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gh-69011: ctypes.create_*_buffer clarify NUL termination character inclusion #132858

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gh-69011: ctypes.create_*_buffer clarify NUL termination character inclusion #132858

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79 changes: 64 additions & 15 deletions Doc/library/ctypes.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -2031,35 +2031,84 @@ Utility functions
pointer.


.. function:: create_string_buffer(init_or_size, size=None)
.. function:: create_string_buffer(init, size=None)
create_string_buffer(size)

This function creates a mutable character buffer. The returned object is a
ctypes array of :class:`c_char`.

*init_or_size* must be an integer which specifies the size of the array, or a
bytes object which will be used to initialize the array items.
If *size* is given (and not ``None``), it must be an :class:`int`.
It specifies the size of the returned array.

If a bytes object is specified as first argument, the buffer is made one item
larger than its length so that the last element in the array is a NUL
termination character. An integer can be passed as second argument which allows
specifying the size of the array if the length of the bytes should not be used.
If the *init* argument is given, it must be :class:`bytes`. It is used
to initialize the array items. Bytes not initialized this way are
set to zero (NUL).

If *size* is not given (or if it is ``None``), the buffer is made one byte
larger than *init*, effectively adding a NUL termination byte.

If both arguments are given, *size* must not be less than ``len(init)``.

.. warning::

If *size* is equal to ``len(init)``, a NUL terminator is
not added. Do not treat such a buffer as a C string.

For example::

>>> bytes(create_string_buffer(2))
b'\x00\x00'
>>> bytes(create_string_buffer(b'ab'))
b'ab\x00'
>>> bytes(create_string_buffer(b'ab', 2))
b'ab'
>>> bytes(create_string_buffer(b'ab', 4))
b'ab\x00\x00'
>>> bytes(create_string_buffer(b'abcdef', 2))
Traceback (most recent call last):
...
ValueError: byte string too long

.. audit-event:: ctypes.create_string_buffer init,size ctypes.create_string_buffer


.. function:: create_unicode_buffer(init_or_size, size=None)
.. function:: create_unicode_buffer(init, size=None)
create_unicode_buffer(size)

This function creates a mutable unicode character buffer. The returned object is
a ctypes array of :class:`c_wchar`.

*init_or_size* must be an integer which specifies the size of the array, or a
string which will be used to initialize the array items.
If *size* is given (and not ``None``), it must be an :class:`int`.
It specifies the size of the returned array.

If the *init* argument is given, it must be a string. It is used
to initialize the array items. Bytes not initialized this way are
set to zero (NUL).

If *size* is not given (or if it is ``None``), the buffer is made one byte
larger than *init*, effectively adding a NUL termination byte.

If both arguments are given, *size* must not be less than ``len(init)``.

.. warning::

If *size* is equal to ``len(init)``, a NUL terminator is
not added. Do not treat such a buffer as a C string.

For example::

If a string is specified as first argument, the buffer is made one item
larger than the length of the string so that the last element in the array is a
NUL termination character. An integer can be passed as second argument which
allows specifying the size of the array if the length of the string should not
be used.
>>> bytes(create_unicode_buffer(2))
b'\x00\x00\x00\x00\x00\x00\x00\x00'
>>> bytes(create_unicode_buffer('ab'))
b'a\x00\x00\x00b\x00\x00\x00\x00\x00\x00\x00'
>>> bytes(create_unicode_buffer('ab', 2))
b'a\x00\x00\x00b\x00\x00\x00'
>>> bytes(create_unicode_buffer('ab', 4))
b'a\x00\x00\x00b\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
>>> bytes(create_unicode_buffer('abcdef', 2))
Traceback (most recent call last):
...
ValueError: string too long

.. audit-event:: ctypes.create_unicode_buffer init,size ctypes.create_unicode_buffer

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