static char help[] = "Tests MatMult() on MatLoad() matrix \n\n";

#include <petscmat.h>

int main(int argc,char **args)
{
  Mat            A;
  Vec            x,b;
  PetscViewer    fd;              /* viewer */
  char           file[PETSC_MAX_PATH_LEN]; /* input file name */
  PetscBool      flg;

  PetscCall(PetscInitialize(&argc,&args,(char*)0,help));
  /* Determine file from which we read the matrix A */
  PetscCall(PetscOptionsGetString(NULL,NULL,"-f",file,sizeof(file),&flg));
  PetscCheck(flg,PETSC_COMM_WORLD,PETSC_ERR_USER,"Must indicate binary file with the -f option");

  /* Load matrix A */
  PetscCall(PetscViewerBinaryOpen(PETSC_COMM_WORLD,file,FILE_MODE_READ,&fd));
  PetscCall(MatCreate(PETSC_COMM_WORLD,&A));
  PetscCall(MatLoad(A,fd));
  flg  = PETSC_FALSE;
  PetscCall(VecCreate(PETSC_COMM_WORLD,&x));
  PetscCall(PetscOptionsGetString(NULL,NULL,"-vec",file,sizeof(file),&flg));
  if (flg) {
    if (file[0] == '0') {
      PetscInt    m;
      PetscScalar one = 1.0;
      PetscCall(PetscInfo(0,"Using vector of ones for RHS\n"));
      PetscCall(MatGetLocalSize(A,&m,NULL));
      PetscCall(VecSetSizes(x,m,PETSC_DECIDE));
      PetscCall(VecSetFromOptions(x));
      PetscCall(VecSet(x,one));
    }
  } else {
    PetscCall(VecLoad(x,fd));
    PetscCall(PetscViewerDestroy(&fd));
  }
  PetscCall(VecDuplicate(x,&b));
  PetscCall(MatMult(A,x,b));

  /* Print (for testing only) */
  PetscCall(MatView(A,0));
  PetscCall(VecView(b,0));
  /* Free data structures */
  PetscCall(MatDestroy(&A));
  PetscCall(VecDestroy(&x));
  PetscCall(VecDestroy(&b));
  PetscCall(PetscFinalize());
  return 0;
}