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# MatMFFDSetFunction
Sets the function used in applying the matrix free `MATMFFD` matrix. 
## Synopsis
```
#include "petscmat.h"   
PetscErrorCode MatMFFDSetFunction(Mat mat, PetscErrorCode (*func)(void *, Vec, Vec), void *funcctx)
```
Logically Collective


## Input Parameters

- ***mat -*** the matrix free matrix `MATMFFD` created via `MatCreateSNESMF()` or `MatCreateMFFD()`
- ***func -*** the function to use
- ***funcctx -*** optional function context passed to function



## Calling Sequence of `func`
```none
PetscErrorCode func(void *funcctx, Vec x, Vec f)
```

- ***funcctx -*** user provided context
- ***x -*** input vector
- ***f -*** computed output function





## Notes
If you use this you MUST call `MatAssemblyBegin()` and `MatAssemblyEnd()` on the matrix free
matrix inside your compute Jacobian routine

If this is not set then it will use the function set with `SNESSetFunction()` if `MatCreateSNESMF()` was used.


## See Also
 [](ch_matrices), `Mat`, `MATMFFD`, `MatCreateSNESMF()`, `MatMFFDGetH()`, `MatCreateMFFD()`, `MATMFFD`,
`MatMFFDSetHHistory()`, `MatMFFDResetHHistory()`, `SNESetFunction()`

## Level
advanced

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/mffd/mffd.c.html#MatMFFDSetFunction">src/mat/impls/mffd/mffd.c</A>

## Examples
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/snes/tutorials/ex22.c.html">src/snes/tutorials/ex22.c</A><BR>

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/mffd/mffd.c.html#MatMFFDSetFunction_MFFD(Mat mat, PetscErrorCode (*func)">MatMFFDSetFunction_MFFD(Mat mat, PetscErrorCode (*func) in src/mat/impls/mffd/mffd.c</A><BR>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/mat/impls/mffd/mffd.c)


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