .true.)                 !write(24,*)'uomo 0 recv',i,'non-blocking success'             !end if         !end          close(24)       else           open(23,file='uomo_1.txt', status="unknown", form= "formatted")         open(25,file='uomo_11.txt', status="unknown", form= "formatted")         n=1, numbers              call mpi_recv(num,1,mpi_integer,0,myid,mpi_comm_world,istatus,ierr)             write(23,*)'uomo 1 recv',num             write(25,*)'uomo 1 recv',num             if (num==-1)                 write(23,*)'abort'                 write(25,*)'abort'                 call exit(0)             end if              res=.true.             num_copy=nint(sqrt(real(num)))             write(25,*)'sqrt',num_copy,'di num',num             i=3, num_copy, +2                  resto=mod(num,i)                 if (resto==0)                     res= .false.                     write(25,*)'num',num,'no numero primo'                     exit     !not numero primo                 end if             end              if (res .eqv. .true.)                 write(25,*)'num',num,'primo'                 write(25,*)'uomo 1 send',num,'non-blocking'                  call mpi_isend(num,1,mpi_integer,0,0,mpi_comm_world,req11,ierr)             end if          end         close(23)         close(25)      end if  end if  call mpi_finalize(ierr) end program 

the problem is, overwite happens, process 1 continues receive last number, sent process 0, multiple times: continues receive until reaches number of isend calls of process 0.

 105) uomo 0 send  1310474405 non-blocking  106) uomo 0 send   551041203 non-blocking  107) uomo 0 send  1400012957 non-blocking  108) uomo 0 send   907501537 non-blocking  109) uomo 0 send   949471365 non-blocking   61) uomo 1 recv   792320847  62) uomo 1 recv  2137864129  63) uomo 1 recv  1888232175  64) uomo 1 recv     4829859  65) uomo 1 recv  2082851615  66) uomo 1 recv   949471365  67) uomo 1 recv   949471365  68-109) uomo 1 recv   949471365  110) uomo1 recv   -1  111) abort