## 3.2.4. Clarification of MPI_INTERCOMM_CREATE

Up: MPI-1.0 and MPI-1.1 Clarifications Next: Clarification of MPI_INTERCOMM_MERGE Previous: Clarification of status after MPI_WAIT and MPI_TEST

The Problem: The MPI-1.1 standard says, in the discussion of MPI_INTERCOMM_CREATE, both that <QUOTE> The groups must be disjoint </QUOTE> and that <QUOTE> The leaders may be the same process. </QUOTE> To further muddy the waters, the reason given for ``The groups must be disjoint'' is based on concerns about the implementation of MPI_INTERCOMM_CREATE that are not applicable for the case where the leaders are the same process.

The Fix: Delete the text: <QUOTE> (the two leaders could be the same process) </QUOTE> from the discussion of MPI_INTERCOMM_CREATE.

Replace the text: <QUOTE> All inter-communicator constructors are blocking and require that the local and remote groups be disjoint in order to avoid deadlock. </QUOTE> with <QUOTE>

All inter-communicator constructors are blocking and require that the local and remote groups be disjoint.

Advice to users.

The groups must be disjoint for several reasons. Primarily, this is the intent of the intercommunicators --- to provide a communicator for communication between disjoint groups. This is reflected in the definition of MPI_INTERCOMM_MERGE, which allows the user to control the ranking of the processes in the created intracommunicator; this ranking makes little sense if the groups are not disjoint. In addition, the natural extension of collective operations to intercommunicators makes the most sense when the groups are disjoint. ( End of advice to users.)
</QUOTE>

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MPI-2.0 of July 18, 1997
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