rq - persistent request (handle)
The MPI_Start() function is used to start a persistent request. The request, rq must be in a completed (or initialized) state. Once started, the request will transition into an "active" state until it is completed via a call to a function similar to MPI_Wait().
The advantage of using a persistent request is that most error-checking is done only once, when the request is initially created. This may cut down on overhead, and improve performance.
When the persistent request is no longer needed, it should be freed with MPI_Request_free().
The request must not be MPI_REQUEST_NULL and must also be a valid persistent request. PMPI_Start() is the profiling version of this function.
All MPI routines except for MPI_Wtime and MPI_Wtick return an error code. The the current MPI error handler is invoked if the return value is not MPI_SUCCESS. The default error handler aborts, but this may be changed with by using the MPI_Errhandler_set() function. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned instead. Note that MPI does not guarentee that an MPI program can continue past an error. In this implementation, all errors except MPI_ERR_INTERN or MPI_ERR_OTHER should always be recoverable.
In C, the error code is passed as the return value. In FORTRAN, all functions have an parameter ierr which returns the error code. MPI C++ functions do not directly return an error code. However, C++ users may want to use the MPI::ERRORS_THROW_EXCEPTIONS handler. This will throw an MPI::Exception with the corresponding error code. To prevent exceptions from being raised from within C and Fortran code, they will see all error return values as MPI_ERR_PENDING when this handler is chosen. In this implementation, call MPI::throw_exception() to throw the correct exception if this occurs.
MPI_SUCCESS - No error;
MPI_ERR_PENDING - Pending exception;
MPI_ERR_REQUEST - Invalid request;
MPI_ERR_INTERN - Out of Memory.
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