pointer(x) doesn't update `x` in CPython

[certik]

In CPython, when we do:

x: i32
x = 5
y: CPtr[i32] = pointer(x, i32)

And then we pass y into C, and C updates "*y", it does not update x.

This is caused by this line:

lpython/src/runtime/ltypes/ltypes.py

Line 195 in f49908e

return ctypes.cast(ctypes.pointer(ctypes.c_int32(x)),

Which wraps the variable x into a ctypes.c_int32(x) wrapper. In that wrapper, the value gets updated, but this updated value will not propagate back to x itself, since in Python the semantics does not allow that.

If we want to support this feature, we would need to do something like this:

from ltypes import Target, target
x: Target[i32] = target(i32)
x = 5
y: CPtr[i32] = pointer(x, i32)

And the target creates the wrapper class, that overloads the = operator (if it is allowed) and x=5 then puts it into the wrapper. Then pointer(x, i32) should be able to update this wrapper. This is really quite hackish, and almost does not seem worth doing.

A better approach might be to use a numpy array, and make CPtr point to the first element. Then I think the first element should update.

All these C interop things are meant to be used when interacting with C, and one can create a small Python function to hide this in it. And if we have the freedom to design the C interface, it's better to return the integer as a result of the function, then there is no problem.

[certik]

To fix this issue, let's add the workaround using a numpy array as a test in our integration_tests, using cpython, llvm and c.