:orphan:
# DMComputeL2Diff
This function computes the L_2 difference between a function u and an FEM interpolant solution u_h. 
## Synopsis
```
#include "petscdm.h"          
#include "petscdmlabel.h"     
#include "petscds.h"     
PetscErrorCode DMComputeL2Diff(DM dm, PetscReal time, PetscErrorCode (**funcs)(PetscInt, PetscReal, const PetscReal[], PetscInt, PetscScalar *, void *), void **ctxs, Vec X, PetscReal *diff)
```
Collective


## Input Parameters

- ***dm    -*** The `DM`
- ***time  -*** The time
- ***funcs -*** The functions to evaluate for each field component
- ***ctxs  -*** Optional array of contexts to pass to each function, or NULL.
- ***X     -*** The coefficient vector u_h, a global vector



## Output Parameter

- ***diff -*** The diff ||u - u_h||_2





## Developer Notes
This API is specific to only particular usage of `DM`

The notes need to provide some information about what has to be provided to the `DM` to be able to perform the computation.


## See Also
 [](ch_dmbase), `DM`, `DMProjectFunction()`, `DMComputeL2FieldDiff()`, `DMComputeL2GradientDiff()`

## Level
developer

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/dm/interface/dm.c.html#DMComputeL2Diff">src/dm/interface/dm.c</A>

## Examples
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/snes/tutorials/ex12.c.html">src/snes/tutorials/ex12.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/snes/tutorials/ex13.c.html">src/snes/tutorials/ex13.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/snes/tutorials/ex63.c.html">src/snes/tutorials/ex63.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/snes/tutorials/ex75.c.html">src/snes/tutorials/ex75.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/tao/tutorials/ex1.c.html">src/tao/tutorials/ex1.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/tao/tutorials/ex2.c.html">src/tao/tutorials/ex2.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ts/tutorials/ex47.c.html">src/ts/tutorials/ex47.c</A><BR>

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/dm/impls/forest/p4est/pforest.h.html#DMComputeL2Diff_pforest(DM dm, PetscReal time, PetscErrorCode (**funcs)">DMComputeL2Diff_pforest(DM dm, PetscReal time, PetscErrorCode (**funcs) in src/dm/impls/forest/p4est/pforest.h</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/dm/impls/plex/plexfem.c.html#DMComputeL2Diff_Plex(DM dm, PetscReal time, PetscErrorCode (**funcs)">DMComputeL2Diff_Plex(DM dm, PetscReal time, PetscErrorCode (**funcs) in src/dm/impls/plex/plexfem.c</A><BR>


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


[Index of all DM routines](index.md)  
[Table of Contents for all manual pages](/manualpages/index.md)  
[Index of all manual pages](/manualpages/singleindex.md)