从 Python 3.11 开始执行 ctypes.memset？

Question

我正在实现一个 memset 函数，该函数应该将字节对象缓冲区设置为零。

从 Python 3.11 开始，缓冲区 api 函数PyObject_GetBuffer() 和 PyBuffer_Release() 现在是稳定 ABI 的一部分。

下面的代码有效，但是：

• 我觉得很奇怪 定义我自己的 Py_buffer 类。不是有预定义的吗 某处？
• 我是否正确使用 PyBuffer_Release？

def memset(bytes_object):
    import ctypes

    # Define the Py_buffer structure
    class Py_buffer(ctypes.Structure):
        _fields_ = [
            ('buf', ctypes.c_void_p),
            ('obj', ctypes.py_object),
            ('len', ctypes.c_ssize_t),
            ('itemsize', ctypes.c_ssize_t),
            ('readonly', ctypes.c_int),
            ('ndim', ctypes.c_int),
            ('format', ctypes.c_char_p),
            ('shape', ctypes.POINTER(ctypes.c_ssize_t)),
            ('strides', ctypes.POINTER(ctypes.c_ssize_t)),
            ('suboffsets', ctypes.POINTER(ctypes.c_ssize_t)),
            ('internal', ctypes.c_void_p),
        ]
    
    buf = Py_buffer()
    ctypes.pythonapi.PyObject_GetBuffer(ctypes.py_object(bytes_object), ctypes.byref(buf), ctypes.c_int(0))
    try:    
        ctypes.memset(buf.buf, 0, buf.len)
    finally:
        ctypes.pythonapi.PyBuffer_Release(ctypes.byref(buf))


obj = bytes("hello world", "ascii")
print("before:", repr(obj))
memset(obj)
print("after:", repr(obj))

给出这个输出：

before: b'hello world'
after: b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'

作为参考，这里有一种使用 已弃用的 PyObject_AsCharBuffer 函数在缓冲区上执行 memset 的旧方法。它仍然适用于 Python 3.11：

def memset_old(bytes_object: bytes):
    import ctypes

    if not isinstance(bytes_object, bytes):
        raise TypeError(f"expected bytes, not {type(bytes_object)}")

    data = ctypes.POINTER(ctypes.c_char)()
    size = ctypes.c_int()
    ctypes.pythonapi.PyObject_AsCharBuffer(ctypes.py_object(bytes_object), ctypes.pointer(data), ctypes.pointer(size))
    ctypes.memset(data, 0, size.value)


obj = bytes("hello world", "ascii")
print("old before:", repr(obj))
memset_old(obj)
print("old after:", repr(obj))

Answer 1

感觉很奇怪，自己要定义自己的

Py_buffer

类。某处没有预定义的吗？

Py_buffer

没有在任何地方定义，而是在 Python

pybuffer.h

目录的

include

标头中定义。

我使用

PyBuffer_Release

正确吗？

是的，但是定义

.argtypes

和

.restype

是个好习惯，这样

ctypes

就可以对参数进行类型检查。另外如前所述

bytes

对象缓冲区是只读的，因此更改它是未定义的行为。

带有类型检查的完整示例：

import ctypes as ct

PyBUF_SIMPLE   = 0
PyBUF_WRITABLE = 0x0001

class Py_buffer(ct.Structure):
    _fields_ = (('buf', ct.c_void_p),
                ('obj', ct.py_object),
                ('len', ct.c_ssize_t),
                ('itemsize', ct.c_ssize_t),
                ('readonly', ct.c_int),
                ('ndim', ct.c_int),
                ('format', ct.c_char_p),
                ('shape', ct.POINTER(ct.c_ssize_t)),
                ('strides', ct.POINTER(ct.c_ssize_t)),
                ('suboffsets', ct.POINTER(ct.c_ssize_t)),
                ('internal', ct.c_void_p))

    def __repr__(self):
        '''Display representation of a buffer.
        '''
        return f'Py_buffer(obj={self.obj!r}, readonly={self.readonly})'

# Explicitly define arguments and return values so ctypes can typecheck.
PyObject_CheckBuffer = ct.pythonapi.PyObject_CheckBuffer
PyObject_CheckBuffer.argtypes = ct.py_object,
PyObject_CheckBuffer.restype = ct.c_int
PyObject_GetBuffer = ct.pythonapi.PyObject_GetBuffer
PyObject_GetBuffer.argtypes = ct.py_object, ct.POINTER(Py_buffer), ct.c_int
PyObject_GetBuffer.restype = ct.c_int
PyBuffer_Release = ct.pythonapi.PyBuffer_Release
PyBuffer_Release.argtypes = ct.POINTER(Py_buffer),
PyBuffer_Release.restype = None

def memset(obj):
    if PyObject_CheckBuffer(obj):  # ensure object passed supports the buffer interface
        buf = Py_buffer()
        try:
            # PyObject_GetBuffer  can throw an exception if it fails.
            PyObject_GetBuffer(obj, ct.byref(buf), PyBUF_SIMPLE)
            print(buf) # View resulting buffer
            ct.memset(buf.buf, 0, buf.len)  # zero it
        finally:
            PyBuffer_Release(ct.byref(buf))

obj = b'hello, world!'
print("before:", repr(obj))
memset(obj)
print("after:", repr(obj))

输出：

before: b'hello, world!'
Py_buffer(obj=b'hello, world!', readonly=1)
after: b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'