Cython:将指针传递给函数中的指针，采用 void **

Question

我正在尝试编写一个函数，在某些条件下，将指向结构的指针更改为指向不同的结构。

我的限制是我想保留初始函数签名，该签名将指向指针(而不是特定结构类型)的通用指针作为参数。

这行不通:

[nav] In [5]: %%cython -f 
         ...: ctypedef struct A: 
         ...:     int x 
         ...:     int y 
         ...:  
         ...: cdef fn(void **m): 
         ...:     # Arbitrary code that changes m[0] to point to another structure
         ...:     pass
         ...:      
         ...: cdef A a 
         ...: cdef A *ap = &a 
         ...: a.x = 2 
         ...: a.y = 3 
         ...: print(ap.x) 
         ...: fn(&ap) 
         ...: print(ap.x) 

Error compiling Cython file:
------------------------------------------------------------
...
cdef A a
cdef A *ap = &a
a.x = 2
a.y = 3
print(a.x)
fn(&ap)
  ^
------------------------------------------------------------

/home/me/.cache/ipython/cython/_cython_magic_3c3902694eafae18c66cb761d4a6b210.pyx:20:3: Cannot assign type 'A **' to 'void **'

我猜这是因为即使我可以编写一个以 void * 作为参数的函数，并自动将任何传递的指针转换为 void *，这也会获胜不能使用指向 void 指针的指针，对吗？

如果是这样，我如何传递结构指针，以便 ap 可以指向不同的结构？

谢谢。

编辑:

进一步阅读后，我意识到这是一个 C 架构“功能”。 This useful article给出一些简短的解释:

One side point about pointers to pointers and memory allocation: although the void * type, as returned by malloc, is a "generic pointer", suitable for assigning to or from pointers of any type, the hypothetical type void ** is not a "generic pointer to pointer." Our allocstr example can only be used for allocating pointers to char. It would not be possible to use a function which returned generic pointers indirectly via a void ** pointer, because when you tried to use it, for example by declaring and calling

double *dptr;
if(!hypotheticalwrapperfunc(100, sizeof(double), &dptr))
    fprintf(stderr, "out of memory\n");

you would not be passing a void **, but rather a double **.

我认为此时我能做的就是在函数签名中指定数据类型，或者返回新分配的值？

Answer 1

我通过传递函数所要求的内容来解决这个问题，即 void **，然后在需要时进行转换:

[nav] In [5]: %%cython -f 
         ...: ctypedef struct A: 
         ...:     int x 
         ...:     int y 
         ...:  
         ...: cdef fn(void **m): 
         ...:     # Arbitrary code that changes m[0] to point to another structure
         ...:     pass
         ...:      
         ...: cdef A a 
         ...: cdef void *avp = <void *>&a
         ...: cdef A *ap = <A*>avp
         ...: a.x = 2 
         ...: a.y = 3 
         ...: print(ap.x) 
         ...: fn(&ap)
         ...: ap = <A*>avp # Must reassign
         ...: print(ap.x)