Re: ctypes free memory which is allocated in C DLL

[zlchen . ken]

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!
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ctypes free memory which is allocated in C DLL

[zlchen . ken]

Hi Guys,

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.

Thanks !
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Re: How to correctly pass “pointer-to-pointer” into DLL via ctypes?

[zlchen . ken]

On Thursday, November 18, 2010 8:03:57 PM UTC+8, Grigory Petrov wrote:
> Hello.
> 
> I have a DLL that allocates memory and returns it. Function in DLL is like 
> this:
> 
> void Foo( unsigned char** ppMem, int* pSize )
> {
>   * pSize = 4;
>   * ppMem = malloc( * pSize );
>   for( int i = 0; i < * pSize; i ++ ) (* pMem)[ i ] = i;
> }
> 
> Also, i have a python code that access this function from my DLL:
> 
> from ctypes import *
> Foo = windll.mydll.Foo
> Foo.argtypes = [ POINTER( POINTER( c_ubyte ) ), POINTER( c_int ) ]
> mem = POINTER( c_ubyte )()
> size = c_int( 0 )
> Foo( byref( mem ), byref( size ) ]
> print size, mem[ 0 ], mem[ 1 ], mem[ 2 ], mem[ 3 ]
> 
> I'm expecting that print will show "4 0 1 2 3" but it shows "4 221 221
> 221 221" O_O. Any hints what i'm doing wrong?

I am wondering in Python how you free the memory which is allocated in your DLL 
?
Thanks 
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