Question

我已经构建了一个带有两个字段的简单Structure子类，它包含一个指向数组的void指针和数组长度。但是，当我尝试使用相同长度的输入列表创建这些列表时，返回的void指针的值与用于创建实例的最后一个数组相同：

from ctypes import POINTER, c_double, c_size_t, c_void_p, Structure, cast
import numpy as np

class External(Structure):

    _fields_ = [("data", c_void_p),
                ("length", c_size_t)]
    @classmethod
    def from_param(cls, seq):
        return seq if isinstance(seq, cls) else cls(seq)

    def __init__(self, seq):
        self.ptr = cast(
            np.array(seq, dtype=np.float64).ctypes.data_as(POINTER(c_double)),
            c_void_p
        )
        self.data = self.ptr
        self.length = len(seq)
        # recreate array from void pointer
        # shows the correct values
        shape = self.length, 2
        ptr = cast(self.data, POINTER(c_double))
        array = np.ctypeslib.as_array(ptr, shape)
        print "Correct array", array.tolist()

if __name__ == "__main__":
    interiors = [
            [[3.5, 3.5], [4.4, 2.0], [2.6, 2.0], [3.5, 3.5]],
            [[4.0, 3.0], [4.0, 3.2], [4.5, 3.2], [4.0, 3.0]],
            ]
    wrong = [External(s) for s in interiors]
    for w in wrong:
        # perform same cast back to array as before
        shape = w.length, 2
        ptr = cast(w.data, POINTER(c_double))
        array = np.ctypeslib.as_array(ptr, shape)
        print "Wrong array", array.tolist()

如果我使用不同长度的输入列表创建我的External实例，那么一切都按预期工作。我在这里做错了什么？

Answer 1

问题是numpy数组立即被垃圾收集，底层内存被释放，导致悬空指针。

解决方案是保留对基础buffer对象的引用：

def __init__(self, seq):
    array = np.array(seq, dtype=np.float64)
    self._buffer = array.data
    self.ptr = cast(
        array.ctypes.data_as(POINTER(c_double)),
        c_void_p
    )
    ...

现在只有当持有引用的External实例被删除时才会释放数组的内存。

当转换为numpy数组时，Structure子类列表返回错误的值

1 个答案: