Problem passing structs to dll's

  • Thread starter Thread starter Guest
  • Start date Start date
G

Guest

Hi!

I am confused about the abnormal behaviour of my project.

Please consider the following situation:
I have written an application in C++.NET 2003 (managed) which loads and
executes an unmanaged function from an unmanaged dll.

On the dll-side i export this function:


typedef struct {
int a;
int b;...} mystruct;

__declspec(dllexport) int dllfn(mystruct* x);


Ok.
This all compiles (and also executes, I can test this via logfiles, etc).
BUT:
If I assign values to the given struct, they "do not arrive" in the main
(managed) application!!!

So, something like


x->a=17;


in the dll surprisingly does not set the value to 17 in the "real" struct
allociated in the main application. On the other hand, the same instruction
in the dll with


x->a=17; printf("a: %ld", x->a);


Works!!!

I do not have any idea, why I apparently do not have access to the struct I
want.


PLEASE HELP ME!

Thank you very much for your great help in advance!!!!

Kind regards!
 
Another question:
Do I have to export the definition of this struct explicitly, or is it
enough to just define the same struct in both the dll and the client
application?

Thanks in advance!
 
olidem said:
My general question:

Is is correct code to call a (extern dll) function with the adress of an
local structure ?
So, in the managed vc++ .net program:

in header file:

typedef struct {
int a;
int b;...} mystruct;

__declspec(dllimport) int dllfn(mystruct* x);

an in cpp file:

main(){
mystruct foo;
int bar;

bar = dllfn(&foo);
}

Is this doable?
Remember that the dll is unmanaged and even ANSI C compatible.
Click to expand...

Yes, that should work. You have a native struct (not ref class or value
class), so the binary layout is compatible as long as the same packing is
used (controlled with pragma pack).
 
Hi Ben!
I tryed your hint, using #pragma pack (show) and got in BOTH, the native and
the managed, program the same following result:

warning C4810: value of pragma pack(show) == 8

It is as if the struct would be cleaned up by the gc if the dll-function
returns.
I also allocated the struct on the heap and gave a pointer to this adress in
the heap to the dll, with the same result.


An intersting observation:
If I assign a special value to one struct member in the managed client app,

foo.a = 815;

Then this 815 is visible in the dll. But if I change this in the dll to,
let's say 816, it does not arrive in the client again. But after the
816-assignment in the dll, this variable is really set to 816, I can work
with it.
 
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