Fix a tricky bug in Cython stream casting

python/kvikio/kvikio/_lib/stream.pyx

@@ -4,6 +4,8 @@
 # distutils: language = c++
 # cython: language_level=3
 
+from libc.stdint cimport uintptr_t
+
 
 cdef extern from "cuda.h":
     ctypedef void* CUstream
@@ -14,14 +16,29 @@ cdef extern from "<kvikio/stream.hpp>" nogil:
     void cpp_stream_deregister "kvikio::stream_deregister"(CUstream stream) except +
 
 
-def stream_register(stream: int, flags: int) -> None:
+# Here stream's type can be annotated in two ways:
+#
+# - Case 1: stream: int
+#   `stream` is a Python object (int is always a Python type hint).
+#   Requires two-level casting to extract the integer value first:
+#       cdef CUstream cpp_stream = <CUstream><uintptr_t>stream
+#
+# - Case 2: stream: uintptr_t
+#   `stream` is a C uintptr_t (because uintptr_t is cimported from libc.stdint).
+#   Cython auto-converts the Python int to C at function entry, so only one cast needed:
+#       cdef CUstream cpp_stream = <CUstream>stream
+#
+# Note: In Case 1, using a single-level cast `<CUstream>stream` will compile, but
+# it casts the Python object's memory address rather than extracting the integer
+# value, resulting in a wrong pointer being passed silently.
+def stream_register(stream: uintptr_t, flags: int) -> None:
     cdef CUstream cpp_stream = <CUstream>stream
     cdef unsigned int cpp_flags = flags
     with nogil:
         cpp_stream_register(cpp_stream, cpp_flags)
 
 
-def stream_deregister(stream: int) -> None:
+def stream_deregister(stream: uintptr_t) -> None:
     cdef CUstream cpp_stream = <CUstream>stream
     with nogil:
         cpp_stream_deregister(cpp_stream)