I made the change in the function so I now get this error
PetscErrorCode ComputeRho(DMMG *dmmg, Vec rho)
{
DA distA = (DA)dmmg->dm;

In function ‘PetscErrorCode Myfunc(_n_DMMG**, _p_Vec*)’:
error: request for member ‘dm’ in ‘* dmmg’, which is of non-class type ‘_n_DMMG*’

It looks like I have a null pointer to _n_DMMG** instead of _n_DMMG*

It seems like your code expects you to pass a DMMG object not a DMMG* object. There are two ways to fix this:

1) In your function 'ComputeRho' change all references of dmmg to *dmmg, i.e.:

DA distA = (DA)(*dmmg)->dm;

2) Change your function declaration back to what you had:

PetscErrorCode ComputeRho(DMMG dmmg, Vec rho)

{...}

and then pass the object to your function as so:

ComputeRho(*dmmg, b);

Do you need your dmmg to be a pointer in your 'main' function? It looks like your declaration could just be:

extern PetscErrorCode Myfunc(DMMG,Vec);

>>>>>>>>>>>>>>>>

int main(int argc,char **argv)

{

DMMG dmmg;

...Vec b;

Myfunc(dmmg,b)

}

PetscErrorCode Myfunc(DMMG dmmg, Vec b)

{

DA da = (DA)dmmg->dm;

.....

}

<<<<<<<<<<<<<<<<<

and that would fix everything.

Sean