int MPI_Buffer_detach(void *buffer_addr, int *size)
int size, mysize, idummy; void *ptr, *myptr, *dummy; MPI_Buffer_detach(&ptr, &size); MPI_Buffer_attach(myptr, mysize); ... ... library code ... ... MPI_Buffer_detach(&dummy, &idummy); MPI_Buffer_attach(ptr, size);
This is much like the action of the Unix signal routine and has the same strengths (it is simple) and weaknesses (it only works for nested usages).
Note that for this approach to work, MPI_Buffer_detach must return MPI_SUCCESS even when there is no buffer to detach. In that case, it returns a size of zero. The MPI 1.1 standard for MPI_BUFFER_DETACH contains the text
The statements made in this section describe the behavior of MPI for buffered-mode sends. When no buffer is currently associated, MPI behaves as if a zero-sized buffer is associated with the process.
This could be read as applying only to the various Bsend routines. This implementation takes the position that this applies to MPI_BUFFER_DETACH as well.
The user is responsible for ensuring that multiple threads do not try to update the same MPI object from different threads. This routine should not be used from within a signal handler.
The MPI standard defined a thread-safe interface but this does not mean that all routines may be called without any thread locks. For example, two threads must not attempt to change the contents of the same MPI_Info object concurrently. The user is responsible in this case for using some mechanism, such as thread locks, to ensure that only one thread at a time makes use of this routine. Because the buffer for buffered sends (e.g., MPI_Bsend) is shared by all threads in a process, the user is responsible for ensuring that only one thread at a time calls this routine or MPI_Buffer_attach.
All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.
The Fortran binding for this routine is different. Because Fortran does not have pointers, it is impossible to provide a way to use the output of this routine to exchange buffers. In this case, only the size field is set.