In some of my work, I came upon this passage in the standard describing how to create a custom MPI_Errhandler: The user routine should be, in C, a function of type MPI_Comm_errhandler_function, which is defined as typedef void MPI_Comm_errhandler_function(MPI_Comm *, int *, ...); The first argument is the communicator in use. The second is the error code to be returned by the MPI routine that raised the error. If the routine would have returned MPI_ERR_IN_STATUS, it is the error code returned in the status for the request that caused the error handler to be invoked. The remaining arguments are “varargs” arguments whose number and meaning is implementation-dependent. An implementation should clearly doc- ument these arguments. The last sentence is bolded because AFAICT, we haven't documented this anywhere. Digging through the code, it looks like we don't provide any extra arguments so the function is just: void MPI_Comm_errhandler_function(MPI_Comm *, int *); I'm happy to write this, but does anyone have a suggestion where the best place to document this would be? Thanks, Wesley