fortran - Why this MPI_SENDRECV is deadlock? -

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this first post. thank in advance kind help. short code testing mpi_sendrecv,which did not understand. deadlock can tell me why?

 program sendrecv   include "mpif.h"   integer ibuf(20)   call mpi_init(ierr)   call mpi_comm_size(mpi_comm_world, nprocs, ierr)   call mpi_comm_rank(mpi_comm_world, myrank, ierr)   a=1   b=2   if (myid == 0)      call mpi_sendrecv(a,1,mpi_real,1,0,  .                     b,1,mpi_real,1,0,  .                     mpi_comm_world, status,ierr)   elseif (myid == 1)      call mpi_sendrecv(b,1,mpi_real,0,0,  .                     a,1,mpi_real,0,0,  .                     mpi_comm_world,status,ierr)   end if   if (myid.eq.0)      write(*,*)   endif   if (myid.eq.1)      write(*,*) b   endif   call mpi_finalize(ierr)   end

as pointed out @steveblackwell, you're using myid instead of myrank. if use:

call mpi_comm_rank(mpi_comm_world, myid, ierr)

instead you'll halfway there. deadlock arises here because both processors (probably) have myid = 0 (note, behavior compiler dependent -- other compilers might set strange number , program appear work, no messages passed).

your second problem status implicitly declared real variable, mpi expecting integer array size mpi_status_size. have sorts of effects -- segfault, or worse, strange memory error since mpi writing buffer shouldn't. (alternatively, use mpi_status_ignore since you're not doing status anyway).

as pointed out @highperformancemark, best practice explicitly type in program , use implicit none avoid these types of problems. in other words, should have reason if subroutine/module/function/main program don't have implicit none in declaration.


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