:orphan:
# PCShellSetPreSolve
Sets routine to apply to the operators/vectors before a `KSPSolve()` is applied. This usually does something like scale the linear system in some application specific way. 
## Synopsis
```
#include "petscpc.h" 
PetscErrorCode PCShellSetPreSolve(PC pc, PetscErrorCode (*presolve)(PC, KSP, Vec, Vec))
```
Logically Collective


## Input Parameters

- ***pc -*** the preconditioner context
- ***presolve -*** the application-provided presolve routine



## Calling sequence of `presolve`
```none
   PetscErrorCode presolve(PC pc, KSP ksp, Vec b, Vec x)
```


- ***pc -*** the preconditioner, get the application context with `PCShellGetContext()`
- ***xin -*** input vector
- ***xout -*** output vector





## See Also
 `PCSHELL`, `PCShellSetApplyRichardson()`, `PCShellSetSetUp()`, `PCShellSetApplyTranspose()`, `PCShellSetPostSolve()`, `PCShellSetContext()`

## Level
advanced

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/impls/shell/shellpc.c.html#PCShellSetPreSolve">src/ksp/pc/impls/shell/shellpc.c</A>

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/impls/shell/shellpc.c.html#PCShellSetPreSolve_Shell(PC pc, PetscErrorCode (*presolve)">PCShellSetPreSolve_Shell(PC pc, PetscErrorCode (*presolve) in src/ksp/pc/impls/shell/shellpc.c</A><BR>


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


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