Binding to C - Arrays and Access Violation

[jmh530 via Digitalmars-d-learn]

I'm working on generating a binding to a C library. I've got the 
.h file converted and can call some parts of the library with no 
errors. However, I have reached a stumbling block in a critical 
part.

The library requires passing function pointers to various 
functions in the library. When I try to run these functions, I 
get an Access Violation error. I enabled additional DMD warnings, 
which helped pinpoint the issue.

My D code calls a C function. One of the parameters to the C 
function is a function pointer to a D function. This D function 
(below) is one that I copied  from the C library's tutorial. I 
only slightly changed the signature. This function is eventually 
called in other functions in the C library.

double myfunc(uint n, const double* x, double* grad, void* 
my_func_data)
{
     if (grad)
     {
         grad[0] = 0.0;
         grad[1] = 0.5 / sqrt(x[1]);
     }
     return sqrt(x[1]);
}

The line (though likely the next will too) that causes a problem 
is

grad[0] = 0.0;

Thus, as it is an Access Violation, I'm guessing the issue is 
with accessing elements of arrays in the D function from the C 
function. I don't know. When I try to call the D function in D, 
it works, but I have to refer to x and grad as x.ptr and grad.ptr.

I'm not sure how to go about fixing this...