# DMSubDomainHookAdd
adds a callback to be run when restricting a problem to the coarse grid 
## Synopsis
```
#include "petscdm.h"          
#include "petscdmlabel.h"     
#include "petscds.h"     
PetscErrorCode DMSubDomainHookAdd(DM global, PetscErrorCode (*ddhook)(DM, DM, void *), PetscErrorCode (*restricthook)(DM, VecScatter, VecScatter, DM, void *), void *ctx)
```
Logically Collective on global


## Input Parameters

- ***global -*** global `DM`
- ***ddhook -*** function to run to pass data to the decomposition `DM` upon its creation
- ***restricthook -*** function to run to update data on block solve (at the beginning of the block solve)
- ***ctx -*** [optional] user-defined context for provide data for the hooks (may be NULL)



## Calling sequence for ddhook
```none
ddhook(DM global,DM block,void *ctx)
```


- ***global -*** global `DM`
- ***block  -*** block `DM`
- ***ctx -*** optional user-defined function context



## Calling sequence for restricthook
```none
restricthook(DM global,VecScatter out,VecScatter in,DM block,void *ctx)
```


- ***global -*** global `DM`
- ***out    -*** scatter to the outer (with ghost and overlap points) block vector
- ***in     -*** scatter to block vector values only owned locally
- ***block  -*** block `DM`
- ***ctx -*** optional user-defined function context





## Notes
This function is only needed if auxiliary data needs to be set up on subdomain `DM`s.

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 global information from its context (instead of from the `SNES`).


## Fortran Note
This function is not available from Fortran.


## See Also
 `DMSubDomainHookRemove()`, `DMRefineHookAdd()`, `SNESFASGetInterpolation()`, `SNESFASGetInjection()`, `PetscObjectCompose()`, `PetscContainerCreate()`

## Level
advanced

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/dm/interface/dm.c.html#DMSubDomainHookAdd">src/dm/interface/dm.c</A>

## Examples
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex29.c.html">src/ts/tutorials/ex29.c.html</A><BR>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/dm/interface/dm.c)


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