MPI_Info_get_nthkey returns the \fIn\fPth defined key in \fIinfo\fP. Keys are numbered 0\...\fIN\fP - 1 where \fIN\fP is the value returned by MPI_Info_get_nkeys. All keys between 0 and \fIN\fP - 1 are guaranteed to be defined. The number of a given key does not change as long as \fIinfo\fP is not modified with MPI_Info_set or MPI_Info_delete.
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI::Exception object.
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.