I am experimenting with an interface for doing generic vector operations, which would have MPI as a potential backend. To implement a reduce over a user-provided function, I would wrap their function inside a wrapper compatible with the MPI_User_function signature, and then call MPI_Op_create. But I can only make a single wrapper as standard C does not provide a way to create function pointers out of thin air, so the pointer to user's original function needs to be somehow passed in as “context” into the wrapper. Normally, C APIs that accept callbacks also accept an opaque void pointer which is then passed into the callback function – this would serve as the context. But I do not see that in the declaration of MPI_User_function: typedef void MPI_User_function(void *invec, void *inoutvec, int *len, MPI_Datatype *datatype); This leaves me with several undesirable options: - Use global data, which, even ignoring the thread-safety implications, means that I can only wrap a single user function at any given time. - Use thread-local data, which I'm not sure is sensible either since there's no guarantee MPI is going to call my wrapper function on the same thread (and it still has the one-instance limitation). - Use invec or inoutvec to pass in the contextual data, which is awkward because it means now the data is complicated by the addition of unrelated contextual data, and it also slightly bloats the data with stuff that needn't be transmitted. - Create a wrapper function dynamically at run time, which means throwing away portability. Is there another more sensible option for this dilemma? Thanks, Phil