# MPI_Attr_get

Retrieves attribute value by key

## Synopsis

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

## Input Parameters

comm
  communicator to which attribute is attached (handle)
keyval
  key value (integer)

## Output Parameters

attribute_val
  attribute value, unless `flag` = false
flag
  true if an attribute value was extracted; false if no attribute is associated with the key

## Notes

Attributes must be extracted from the same language as they were inserted in with `MPI_ATTR_PUT`. The notes for C and Fortran below explain why.

## 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.

## 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.

## 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 `MPI_Comm_get_attr`.

## 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 integer, 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 communicators), `MPI_File_set_errhandler` (for files), and `MPI_Win_set_errhandler` (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 whenever possible.

MPI_SUCCESS
  No error; MPI routine completed successfully.

MPI_ERR_COMM
  Invalid communicator. A common error is to use a null communicator in a call (not even allowed in `MPI_Comm_rank`).

MPI_ERR_KEYVAL
  Invalid keyval

## See Also

MPI_Attr_put, MPI_Keyval_create, MPI_Attr_delete, MPI_Comm_get_attr