ctypes free memory which is allocated in C DLL

[zlchen.ken at gmail.com]

On Sunday, October 28, 2012 6:26:28 AM UTC+8, Nobody wrote:
> 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.

Thank you for the details.
This is really useful!