:orphan:
# DMSetDefaultConstraints
Set the `PetscSection` and `Mat` that specify the local constraint interpolation. 
## Synopsis
```
#include "petscdm.h"          
#include "petscdmlabel.h"     
#include "petscds.h"     
PetscErrorCode DMSetDefaultConstraints(DM dm, PetscSection section, Mat mat, Vec bias)
```
Collective


## Input Parameters

- ***dm -*** The `DM`
- ***section -*** The `PetscSection` describing the range of the constraint matrix: relates rows of the constraint matrix to dofs of the default section.  Must have a local communicator (`PETSC_COMM_SELF` or derivative).
- ***mat -*** The `Mat` that interpolates local constraints: its width should be the layout size of the default section:  `NULL` indicates no constraints.  Must have a local communicator (`PETSC_COMM_SELF` or derivative).
- ***bias -*** A bias vector to be added to constrained values in the local vector.  `NULL` indicates no bias.  Must have a local communicator (`PETSC_COMM_SELF` or derivative).





## Notes
If a constraint matrix is specified, then it is applied during `DMGlobalToLocalEnd()` when mode is `INSERT_VALUES`, `INSERT_BC_VALUES`, or `INSERT_ALL_VALUES`.  Without a constraint matrix, the local vector l returned by `DMGlobalToLocalEnd()` contains values that have been scattered from a global vector without modification; with a constraint matrix A, l is modified by computing c = A * l + bias, l[s[i]] = c[i], where the scatter s is defined by the `PetscSection` returned by `DMGetDefaultConstraints()`.

If a constraint matrix is specified, then its adjoint is applied during `DMLocalToGlobalBegin()` when mode is `ADD_VALUES`, `ADD_BC_VALUES`, or `ADD_ALL_VALUES`.  Without a constraint matrix, the local vector l is accumulated into a global vector without modification; with a constraint matrix A, l is first modified by computing c[i] = l[s[i]], l[s[i]] = 0, l = l + A'*c, which is the adjoint of the operation described above.  Any bias, if specified, is ignored when accumulating.

This increments the references of the `PetscSection`, `Mat`, and `Vec`, so they user can destroy them.


## See Also
 [](ch_dmbase), `DM`, `DMGetDefaultConstraints()`

## Level
advanced

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


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