Manipulating C pointers

[certik]

Here is an example of how pointers should be manipulated, in Fortran, Python, both should generate the same ASR, and the C backend should then generate the output below:

Fortran:

type(c_ptr) :: queries
integer(2), pointer :: x(:)         
c_f_pointer(queries, x)
c_loc(x(idx))       

Python:

from ltypes import c_loc, c_p_pointer, CPtr, Pointer
from typing import Annotate
queries: CPtr
x: Annotate[i16[:], Pointer]
x = c_p_pointer(queries, i16)                      
c_loc(x[idx])           

C output:

void *queries;
int16_t *x;
x = (int16_t*)queries;
&x[idx];

Also allow passing void * pointers as arguments to functions.

[certik]

To pass void * as function arguments, in Python:

@ccaller
def f(A_void: CPtr, idx: i32) -> None:
    A: Annotate[i16[:], Pointer]   
    A = c_p_pointer(A_void, i16)
    g(c_loc(A[idx]))

translates to C:

void f(void *A_void, int32_t idx) {
    int16_t *A;
    A = (int16_t *) A_void;
    g(&A[idx])

[certik]

Note: I think c_loc(x[idx]) will not work in CPython, since x[idx] will return an integer as a value, so we can't take pointer to it from inside c_loc. We'll have to use some syntax from https://docs.python.org/3/library/ctypes.html#pointers.

[czgdp1807]

In Python, instead of x = c_p_pointer(queries, i16) can we do, c_p_pointer(queries, x)? @certik

[certik]

Yes, that's fine, if it can be done.

[certik]

Most of these are fixed. The remaining issue is #609.

[ubaidsk]

can we do, c_p_pointer(queries, x)

Can this be done @certik , @czgdp1807 ? Probably not. I think Python does not allow/support updating a variable passed as argument.

[certik]

Right, Python does not support it.