Get the memory address pointed to by a ctypes pointer

Question

Short version: How can I get the address that a ctypes pointer points to?

Long version: I have registered a python function as a callback with a C library. The C library expects function signature of void (*p_func)(char stat, char * buf, short buf_len) so I register an appropriate python function. When I get into the python function, I want to know the memory address pointed to by buf. How can I do this?

It would help if you posted some source code using the {} tags. Thanks. — octopusgrabbus, Mar 13 '12 at 15:31
I have fixed this myself with `ctypes.addressof(buf.contents)` but I can't answer my own question yet. — Iain Rist, Mar 13 '12 at 15:48
I think you are looking for the "id" of the function http://docs.python.org/library/functions.html#id In [1]: a = 4 In [2]: id(a) Out[2]: 10019764 — Garrett Berg, Mar 13 '12 at 15:43
I am after the address of a bit of memory pointed to by a ctypes.POINTER object that has been passed to me by a c library, not the id of a purely python object. — Iain Rist, Mar 13 '12 at 15:48

score 18 · Accepted Answer · answered Mar 20 '12 at 09:52

18

I have fixed this myself by reading the documentation.

I wanted to know the memory location of a block of memory allocated by a library. I had the ctypes pointer that pointed to said block. To get the memory address of the block I used ctypes.addressof(p_block.contents).

The confusion arose around my understanding that p_block.contents != p_block.contents, but then I realised all p_block.contents objects have the same underlying buffer. The address of the underlying buffer is obtained with ctypes.addressof.

answered Mar 20 '12 at 09:52

Iain Rist

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That's a bit like `&(*p)` in C where `p` is a pointer. What is strange is that ctypes does not define a `value` attribute for arbitrary pointer type as returned by `ctypes.POINTER` function. The `ctypes.c_void_p` class *does* have the `value` attribute, but the class returned by `ctypes.POINTER` does not. Very odd, counterintuitive, and almost reeks of incomplete implementation. – Armen Michaeli Oct 22 '15 at 17:43
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FYI https://bugs.python.org/issue26565, "Add value attribute to non basic pointers": >> I know one can do addressof(p.contents), but it's a bit inconsistent that c_void_p and c_char_p contain the same information in the value attribute. > The `value` of a c_char_p or c_wchar_p pointer is a Python bytes or str object. Since `value` won't consistently be the address value, it may be better to introduce a read-only `as_void` attribute that can be implemented consistently for all pointer types (including function pointers). – memeplex May 24 '16 at 21:46
Gotcha! Hair pulling is over. Matsalams! – daparic Jun 26 '20 at 18:09

score 6 · Answer 2 · answered Jun 10 '19 at 15:30

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real_adr = cast(anyctypespointer, c_void_p).value

Works for any kind of ctypes pointer - even function pointers etc. which lack .content

answered Jun 10 '19 at 15:30

kxr

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Get the memory address pointed to by a ctypes pointer

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