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# KSPPIPEGCRSetModifyPC
Sets the routine used by `KSPPIPEGCR` to modify the preconditioner at each iteration 
## Synopsis
```
#include "petscksp.h"  
PetscErrorCode KSPPIPEGCRSetModifyPC(KSP ksp, PetscErrorCode (*function)(KSP, PetscInt, PetscReal, void *), void *data, PetscErrorCode (*destroy)(void *))
```
Logically Collective


## Input Parameters
+  ksp      - iterative context obtained from KSPCreate()
.  function - user defined function to modify the preconditioner
.  ctx      - user provided context for the modify preconditioner function
-  destroy  - the function to use to destroy the user provided application context.


## Calling Sequence of `function`
```none
PetscErrorCode function(KSP ksp, PetscInt n, PetscReal rnorm, void *ctx)
```

- ***ksp   -*** iterative context
- ***n     -*** the total number of PIPEGCR iterations that have occurred
- ***rnorm -*** 2-norm residual value
- ***ctx   -*** the user provided application context



## Calling Sequence of `destroy`
```none
PetscErrorCode destroy(void *ctx)
```




## Notes
The default modifypc routine is `KSPPIPEGCRModifyPCNoChange()`


## See Also
 [](ch_ksp), `KSPPIPEGCR`, `KSPPIPEGCRModifyPCNoChange()`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/impls/gcr/pipegcr/pipegcr.c.html#KSPPIPEGCRSetModifyPC">src/ksp/ksp/impls/gcr/pipegcr/pipegcr.c</A>

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/impls/gcr/pipegcr/pipegcr.c.html#KSPPIPEGCRSetModifyPC_PIPEGCR">KSPPIPEGCRSetModifyPC_PIPEGCR in src/ksp/ksp/impls/gcr/pipegcr/pipegcr.c</A><BR>


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


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