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MPI_Comm_create_keyval

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Create a new attribute key 
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Synopsis

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int MPI_Comm_create_keyval(MPI_Comm_copy_attr_function * comm_copy_attr_fn,
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                           MPI_Comm_delete_attr_function * comm_delete_attr_fn,
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                           int *comm_keyval, void *extra_state)
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Input Parameters

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comm_copy_attr_fn
Copy callback function for <tt>keyval
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</tt>
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comm_delete_attr_fn
Delete callback function for <tt>keyval
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</tt>
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extra_state
Extra state for callback functions
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Output Parameters

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comm_keyval
key value for future access (integer)
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Notes

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Key values are global (available for any and all communicators).
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Default copy and delete functions are available.  These are
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MPI_COMM_NULL_COPY_FN
empty copy function
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MPI_COMM_NULL_DELETE_FN
empty delete function
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MPI_COMM_DUP_FN
simple dup function
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There are subtle differences between C and Fortran that require that the
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copy_fn be written in the same language from which <tt>MPI_Comm_create_keyval
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</tt>is called.
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This should not be a problem for most users; only programmers using both
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Fortran and C in the same program need to be sure that they follow this rule.
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Return value from attribute callbacks

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The MPI-2 versions of the attribute callbacks should return either
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<tt>MPI_SUCCESS</tt> on success or a valid MPI error code or class on failure.
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The MPI standard is ambiguous on this point, but as MPI-2 provides
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the routines <tt>MPI_Add_error_class</tt> and <tt>MPI_Add_error_code</tt> that allow the
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user to define and use MPI error codes and classes.
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Thread and Interrupt Safety

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This routine is thread-safe.  This means that this routine may be
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safely used by multiple threads without the need for any user-provided
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thread locks.  However, the routine is not interrupt safe.  Typically,
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this is due to the use of memory allocation routines such as <tt>malloc
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</tt>or other non-MPICH runtime routines that are themselves not interrupt-safe.
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Notes for Fortran

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All MPI routines in Fortran (except for <tt>MPI_WTIME</tt> and <tt>MPI_WTICK</tt>) have
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an additional argument <tt>ierr</tt> at the end of the argument list.  <tt>ierr
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</tt>is an integer and has the same meaning as the return value of the routine
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in C.  In Fortran, MPI routines are subroutines, and are invoked with the
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<tt>call</tt> statement.
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All MPI objects (e.g., <tt>MPI_Datatype</tt>, <tt>MPI_Comm</tt>) are of type <tt>INTEGER
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</tt>in Fortran.
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Errors

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All MPI routines (except <tt>MPI_Wtime</tt> and <tt>MPI_Wtick</tt>) return an error value;
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C routines as the value of the function and Fortran routines in the last
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argument.  Before the value is returned, the current MPI error handler is
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called.  By default, this error handler aborts the MPI job.  The error handler
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may be changed with <tt>MPI_Comm_set_errhandler</tt> (for communicators),
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<tt>MPI_File_set_errhandler</tt> (for files), and <tt>MPI_Win_set_errhandler</tt> (for
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RMA windows).  The MPI-1 routine <tt>MPI_Errhandler_set</tt> may be used but
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its use is deprecated.  The predefined error handler
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<tt>MPI_ERRORS_RETURN</tt> may be used to cause error values to be returned.
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Note that MPI does not guarentee that an MPI program can continue past
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an error; however, MPI implementations will attempt to continue whenever
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possible.
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MPI_SUCCESS
No error; MPI routine completed successfully.
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See Also

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MPI_Comm_free_keyval
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