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<H1 id="MPI_Comm_compare">MPI_Comm_compare</H1>
Compares two communicators
<H2>Synopsis</H2>
<PRE>
int MPI_Comm_compare(MPI_Comm comm1, MPI_Comm comm2, int *result)
</PRE>
<H2>Input Parameters</H2>
<DL>
<DT><B>comm1 </B> <DD> comm1 (handle)
<DT><B>comm2 </B> <DD> comm2 (handle)
</DL>
<P>
<H2>Output Parameters</H2>
<DL>
<DT><B>result </B> <DD> integer which is <tt>MPI_IDENT</tt> if the contexts and groups are the
same, <tt>MPI_CONGRUENT</tt> if different contexts but identical groups, <tt>MPI_SIMILAR
</tt>if different contexts but similar groups, and <tt>MPI_UNEQUAL</tt> otherwise
</DL>
<P>
<H2>Using 'MPI_COMM_NULL' with 'MPI_Comm_compare'</H2>
<P>
It is an error to use <tt>MPI_COMM_NULL</tt> as one of the arguments to
<tt>MPI_Comm_compare</tt>. The relevant sections of the MPI standard are
<P>
<BR>(2.4.1 Opaque Objects)
A null handle argument is an erroneous <tt>IN</tt> argument in MPI calls, unless an
exception is explicitly stated in the text that defines the function.
<P>
<BR>(5.4.1. Communicator Accessors)
where there is no text in <tt>MPI_COMM_COMPARE</tt> allowing a null handle.
<P>
<H2>Thread and Interrupt Safety</H2>
<P>
This routine is thread-safe. This means that this routine may be
safely used by multiple threads without the need for any user-provided
thread locks. However, the routine is not interrupt safe. Typically,
this is due to the use of memory allocation routines such as <tt>malloc
</tt>or other non-MPICH runtime routines that are themselves not interrupt-safe.
(To perform the communicator comparisions, this routine may need to
allocate some memory. Memory allocation is not interrupt-safe, and hence
this routine is only thread-safe.)
<P>
<H2>Notes for Fortran</H2>
All MPI routines in Fortran (except for <tt>MPI_WTIME</tt> and <tt>MPI_WTICK</tt>) have
an additional argument <tt>ierr</tt> at the end of the argument list. <tt>ierr
</tt>is an integer and has the same meaning as the return value of the routine
in C. In Fortran, MPI routines are subroutines, and are invoked with the
<tt>call</tt> statement.
<P>
All MPI objects (e.g., <tt>MPI_Datatype</tt>, <tt>MPI_Comm</tt>) are of type <tt>INTEGER
</tt>in Fortran.
<P>
<H2>Errors</H2>
<P>
All MPI routines (except <tt>MPI_Wtime</tt> and <tt>MPI_Wtick</tt>) 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 <tt>MPI_Comm_set_errhandler</tt> (for communicators),
<tt>MPI_File_set_errhandler</tt> (for files), and <tt>MPI_Win_set_errhandler</tt> (for
RMA windows). The MPI-1 routine <tt>MPI_Errhandler_set</tt> may be used but
its use is deprecated. The predefined error handler
<tt>MPI_ERRORS_RETURN</tt> may be used to cause error values to be returned.
Note that MPI does <em>not</em> guarentee that an MPI program can continue past
an error; however, MPI implementations will attempt to continue whenever
possible.
<P>
<DL>
<DT><B>MPI_SUCCESS </B> <DD> No error; MPI routine completed successfully.
</DL>
<DL>
<DT><B>MPI_ERR_ARG </B> <DD> Invalid argument. Some argument is invalid and is not
identified by a specific error class (e.g., <tt>MPI_ERR_RANK</tt>).
</DL>
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