Test buffer contents if on CPU

Signed-off-by: Vasily Litvinov <vasilij.n.litvinov@intel.com>

Author: vnlitvinov

pandas/tests/api/test_protocol.py

@@ -1,5 +1,6 @@
 import pytest
 import math
+import ctypes
 
 @pytest.mark.parametrize("test_data",
                          [
@@ -98,7 +99,8 @@ def test_get_columns(df_from_dict):
     assert dfX.get_column(1).dtype[0] == 2
 
 def test_buffer(df_from_dict):
-    df = df_from_dict({"a": [0, 1]})
+    arr = [0, 1, -1]
+    df = df_from_dict({"a": arr})
     dfX = df.__dataframe__()
     colX = dfX.get_column(0)
     bufX = colX.get_buffers()
@@ -107,6 +109,17 @@ def test_buffer(df_from_dict):
 
     assert dataBuf.bufsize > 0
     assert dataBuf.ptr != 0
-    assert dataBuf.__dlpack_device__
+    device, _ = dataBuf.__dlpack_device__
 
     assert dataDtype[0] == 0
+
+    if device == 1: # CPU-only as we're going to directly read memory here
+        bitwidth = dataDtype[1]
+        ctype = {8: ctypes.c_int8,
+                 16: ctypes.c_int16,
+                 32: ctypes.c_int32,
+                 64: ctypes.c_int64}[bitwidth]
+
+        for idx, truth in enumerate(arr):
+            val = ctype.from_address(dataBuf.ptr + idx * (bitwidth // 8)).value
+            assert val == truth, f"Buffer at index {idx} mismatch"