Reduces values on all processes to a single value in a nonblocking way
int MPI_Ireduce(const void *sendbuf, void *recvbuf, int count, MPI_Datatype datatype, MPI_Op op,
int root, MPI_Comm comm, MPI_Request *request)
- address of send buffer (choice)
- number of elements in send buffer (non-negative integer)
- data type of elements of send buffer (handle)
- reduce operation (handle)
- rank of root process (integer)
- communicator (handle)
- address of receive buffer (choice)
- communication request (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
All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER
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
- No error; MPI routine completed successfully.
- Invalid argument. Some argument is invalid and is not
identified by a specific error class (e.g., MPI_ERR_RANK).
- Invalid buffer pointer. Usually a null buffer where
one is not valid.
- Invalid communicator. A common error is to use a null
communicator in a call (not even allowed in MPI_Comm_rank).
- Invalid count argument. Count arguments must be
non-negative; a count of zero is often valid.
- Invalid operation. MPI operations (objects of type MPI_Op)
must either be one of the predefined operations (e.g., MPI_SUM) or
created with MPI_Op_create.
- Invalid datatype argument. Additionally, this error can
occur if an uncommitted MPI_Datatype (see MPI_Type_commit) is used
in a communication call.
- Other error; use MPI_Error_string to get more information
about this error code.