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gh-128182: Add per-object memory access synchronization to ctypes (GH-128490)
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‎Doc/library/ctypes.rst‎

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@@ -870,6 +870,36 @@ invalid non-\ ``NULL`` pointers would crash Python)::
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ValueError: NULL pointer access
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>>>
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.. _ctypes-thread-safety:
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Thread safety without the GIL
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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In Python 3.13, the :term:`GIL` may be disabled on :term:`experimental free threaded <free threading>` builds.
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In ctypes, reads and writes to a single object concurrently is safe, but not across multiple objects:
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.. code-block:: pycon
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>>> number = c_int(42)
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>>> pointer_a = pointer(number)
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>>> pointer_b = pointer(number)
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In the above, it's only safe for one object to read and write to the address at once if the GIL is disabled.
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So, ``pointer_a`` can be shared and written to across multiple threads, but only if ``pointer_b``
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is not also attempting to do the same. If this is an issue, consider using a :class:`threading.Lock`
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to synchronize access to memory:
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.. code-block:: pycon
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>>> import threading
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>>> lock = threading.Lock()
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>>> # Thread 1
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>>> with lock:
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... pointer_a.contents = 24
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>>> # Thread 2
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>>> with lock:
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... pointer_b.contents = 42
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.. _ctypes-type-conversions:
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‎Lib/test/test_ctypes/test_arrays.py‎

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create_string_buffer, create_unicode_buffer,
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c_char, c_wchar, c_byte, c_ubyte, c_short, c_ushort, c_int, c_uint,
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c_long, c_ulonglong, c_float, c_double, c_longdouble)
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from test.support import bigmemtest, _2G
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from test.support import bigmemtest, _2G, threading_helper, Py_GIL_DISABLED
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from ._support import (_CData, PyCArrayType, Py_TPFLAGS_DISALLOW_INSTANTIATION,
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Py_TPFLAGS_IMMUTABLETYPE)
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def test_large_array(self, size):
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c_char * size
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@threading_helper.requires_working_threading()
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@unittest.skipUnless(Py_GIL_DISABLED, "only meaningful if the GIL is disabled")
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def test_thread_safety(self):
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from threading import Thread
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buffer = (ctypes.c_char_p * 10)()
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def run():
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for i in range(100):
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buffer.value = b"hello"
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buffer[0] = b"j"
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with threading_helper.catch_threading_exception() as cm:
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threads = (Thread(target=run) for _ in range(25))
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with threading_helper.start_threads(threads):
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pass
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if cm.exc_value:
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raise cm.exc_value
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if __name__ == '__main__':
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unittest.main()

‎Misc/NEWS.d/next/Library/2025-01-04-11-32-46.gh-issue-128182.SJ2Zsa.rst‎

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Fix crash when using :mod:`ctypes` pointers concurrently on the :term:`free
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threaded <free threading>` build.

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