:orphan:
# KSPGetResidualHistory
Gets the array used to hold the residual history and the number of residuals it contains. 
## Synopsis
```
#include "petscksp.h" 
#include "petscmat.h" 
PetscErrorCode KSPGetResidualHistory(KSP ksp, const PetscReal *a[], PetscInt *na)
```
Not Collective


## Input Parameter

- ***ksp -*** iterative context obtained from `KSPCreate()`



## Output Parameters

- ***a   -*** pointer to array to hold history (or `NULL`)
- ***na  -*** number of used entries in a (or `NULL`)





## Note
This array is borrowed and should not be freed by the caller.

Can only be called after a `KSPSetResidualHistory()` otherwise a and na are set to zero


## Fortran Note
The Fortran version of this routine has a calling sequence
```none
call KSPGetResidualHistory(KSP ksp, integer na, integer ierr)
```
note that you have passed a Fortran array into `KSPSetResidualHistory()` and you need
to access the residual values from this Fortran array you provided. Only the `na` (number of
residual norms currently held) is set.


## See Also
 [](ch_ksp), `KSPSetResidualHistory()`, `KSP`

## Level
advanced

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/interface/itfunc.c.html#KSPGetResidualHistory">src/ksp/ksp/interface/itfunc.c</A>


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


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