.TH MPI_Attr_get 3 "11/12/2019" " " "MPI"

.SH NAME

MPI_Attr_get \-  Retrieves attribute value by key 

.SH SYNOPSIS

.nf

int MPI_Attr_get(MPI_Comm comm, int keyval, void *attribute_val, int *flag)

.fi

.SH INPUT PARAMETERS

.PD 0

.TP

.B comm 

- communicator to which attribute is attached (handle)

.PD 1

.PD 0

.TP

.B keyval 

- key value (integer)

.PD 1

.SH OUTPUT PARAMETERS

.PD 0

.TP

.B attribute_val 

- attribute value, unless 

.I flag

= false

.PD 1

.PD 0

.TP

.B flag 

- true if an attribute value was extracted;  false if no attribute is

associated with the key

.PD 1

.SH NOTES

Attributes must be extracted from the same language as they were inserted

in with 

.I MPI_ATTR_PUT

\&.

The notes for C and Fortran below explain why.

.SH NOTES FOR C

Even though the 

.I attribute_val

argument is declared as 

.I void *

, it is

really the address of a void pointer (i.e., a 

.I void **

).  Using

a 

.I void *

, however, is more in keeping with C idiom and allows the

pointer to be passed without additional casts.

.SH 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 

.I malloc

or other non-MPICH runtime routines that are themselves not interrupt-safe.

.SH DEPRECATED FUNCTION

The MPI-2 standard deprecated a number of routines because MPI-2 provides

better versions.  This routine is one of those that was deprecated.  The

routine may continue to be used, but new code should use the replacement

routine.

The replacement for this routine is 

.I MPI_Comm_get_attr

\&.

.SH NOTES FOR FORTRAN

All MPI routines in Fortran (except for 

.I MPI_WTIME

and 

.I MPI_WTICK

) have

an additional argument 

.I ierr

at the end of the argument list.  

.I 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

.I call

statement.

All MPI objects (e.g., 

.I MPI_Datatype

, 

.I MPI_Comm

) are of type 

.I INTEGER

in Fortran.

The 

.I attribute_val

in Fortran is a pointer to a Fortran integer, not

a pointer to a 

.I void *

\&.

.SH ERRORS

All MPI routines (except 

.I MPI_Wtime

and 

.I 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 

.I MPI_Comm_set_errhandler

(for communicators),

.I MPI_File_set_errhandler

(for files), and 

.I MPI_Win_set_errhandler

(for

RMA windows).  The MPI-1 routine 

.I MPI_Errhandler_set

may be used but

its use is deprecated.  The predefined error handler

.I MPI_ERRORS_RETURN

may be used to cause error values to be returned.

Note that MPI does 

.B not

guarentee that an MPI program can continue past

an error; however, MPI implementations will attempt to continue whenever

possible.

.PD 0

.TP

.B MPI_SUCCESS 

- No error; MPI routine completed successfully.

.PD 1

.PD 0

.TP

.B MPI_ERR_COMM 

- Invalid communicator.  A common error is to use a null

communicator in a call (not even allowed in 

.I MPI_Comm_rank

).

.PD 1

.PD 0

.TP

.B MPI_ERR_KEYVAL 

- Invalid keyval

.PD 1

.SH SEE ALSO

MPI_Attr_put, MPI_Keyval_create, MPI_Attr_delete, MPI_Comm_get_attr

.br