GATHER(3PVM)

NAME
pvm_gather - A specified member of the group receives messages from each member of the group and gathers these messages into a single array.

SYNOPSIS
C int info = pvm_gather( void *result, void *data,
                  int count, int datatype, int msgtag,
                  char *group, int rootginst)
Fortran call pvmfgather(result, data, count, datatype,
                        msgtag, group, rootginst, info)

PARAMETERS

result
    On the root this is a pointer to the starting address of an array datatype of local values which are to be accumulated from the members of the group. If n if the number of members in the group, then this array of datatype should be of length at least n*count. This argument is meaningful only on the root.

data
    For each group member this is a pointer to the starting address of an array of length count of datatype which will be sent to the specified root member of the group.

count
    Integer specifying the number of elements of datatype to be sent by each member of the group to the root.

datatype
    Integer specifying the type of the entries in the result and data arrays. (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 performs the gather of the messages from the members of the group.

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

DESCRIPTION
pvm_gather() performs a send of messages from each member of the group to the specified root member of the group. All group members must call pvm_gather(), each sends its array data of length count of datatype to the root which accumulates these messages into its result array. It is as if the root receives count elements of datatype from the ith member of the group and places these values in its result array starting with offset i*count from the beginning of the result array. The root task is identified by its instance number in the group.

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

In using the scatter and gather routines, keep in mind that C stores multidimensional arrays in row order, typically starting with an initial index of 0; whereas, Fortran stores arrays in column order, typically starting with an offset of 1.

Note: pvm_gather() does not block. If a task calls pvm_gather and then leaves the group before the root has called pvm_gather 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_gather(&getmatrix, &myrow, 10, PVM_INT,
                      msgtag, "workers", rootginst);

Fortran:
   CALL PVMFGATHER(GETMATRIX, MYCOLUMN, COUNT, INTEGER4,
  &                MTAG, 'workers', ROOT, INFO)

ERRORS
These error conditions can be returned by pvm_gather

PvmNoInst
    Calling task is not in the group

PvmBadParam
    The datatype specified is not appropriate

PvmSysErr
    Pvm system error

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