---
name: "pvmfgather"
section: "3"
platform: "linux"
package: "pvm-dev"
version: "3.4.2-8"
release: "Debian 3.0 (Woody)"
platforms: ["linux"]
url: "https://new.carta.tech/pvm_gather"
---

# pvmfgather(3)

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

Part of [pvm\_gather(3)](/pvm_gather/3.md).

## 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)