# MPI_Op_commute

Queries an MPI reduction operation for its commutativity.

## Synopsis

    int MPI_Op_commutative(MPI_Op op, int *commute)

## Input Parameters

op
  operation (handle)

## Output Parameters

commute
  Flag is true if `op` is a commutative operation. (logical)

## Null Handles

The MPI 1.1 specification, in the section on opaque objects, explicitly disallows freeing a null communicator. The text from the standard is:

     A null handle argument is an erroneous IN argument in MPI calls, unless an
     exception is explicitly stated in the text that defines the function. Such
     exception is allowed for handles to request objects in Wait and Test calls
     (sections Communication Completion and Multiple Completions ). Otherwise, a
     null handle can only be passed to a function that allocates a new object and
     returns a reference to it in the handle.

## 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 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.

## 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_ARG
  Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., `MPI_ERR_RANK`).

## See Also

MPI_Op_create