g1 - group (handle)
g2 - group (handle)
out - the resulting union group (handle).
The MPI_Group_union() function is used to construct a new group out that is the set union between the two groups g1 and g2 .
i.e. The processes in the new group will be those in either of g1 and g2 .
If the new group is empty then MPI_GROUP_EMPTY is returned in out .
The new group returned should eventually be freed with the MPI_Group_free() function.
In C++, the new group is passed as the return value to this function.
Note that the groups must be valid (not MPI_GROUP_NULL). PMPI_Group_union() 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_GROUP - Invalid group;
MPI_ERR_ARG - Invalid output pointer;
MPI_ERR_INTERN - Out of Memory, cannot create a new group.
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