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MPI_Comm_get_attr(3)		      MPI		  MPI_Comm_get_attr(3)

NAME
       MPI_Comm_get_attr -  Retrieves attribute	value by key

SYNOPSIS
       int MPI_Comm_get_attr(MPI_Comm comm, int	comm_keyval, void *attribute_val,
       int *flag)

INPUT PARAMETERS
       comm   -	communicator to	which the attribute is attached	(handle)
       comm_keyval
	      -	key value (integer)

OUTPUT PARAMETERS
       attribute_val
	      -	attribute value, unless	flag = false (None)
       flag   -	false if no attribute is associated with the key (logical)

NOTES
       Attributes  must	 be  extracted from the	same language as they were in-
       serted in with MPI_ATTR_PUT .  The notes	for C and  Fortran  below  ex-
       plain why.

THREAD AND INTERRUPT SAFETY
       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 malloc  or
       other  non-MPICH	 runtime  routines  that are themselves	not interrupt-
       safe.

NOTES FOR C
       Even though the attribute_val argument is declared as void *  ,	it  is
       really  the address of a	void pointer (i.e., a void ** ).  Using	a void
       * , however, is more in keeping with C idiom and	allows the pointer  to
       be passed without additional casts.

NOTES FOR FORTRAN
       All  MPI	routines in Fortran (except for	MPI_WTIME and MPI_WTICK	) have
       an additional argument ierr at the end of the argument list.   ierr  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 call	statement.

       All MPI objects (e.g., MPI_Datatype , MPI_Comm )	are of type INTEGER in
       Fortran.	  The attribute_val in Fortran is a pointer to a Fortran inte-
       ger, not	a pointer to a void * .

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 com-
       municators),  MPI_File_set_errhandler  (for files), and MPI_Win_set_er-
       rhandler	(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 guarantee that an MPI program can continue past
       an error; however, MPI implementations will attempt to  continue	 when-
       ever possible.

       MPI_SUCCESS
	      -	No error; MPI routine completed	successfully.
       MPI_ERR_ARG
	      -	Invalid	argument.  Some	argument is invalid and	is not identi-
	      fied by a	specific error class (e.g., MPI_ERR_RANK ).
       MPI_ERR_COMM
	      -	 Invalid communicator.	A common error is to use a null	commu-
	      nicator in a call	(not even allowed in MPI_Comm_rank ).
       MPI_ERR_KEYVAL
	      -	Invalid	keyval
       MPI_ERR_OTHER
	      -	Other error; use  MPI_Error_string  to	get  more  information
	      about this error code.

				   2/3/2025		  MPI_Comm_get_attr(3)

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