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# DMCoarsenHookAdd

adds a callback to be run when restricting a nonlinear problem to the coarse grid

## Synopsis

    #include "petscdm.h"
    #include "petscdmlabel.h"
    #include "petscds.h"
    PetscErrorCode DMCoarsenHookAdd(DM fine, PetscErrorCode (*coarsenhook)(DM fine, DM coarse, PetscCtx ctx), PetscErrorCode (*restricthook)(DM fine, Mat mrestrict, Vec rscale, Mat inject, DM coarse, PetscCtx ctx), PetscCtx ctx)

Logically Collective; No Fortran Support

## Input Parameters

- fine - `DM` on which to run a hook when restricting to a coarser level

- coarsenhook - function to run when setting up a coarser level

- restricthook - function to run to update data on coarser levels (called once per `SNESSolve()`)

- ctx - [optional] application context for provide data for the hooks (may be `NULL`)

## Calling sequence of `coarsenhook`

- fine - fine level `DM`

- coarse - coarse level `DM` to restrict problem to

- ctx - optional application function context

## Calling sequence of `restricthook`

- fine - fine level `DM`

- mrestrict - matrix restricting a fine-level solution to the coarse grid, usually the transpose of the interpolation

- rscale - scaling vector for restriction

- inject - matrix restricting by injection

- coarse - coarse level DM to update

- ctx - optional application function context

## Notes

This function is only needed if auxiliary data, attached to the `DM` with `PetscObjectCompose()`, needs to be set up or passed from the fine `DM` to the coarse `DM`.

If this function is called multiple times, the hooks will be run in the order they are added.

In order to compose with nonlinear preconditioning without duplicating storage, the hook should be implemented to extract the finest level information from its context (instead of from the `SNES`).

The hooks are automatically called by `DMRestrict()`

## See Also

DM Basics, `DM`, `DMCoarsenHookRemove()`, `DMRefineHookAdd()`, `SNESFASGetInterpolation()`, `SNESFASGetInjection()`, `PetscObjectCompose()`, `PetscContainerCreate()`

## Level

advanced

## Location

src/dm/interface/dm.c

## Examples

src/snes/tutorials/ex48.c
src/ts/tutorials/ex29.c

Index of all DM routines
Table of Contents for all manual pages
Index of all manual pages