# MatShellTestMult
Compares the multiply routine provided to the `MATSHELL` with differencing on a given function. 
## Synopsis
```
#include "petscmat.h" 
PetscErrorCode MatShellTestMult(Mat mat, PetscErrorCode (*f)(void *, Vec, Vec), Vec base, void *ctx, PetscBool *flg)
```
Logically Collective on mat


## Input Parameters

- ***mat -*** the `MATSHELL` shell matrix
- ***f -*** the function
- ***base -*** differences are computed around this vector, see `MatMFFDSetBase()`, for Jacobians this is the point at which the Jacobian is being evaluated
- ***ctx -*** an optional context for the function



## Output Parameter

- ***flg -*** `PETSC_TRUE` if the multiply is likely correct



## Options Database Key

- ***-mat_shell_test_mult_view -*** print if any differences are detected between the products and print the difference





## Fortran Note
Not supported from Fortran


## See Also
 `MATSHELL`, `MatCreateShell()`, `MatShellGetContext()`, `MatShellGetOperation()`, `MatShellTestMultTranspose()`

## Level
advanced

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


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


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