int MPI_Comm_get_name(
  MPI_Comm comm,
  char *comm_name,
  int *resultlen
);

int MPI_Comm_get_name(
  MPI_Comm comm,
  wchar_t *comm_name,
  int *resultlen
);

Parameters

comm

[in] Communicator to get name of (handle)

comm_name

[out] On output, contains the name of the communicator. It must be an array of size at least MPI_MAX_OBJECT_NAME.

resultlen

[out] Number of characters in name

Remarks

MPI_COMM_GET_NAME returns the last name which has previously been associated with the given communicator. The name may be set and got from any language. The same name will be returned independent of the language used. name should be allocated so that it can hold a resulting string of length MPI_MAX_OBJECT_NAME characters. MPI_COMM_GET_NAME returns a copy of the set name in name.

If the user has not associated a name with a communicator, or an error occurs, MPI_COMM_GET_NAME will return an empty string (all spaces in Fortran, "" in C and C++). The three predefined communicators will have predefined names associated with them. Thus, the names of MPI_COMM_WORLD, MPI_COMM_SELF, and MPI_COMM_PARENT will have the default of MPI_COMM_WORLD, MPI_COMM_SELF, and MPI_COMM_PARENT. The fact that the system may have chosen to give a default name to a communicator does not prevent the user from setting a name on the same communicator; doing this removes the old name and assigns the new one.

Because MPI specifies that null objects (e.g., MPI_COMM_NULL) are invalid as input to MPI routines unless otherwise specified, using MPI_COMM_NULL as input to this routine is an error.

Rationale..

We provide separate functions for setting and getting the name of a communicator, rather than simply providing a predefined attribute key for the following reasons:

• It is not, in general, possible to store a string as an attribute from Fortran.

   

• It is not easy to set up the delete function for a string attribute unless it is known to have been allocated from the heap.

   

• To make the attribute key useful additional code to call strdup is necessary. If this is not standardized then users have to write it. This is extra unneeded work which we can easily eliminate.

   

• The Fortran binding is not trivial to write (it will depend on details of the Fortran compilation system), and will not be portable. Therefore it should be in the library rather than in user code.

Advice to users.

The above definition means that it is safe simply to print the string returned by MPI_COMM_GET_NAME, as it is always a valid string even if there was no name.

Note that associating a name with a communicator has no effect on the semantics of an MPI program, and will (necessarily) increase the store requirement of the program, since the names must be saved. Therefore there is no requirement that users use these functions to associate names with communicators. However debugging and profiling MPI applications may be made easier if names are associated with communicators, since the debugger or profiler should then be able to present information in a less cryptic manner.

Thread and Interrupt Safety

This routine is thread and interrupt safe only if no MPI routine that updates or frees the same MPI object may be called concurrently with this routine.

The MPI standard defined a thread-safe interface but this does not mean that all routines may be called without any thread locks. For example, two threads must not attempt to change the contents of the same MPI_Info object concurrently. The user is responsible in this case for using some mechanism, such as thread locks, to ensure that only one thread at a time makes use of this routine.

Notes for Fortran

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.

Errors

All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

MPI_SUCCESS

No error; MPI routine completed successfully.

MPI_ERR_COMM

Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).

Example Code

The following sample code illustrates MPI_Comm_get_name.