.\" -*- nroff -*-
.\" Copyright 2010 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines
.\" $COPYRIGHT$
.TH MPI_Comm_get_attr 3 "Jan 31, 2017" "2.0.2" "Open MPI"
.SH NAME
\fBMPI_Comm_get_attr\fP \- Retrieves attribute value by key.

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Comm_get_attr(MPI_Comm \fIcomm\fP, int \fIcomm_keyval\fP,
	void *\fIattribute_val\fP, int *\fIflag\fP)

.fi
.SH Fortran Syntax (see FORTRAN 77 NOTES)
.nf
INCLUDE 'mpif.h'
MPI_COMM_GET_ATTR(\fICOMM, COMM_KEYVAL, ATTRIBUTE_VAL, FLAG, IERROR\fP)
	INTEGER	\fICOMM, COMM_KEYVAL, IERROR \fP
	INTEGER(KIND=MPI_ADDRESS_KIND) \fIATTRIBUTE_VAL\fP
	LOGICAL \fIFLAG\fP

.fi
.SH C++ Syntax
.nf
#include <mpi.h>
bool MPI::Comm::Get_attr(int \fIcomm_keyval\fP, void* \fIattribute_val\fP)
	const

.fi
.SH INPUT PARAMETERS
.ft R
.TP 1i
comm
Communicator to which the attribute is attached (handle).
.TP 1i
comm_keyval
Key value (integer).

.SH OUTPUT PARAMETER
.ft R
.TP 1i
attribute_val
Attribute value, unless f\fIlag\fP = false.
.TP 1i
flag
False if no attribute is associated with the key (logical).
.TP 1i
IERROR
Fortran only: Error status (integer).

.SH DESCRIPTION
.ft R
MPI_Comm_get_attr retrieves an attribute value by key. The call is erroneous if there is no key with value \fIkeyval\fP. On the other hand, the call is correct if the key value exists, but no attribute is attached on \fIcomm\fP for that key; in that case, the call returns \fIflag\fP = false. In particular, MPI_KEYVAL_INVALID is an erroneous key value.
.sp
This function replaces MPI_Attr_get, the use of which is deprecated. The C binding is identical. The Fortran binding differs in that \fIattribute_val\fP is an address-sized integer.

.SH FORTRAN 77 NOTES
.ft R
The MPI standard prescribes portable Fortran syntax for
the \fIATTRIBUTE_VAL\fP argument only for Fortran 90. Sun FORTRAN 77
users may use the non-portable syntax
.sp
.nf
     INTEGER*MPI_ADDRESS_KIND \fIATTRIBUTE_VAL\fP
.fi
.sp
where MPI_ADDRESS_KIND is a constant defined in mpif.h
and gives the length of the declared integer in bytes.

.SH ERRORS
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI::Exception object.
.sp
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; 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.