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5.8.5 Writing new MPI function wrappers

For the most part the wrappers are straightforward. The only significant complexity arises with nonblocking receives.

The issue is that ‘MPI_Irecv’ states the recv buffer and returns immediately, giving a handle (‘MPI_Request’) for the transaction. Later the user will have to poll for completion with ‘MPI_Wait’ etc, and when the transaction completes successfully, the wrappers have to paint the recv buffer. But the recv buffer details are not presented to ‘MPI_Wait’---only the handle is. The library therefore maintains a shadow table which associates uncompleted ‘MPI_Request’s with the corresponding buffer address/count/type. When an operation completes, the table is searched for the associated address/count/type info, and memory is marked accordingly.

Access to the table is guarded by a (POSIX pthreads) lock, so as to make the library thread-safe.

The table is allocated with ‘malloc’ and never ‘free’d, so it will show up in leak checks.

Writing new wrappers should be fairly easy. The source file is ‘auxprogs/libmpiwrap.c’. If possible, find an existing wrapper for a function of similar behaviour to the one you want to wrap, and use it as a starting point. The wrappers are organised in sections in the same order as the MPI 1.1 spec, to aid navigation. When adding a wrapper, remember to comment out the definition of the default wrapper in the long list of defaults at the bottom of the file (do not remove it, just comment it out).