Hi Jeffrey, Thanks a lot for answering my message. The &work io work thing escaped my attention. (Usually the compiler catches these things but of course here any pointer type should work. I suppose with C++ MPI it's possible to enforce more strict type checking using template or so.)
Anyway in the meantime I rewrote everything without this mistake, but still wasn't aware of it. Everything works as expected now. thanks, Peter On 17 April 2012 22:15, Jeffrey Squyres <jsquy...@cisco.com> wrote: > Sorry for the delay in replying; I was out last week. > > MPI_SEND and MPI_RECV take pointers to the buffer to send and receive, > respectively. > > When you send a scalar variable, like an int, you get the address of the > buffer via the & operator (e.g., MPI_Send(&i, ...)). When you send a > new'ed/malloc'ed array, you only need to send the pointer value -- i.e., the > address pointing to the buffer. Don't send the address of the pointer, > because then you're telling MPI to overwrite the pointer itself. I.e.,: > > work = new char[...]; > MPI_Send(work, ...) > > not > > work = new char[...]; > MPI_Send(&work, ...); > > More below. > > On Apr 11, 2012, at 3:03 PM, Peter Sels wrote: > >> Probably a buffer overrun or so, but I cannot see where. > > The buffer overrun is where you specify &array in your MPI_SEND/MPI_RECV > calls. > >> On linux I get: Segmentation fault (11) >> >> Increasing the length gives more problems... >> >> How can I get this code stable? >> What am I doing wrong? >> Is there a maximum length to MPI messages? > > No. > >> For sending a string, do I use MPI_CHARACTER or MPI_BYTE or ...? > > MPI_BYTE. MPI_CHARACTER is for Fortran CHARACTERs. > >> How come I cannot assert that my messages end in '\0' when received? >> And how come that when I print them, I also get a segmentation fault? > > I think these two issues are symptoms of (work) vs. (&work), from above. > >> Can I send two subsequent messages using MPI_Send, or do I have to do > > Sure. > >> the first as MPI_Isend and then do a MPI_Wait before the next >> MPI_Send?... > > You can do multiple Isend's and then a Waitall, if you want. > >> Why do I not find code online for receiving the length first and then >> allocating a buffer of this size and then receiving the next message? > > I don't know. Perhaps you didn't google enough? :-) > > FWIW, the new MPI-3 functions MPI_MPROBE and MPI_IMPROBE will help with > unknown-length messages, too. We have that implemented on the Open MPI SVN > trunk, but they are not yet available in a stable release. They'll debut in > OMPI v1.7. > > -- > Jeff Squyres > jsquy...@cisco.com > For corporate legal information go to: > http://www.cisco.com/web/about/doing_business/legal/cri/ > > > _______________________________________________ > users mailing list > us...@open-mpi.org > http://www.open-mpi.org/mailman/listinfo.cgi/users -- Peter Sels www.LogicallyYours.com
