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# DMGetCompatibility

determine if two DMs are compatible

### Synopsis

    #include "petscdm.h"
    #include "petscdmlabel.h"
    #include "petscds.h"
    PetscErrorCode DMGetCompatibility(DM dm1,DM dm2,PetscBool *compatible,PetscBool *set)

Collective

### Input Parameters

| dm1 | - the first DM
| dm2 | - the second DM

### Output Parameters

| compatible | - whether or not the two DMs are compatible
| set | - whether or not the compatible value was set

### Notes

Two DMs are deemed compatible if they represent the same parallel decomposition of the same topology. This implies that the section (field data) on one "makes sense" with respect to the topology and parallel decomposition of the other. Loosely speaking, compatible DMs represent the same domain and parallel decomposition, but hold different data.

Typically, one would confirm compatibility if intending to simultaneously iterate over a pair of vectors obtained from different DMs.

For example, two DMDA objects are compatible if they have the same local and global sizes and the same stencil width. They can have different numbers of degrees of freedom per node. Thus, one could use the node numbering from either DM in bounds for a loop over vectors derived from either DM.

Consider the operation of summing data living on a 2-dof DMDA to data living on a 1-dof DMDA, which should be compatible, as in the following snippet.

      ...
      ierr = DMGetCompatibility(da1,da2,&compatible,&set);CHKERRQ(ierr);
      if (set && compatible)  {
        ierr = DMDAVecGetArrayDOF(da1,vec1,&arr1);CHKERRQ(ierr);
        ierr = DMDAVecGetArrayDOF(da2,vec2,&arr2);CHKERRQ(ierr);
        ierr = DMDAGetCorners(da1,&x,&y,NULL,&m,&n,NULL);CHKERRQ(ierr);
        for (j=y; j<y+n; ++j) {
          for (i=x; i<x+m, ++i) {
            arr1[j][i][0] = arr2[j][i][0] + arr2[j][i][1];
          }
        }
        ierr = DMDAVecRestoreArrayDOF(da1,vec1,&arr1);CHKERRQ(ierr);
        ierr = DMDAVecRestoreArrayDOF(da2,vec2,&arr2);CHKERRQ(ierr);
      } else {
        SETERRQ(PetscObjectComm((PetscObject)da1,PETSC_ERR_ARG_INCOMP,"DMDA objects incompatible");
      }
      ...

Checking compatibility might be expensive for a given implementation of DM, or might be impossible to unambiguously confirm or deny. For this reason, this function may decline to determine compatibility, and hence users should always check the "set" output parameter.

A DM is always compatible with itself.

In the current implementation, DMs which live on "unequal" communicators (MPI_UNEQUAL in the terminology of MPI_Comm_compare()) are always deemed incompatible.

This function is labeled "Collective," as information about all subdomains is required on each rank. However, in DM implementations which store all this information locally, this function may be merely "Logically Collective".

### Developer Notes

Compatibility is assumed to be a symmetric concept; DM A is compatible with DM B iff B is compatible with A. Thus, this function checks the implementations of both dm and dmc (if they are of different types), attempting to determine compatibility. It is left to DM implementers to ensure that symmetry is preserved. The simplest way to do this is, when implementing type-specific logic for this function, is to check for existing logic in the implementation of other DM types and let *set = PETSC_FALSE if found.

### See Also

DM, DMDACreateCompatibleDMDA(), DMStagCreateCompatibleDMStag()

### Level

advanced

### Location

src/dm/interface/dm.c

### Implementations

DMGetCompatibility_DA in src/dm/impls/da/da.c
DMGetCompatibility_Stag in src/dm/impls/stag/stag.c
Index of all DM routines
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