lpython emulation mode segfaults

[certik]

This fixes it:

--- a/src/runtime/lpython/lpython.py
+++ b/src/runtime/lpython/lpython.py
@@ -729,14 +729,9 @@ def c_p_pointer(cptr, targettype, targetshape=None):
         newa = ctypes.cast(cptr, targettype_ptr)
         return newa
     else:
-        if py_is_dataclass(targettype):
-            if cptr.value is None:
-                return None
-            return ctypes.cast(cptr, ctypes.py_object).value
-
         targettype_ptr = ctypes.POINTER(targettype_ptr)
         newa = ctypes.cast(cptr, targettype_ptr)
-        if is_ctypes_Structure(targettype):
+        if is_dataclass(targettype):
             # return after wrapping newa inside PointerToStruct
             return PointerToStruct(newa)
         return newa

I need to find a reproducer still. This is the bug:

(HOST)  C BEFORE : { 0 ,  0 ,  0 , ...,  0 ,  0 }
./run_full_emulation.sh: line 6: 31565 Segmentation fault: 11

[certik]

Everything passes except:

  File "/Users/ondrej/repos/lpython/integration_tests/bindc_09.py", line 36, in main
    assert foo.value == Value.FIVE
AssertionError

[certik]

Here is a minimal reproducible example that fails in master, but succeeds with #2124 (but this PR break another important test):

import numpy, ctypes
from lpython import (i64, i16, CPtr, c_p_pointer, Pointer, sizeof, packed,
        dataclass)


global_arrays = []


def alloc(buf_size:i64) -> i64:
    xs = numpy.empty(buf_size, dtype=numpy.uint8)
    global_arrays.append(xs)
    p = ctypes.c_void_p(xs.ctypes.data)
    return ctypes.cast(p.value, ctypes.c_void_p)


@packed
@dataclass
class S:
    a: i16
    b: i64


def main():
    p1: CPtr = alloc(sizeof(S))
    p2: Pointer[S] = c_p_pointer(p1, S)
    p2.a = i16(5)
    p2.b = i64(4)
    assert p2.a == i16(5)
    assert p2.b == i64(4)


main()

In master:

$ PYTHONPATH=src/runtime/lpython/ python a.py
zsh: segmentation fault  PYTHONPATH=src/runtime/lpython/ python a.py

In #2124:

$ PYTHONPATH=src/runtime/lpython/ python a.py
$

[certik]

The alloc is just a Python implementation of a C-style alloc, it returns a pointer to a preallocated memory.

We need to be able to cast it to a struct correctly. We are with #2124, but then a dataclass test stops working.

[certik]

I think the above example should NOT work. We can't take a pointer to a non bind(c) quantity. The c_loc and c_funcloc should only work on bind(c), nothing else.

[certik]

It must be done like this:

@ccallable
@packed
@dataclass
class S:
    a: i16
    b: i64

and then it works.

[certik]

I added tests that have to work correctly in #2126, and they seem to work fine. So that's good news.

Now the next step is to give an error message for usage that should not work.

[rebcabin]

approved

[certik]

The error message is done here: #2133. It found many incorrect tests that I fixed.

However, in the process I discovered further bugs with structs, such as #2134.