Dear MPI forum, I posted a technical question on StackOverflow and was advised to ask it in this mailing list. The original post is here: https://stackoverflow.com/questions/70697604/is-mpi-igather-thread-safe In a nutshell, I am trying to call independent  MPI_Igathers in an OpenMP-parallel for-loop (after testing for MPI_THREAD_MULTIPLE), and am getting a deadlock apparently caused by mismatched sends and receives, even in a very much stripped down example. Any help on wether my code is not standard-conforming would be greatly appreciated. I attached two versions of the code for convenience, one of them is also in the StackOverflow post. The _v2 version does not deadlock but produces incorrect results when run with multiple MPI processes and multiple OpenMP thread per process. Thanks and best wishes, Jonas -- *J. Thies* Assistant Professor TU Delft Faculty Electrical Engineering, Mathematics and Computer Science Institute of Applied Mathematics and High Performance Computing Center Mekelweg 4 2628 CD Delft T +31 15 27 XXXX *[email protected]*