CODEWITHSUNDEEP
fortranmpi
Vladimir F Героям слава
Vladimir F Героям слава

Reputation: 60008

MPI_Recv overwrites parts of memory it should not access

In the following code the value of xysize gets changed, if I do not declare it as a parameter (which I generally cannot do). It happens only with optimizations -O2 and more in gfortran 4.7.2 and OpenMPI 1.6. How is it possible? I cannot find the exact interface, that I import from mpi.mod, but the C prototype clearly states that count is passed by value, hence it cannot change. 

     write(*,*) im,"receiving from",image_index([iim,jim,kim+1]),"size",&
      size(D%A(D%starti:D%endi,D%startj:D%endj,D%endk)),xysize

    call MPI_RECV(D%A(D%starti:D%endi,D%startj:D%endj,D%endk+1),xysize , MPI_REAL, image_index([iim,jim,kim+1])-1,&
           5000, comm, status, ierr)

    write(*,*) im,"received size",&
      size(D%A(D%starti:D%endi,D%startj:D%endj,D%endk)),xysize

output:

1 receiving from           2 size        4096        4096
1 received size        4096        5000

Upvotes: 2

Views: 902

Answers (1)

mgilson
mgilson

Reputation: 309929

For the sake of future visitors, I suppose I'll answer this even though it's all answered in the comments above.

As far as I'm aware, if your program is behaving properly, you cannot change the value of that parameter ("count") in a call to MPI_Recv.

Your argument status is too small, it should be an array status(MPI_STATUS_SIZE), and you're getting a buffer overflow -- This often results in a segmentation fault, but at times (depending on how the compiler packed the variables in memory), it can result in funny behavior like this.

Upvotes: 5

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