ctypes free memory which is allocated in C DLL

[Nobody]

On Sat, 27 Oct 2012 07:42:01 -0700, zlchen.ken wrote:

> I have a DLL which written in C language, one of the function is to
> allocate a structure, fill the members and then return the pointer of
> the structure. 
> 
> After Python called this function, and done with the returned structure,
> I would like to free the returned structure. How can I achieve this ?
> Basically, I tried that I wrote a corresponding free interface in the
> DLL, it works, but calling the libc.free in Python doesn't work.
> 
> my_dll.return_structure_ptr.restypes = POINTER(Dummy) res_ptr =
> my_dll.return_structure_ptr() windll.msvcrt.free(res_ptr)  <==== doesn't
> work, memory violation my_dll.free_dummy_struture(res_ptr)  <== This
> works.

On Windows, a process may have multiple versions of the MSVCRT DLL (which
provides malloc/free). If an executable or DLL is linked against multiple
DLLs, each DLL could be using a different version of MSVCRT.

Different versions of MSVCRT may have separate heaps, so anything which
is allocated with malloc() (or calloc() or realloc()) from a specific
version of MSVCRT must be passed to free() from the same version of MSVCRT.
windll.msvcrt refers to the version of MSVCRT against which the Python DLL
is linked, which isn't necessarily the version against which my_dll is
linked.

If a function in a DLL returns a pointer to memory which it allocated
with malloc(), that DLL must also provide a function which can be used to
free that memory. It can't leave it to the application (or higher-level
DLL) to call free(), because the application may not be using the same
version of MSVCRT as the DLL.