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# PCShellSetMatApply
Sets routine to use as preconditioner on a block of vectors.
## Synopsis
```
#include "petscpc.h"
PetscErrorCode PCShellSetMatApply(PC pc, PetscErrorCode (*matapply)(PC, Mat, Mat))
```
Logically Collective
## Input Parameters
- ***pc -*** the preconditioner context
- ***apply -*** the application-provided preconditioning routine
## Calling sequence of `apply`
```none
PetscErrorCode apply(PC pc, Mat Xin, Mat Xout)
```
- ***pc -*** the preconditioner, get the application context with `PCShellGetContext()`
- ***Xin -*** input block of vectors
- ***Xout -*** output block of vectors
## See Also
`PCSHELL`, `PCShellSetApply()`
## Level
advanced
## Location
src/ksp/pc/impls/shell/shellpc.c
## Examples
src/ksp/ksp/tutorials/ex79.c
## Implementations
PCShellSetMatApply_Shell(PC pc, PetscErrorCode (*matapply) in src/ksp/pc/impls/shell/shellpc.c
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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/ksp/pc/impls/shell/shellpc.c)
[Index of all PC routines](index.md)
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