# VecScatterCreate

Creates a vector scatter context.

### Synopsis

    #include "petscvec.h"
    PetscErrorCode PETSCVEC_DLLEXPORT VecScatterCreate(Vec xin,IS ix,Vec yin,IS iy,VecScatter *newctx)

Collective on Vec

### Input Parameters

| xin | - a vector that defines the shape (parallel data layout of the vector) of vectors from which we scatter
| yin | - a vector that defines the shape (parallel data layout of the vector) of vectors to which we scatter
| ix | - the indices of xin to scatter (if PETSC_NULL scatters all values)
| iy | - the indices of yin to hold results (if PETSC_NULL fills entire vector yin)

### Output Parameter

newctx -location to store the new scatter context

### Options Database Keys

| -vecscatter_merge | - VecScatterBegin() handles all of the communication, VecScatterEnd() is a nop eliminates the chance for overlap of computation and communication
| -vecscatter_ssend | - Uses MPI_Ssend_init() instead of MPI_Send_init()
| -vecscatter_sendfirst | - Posts sends before receives
| -vecscatter_rr | - use ready receiver mode for MPI sends
| -vecscatter_packtogether | - Pack all messages before sending, receive all messages before unpacking ONLY implemented for BLOCK SIZE of 4 and 12! (others easily added)

### C++ variants

| VecScatter VecScatterCreate(Vec x,Vec y,IS is) |  | -> |  | VecScatterCreate(x,PETSC_NULL,y,is,&s); return s;
| VecScatterCreate(Vec x,Vec y,IS is,VecScatter *s) |  | -> |  | VecScatterCreate(x,PETSC_NULL,y,is,s)
| VecScatter VecScatterCreate(Vec x,IS is,Vec y) |  | -> |  | VecScatterCreate(x,is,y,PETSC_NULL,&s); return s;
| VecScatterCreate(Vec x,IS is,Vec y,VecScatter *s) |  | -> |  | VecScatterCreate(x,is,y,PETSC_NULL,s)
| VecScatter VecScatterCreate(Vec x,IS is1,Vec y,IS is2) |  | -> |  | VecScatterCreate(x,is1,y,is2,&s); return s;

### Notes

In calls to VecScatter() you can use different vectors than the xin and yin you used above; BUT they must have the same parallel data layout, for example, they could be obtained from VecDuplicate(). A VecScatter context CANNOT be used in two or more simultaneous scatters; that is you cannot call a second VecScatterBegin() with the same scatter context until the VecScatterEnd() has been called on the first VecScatterBegin(). In this case a separate VecScatter is needed for each concurrent scatter.

### See Also

VecScatterDestroy(), VecScatterCreateToAll(), VecScatterCreateToZero()

Level:intermediate
Location:src/vec/utils/vscat.c
Index of all Vec routines
Table of Contents for all manual pages
Index of all manual pages

### Examples

src/dm/ao/examples/tutorials/ex2.c.html
src/dm/da/examples/tutorials/ex6.c.html
src/dm/da/examples/tutorials/ex6f90.F.html