REDUCE(3PVM)

NAME
pvm_reduce - Performs a reduction operation over members of the specified group.

SYNOPSIS
C int info = pvm_reduce( void (*func)(),
                  void *data, int count, int datatype,
                  int msgtag, char *group, int rootginst)
Fortran call pvmfreduce(func, data, count, datatype,
                        msgtag, group, rootginst, info)

PARAMETERS

func
    Function which defines the operation performed on the global data. Predefined are PvmMax, PvmMin, PvmSum, and PvmProduct. Users can define their own function.
      SYNOPSIS for func
      C void func(int *datatype, void *x, void *y,
                    int *num, int *info)
      Fortran call func(datatype, x, y, num, info)

data
    Pointer to the starting address of an array of local values. On return, the data array on the root will be overwritten with the result of the reduce operation over the group.

count
    Integer specifying the number of elements of datatype in the data array.

datatype
    Integer specifying the type of the entries in the data array. (See below for defined types.)

msgtag
    Integer message tag supplied by the user. msgtag should be >= 0. It allows the user's program to distinguish between different kinds of messages.

group
    Character string group name of an existing group.

rootginst
    Integer instance number of group member who gets the result.

info
    Integer status code returned by the routine. Values less than zero indicate an error.

DISCUSSION
pvm_reduce() performs global operations such as max, min, and sum over all the tasks in a group. All group members call pvm_reduce() with their local data, and the result of the reduction operation appears on the user specified root task root. The root task is identified by its instance number in the group.

PVM supplies the following predefined functions that can be specified in func.
         PvmMin
         PvmMax
         PvmSum
         PvmProduct
PvmMax and PvmMin are implemented for all the dataypes listed below. For complex values the minimum [maximum] is that complex pair with the minimum [maximum] modulus. PvmSum and PvmProduct are implemented for all the dataypes listed below with the exception of PVM_BYTE and BYTE1.

C and Fortran defined datatypes are:
           C datatypes   FORTRAN datatypes
         -----------------------------------
           PVM_BYTE       BYTE1
           PVM_SHORT      INTEGER2
           PVM_INT        INTEGER4
           PVM_FLOAT      REAL4
           PVM_CPLX       COMPLEX8
           PVM_DOUBLE     REAL8
           PVM_DCPLX      COMPLEX16
           PVM_LONG

A user defined function may be used used in func. The argument func is a function with four arguments. It is the base function used for the reduction operation. Both x and y are arrays of type specified by datatype with num entries. The arguments datatype and info are as specified above. The arguments x and num correspond to data and count above. The argument y contains received values.

Note: pvm_reduce() does not block. If a task calls pvm_reduce and then leaves the group before the root has called pvm_reduce an error may occur.

The current algorithm is very simple and robust. A future implementation may make more efficient use of the architecture to allow greater parallelism.

EXAMPLES
C:
   info =  pvm_reduce(PvmMax, &myvals, 10, PVM_INT,
                      msgtag, "workers", rootginst);

Fortran:
   CALL PVMFREDUCE(PvmMax, MYVALS, COUNT, INTEGER4,
  &                MTAG, 'workers', ROOT, INFO)

ERRORS
These error conditions can be returned by pvm_reduce
	PvmNoInst	Calling task is not in the group
	PvmBadParam	The datatype specified is not appropriate
		for the specified reduction function.
	PvmSysErr	Pvm system error

SEE ALSO
pvm_bcast(3PVM), pvm_barrier(3PVM), pvm_psend(3PVM)