ierr = MatSeqAIJGetArray(ptr->K,&Kc);CHKERRQ(ierr); 
ierr = MatGetLocalSize(ptr->K,&m,&n); CHKERRQ(ierr);
for(i=0;i<m;i++)
    { 
      for(j=0;j<n;j++)
      {
        v  = *(Kc + (i*n+j));
        ierr = MatSetValues(Kloc,1,&i,1,&j,&v,INSERT_VALUES);CHKERRQ(ierr);
      }
    } 
    

I don't understand the purpose of this code fragment. Are you wanting to copy your K into Kloc? What is the purpose of Kloc? With standard usage of SNES one only needs to provide PETSc a global matrix, one does not need to work with "ghosted" matrices at all. 

Also normally one fills the f vector in FormFunction and the Jacobian in FormJacobian and there is no need to access the Jacobians at all in SNES https://www.mcs.anl.gov/petsc/documentation/faq.html#functionjacobian  nor do you need to pass into FormFunction in the context the F or Jacobian matrix. 

Maybe if you explained your use case a bit more we could make suggestions on how to accomplish it.

Barry



On May 19, 2021, at 10:00 AM, Kaushik Vijaykumar <kaushikv318@gmail.com> wrote:

Hi everyone,

We are in the process of integrating nonlinear solution to our FE code through SNES. The first steps that I understood that need to be done is to be able to pass the assembled stiffness matrix K and force vector F to the "formfunction" to calculate the residual and "formjacobian" to calculate the tangent stiffness matrix. To do so, I have defined a struct:

extern PetscErrorCode FormJacobian(SNES,Vec,Mat,Mat,void*);
extern PetscErrorCode FormFunction(SNES,Vec,Vec,void*);

// define a struct type to pass K and f as "user context"
typedef struct
    {
      Mat K;
      Vec f;
    } K_and_f;

In the main program, the struct is declared
int main()
{
K_and_f main_K_and_f; // declare the struct

// SNES - Populate K and f into the struct
main_K_and_f.K = K; // K matrix
main_K_and_f.f = f; // f vector 
....
....
}
In form function

PetscErrorCode FormFunction(SNES snes, Vec x, Vec F, void* ctx) {
    PetscErrorCode ierr;
    PetscReal *ax,*c;
    PetscReal *aF;
    PetscScalar *Kc,v;
    PetscInt nlocal,m,n,i,j,index[100000];

    // Create local Vec and Mat
    Mat Kloc;
    Vec Floc, Uloc, KUloc, resloc;
   
    K_and_f* ptr = (K_and_f*) ctx; // cast the pointer to void into pointer to struct
    // Get local F array, FLOC
    ierr = VecGhostGetLocalForm(ptr->f,&Floc);CHKERRQ(ierr);


I am able to get the f array from the main program using " VecGhostGetLocalForm" and the vec is correct. However, I am having trouble with reading the K matrix in formfunction. 

The following trial gives me incorrect K values in Kloc:
ierr = MatSeqAIJGetArray(ptr->K,&Kc);CHKERRQ(ierr); 
ierr = MatGetLocalSize(ptr->K,&m,&n); CHKERRQ(ierr);
for(i=0;i<m;i++)
    {
      for(j=0;j<n;j++)
      {
        v  = *(Kc + (i*n+j));
        ierr = MatSetValues(Kloc,1,&i,1,&j,&v,INSERT_VALUES);CHKERRQ(ierr);
      }
    } 
    
    When I compare K and Kloc, they are not identical 


Please let me know if there is an equivalent function like VecGhostGetLocalForm() for Matrices. If not, is there a better way to do this.

Any guidance/help is greatly appreciated.

Thanks
Kaushik