I'm a new person to C++ dll import topic and may be my question is very easy but I can not find it on google.
I have a very simple C++ win32 dll:
#include <iostream>
using namespace std;
extern "C"
{
__declspec(dllexport) void __stdcall DisplayHellowFromDLL()
{
cout<<"Hi"<<endl;
}
}
When I call this method from C# I do not have any problem, here is C# code
namespace UnmanagedTester
{
class Program
{
[DllImport(#"C:\CGlobalDll")]
public static extern void DisplayHellowFromDLL();
static void Main(string[] args)
{
Console.WriteLine("This is C# program");
DisplayHellowFromDLL();
}
}
}
As I expected the output is: "This is C# program" "Hi".
Now if I change the declaration of C function as:
__declspec(dllexport) void DisplayHellowFromDLL()
without __stdcall, I do not have any problem as well, and the question is:
When do I really need __declspec(dllexport) TYPE __stdcall and when I can use only __declspec(dllexport) TYPE ?
Thanks a lot.
You can think of it like this:
__declspec(dllexport) declares your function as a public function that your DLL exports;
__stdcall is a rather low-level detail that refers to the "calling convention" adopted by that function; specifically, __stdcall means that the callee cleans the stack;
alternative to __stdcall is __cdecl, which means: the caller cleans the stack.
__cdecl is the "natural" C calling convention; it supports the definition of vararg functions (like printf).
__stdcall is the default calling convention for DLL functions, so you don't need specify it if you are only going to call those functions through their DLL API.
This should explain what you are observing.
It works by accident because the function doesn't take any arguments. As soon as you do this on a function that does take arguments you'll start running out of luck. The call will leave the stack imbalanced, very unhealthy. You'd get the pInvokeStackImbalance MDA warning when you debug. An imbalanced stack can otherwise go unnoticed for a while, it tends to crash your program in the Release build.
You need to specify the calling convention if you compile the calling code with some other convention. Otherwise the default will work.
Ilya, you can also set the default calling convention in Project Properties -> Configuration Properties -> C/C++ -> Advanced -> Calling Convention.
If the default calling convention in your project is already set to __stdcall (/Gz), then adding __std
Related
in my next project I want to implement a GUI for already existing code in C++.
My plan is to wrap the C++ part in a DLL and to implement the GUI in C#. My problem is that I don't know how to implement a callback from the unmanaged DLL into the manged C# code. I've already done some development in C# but the interfacing between managed and unmanaged code is new to me. Can anybody give me some hints or reading tips or a simple example to start from? Unfortunatly I could not find anything helpful.
You don't need to use Marshal.GetFunctionPointerForDelegate(), the P/Invoke marshaller does it automatically. You'll need to declare a delegate on the C# side whose signature is compatible with the function pointer declaration on the C++ side. For example:
using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
class UnManagedInterop {
private delegate int Callback(string text);
private Callback mInstance; // Ensure it doesn't get garbage collected
public UnManagedInterop() {
mInstance = new Callback(Handler);
SetCallback(mInstance);
}
public void Test() {
TestCallback();
}
private int Handler(string text) {
// Do something...
Console.WriteLine(text);
return 42;
}
[DllImport("cpptemp1.dll")]
private static extern void SetCallback(Callback fn);
[DllImport("cpptemp1.dll")]
private static extern void TestCallback();
}
And the corresponding C++ code used to create the unmanaged DLL:
#include "stdafx.h"
typedef int (__stdcall * Callback)(const char* text);
Callback Handler = 0;
extern "C" __declspec(dllexport)
void __stdcall SetCallback(Callback handler) {
Handler = handler;
}
extern "C" __declspec(dllexport)
void __stdcall TestCallback() {
int retval = Handler("hello world");
}
That's enough to get you started with it. There are a million details that can get you into trouble, you are bound to run into some of them. The much more productive way to get this kind of code going is writing a wrapper in the C++/CLI language. That also lets you wrap a C++ class, something you can't do with P/Invoke. A decent tutorial is available here.
P/Invoke can handle marshaling a managed delegate to a function pointer. So if you expose API's that register a call back function from your DLL and in C# pass a delegate to that function.
There is an example on MSDN of doing this with the EnumWindows function. In that article be careful to pay attention to the line in point 4 that states:
If, however, the callback function can
be invoked after the call returns, the
managed caller must take steps to
ensure that the delegate remains
uncollected until the callback
function finishes. For detailed
information about preventing garbage
collection, see Interop Marshaling
with Platform Invoke.
What that is saying is that you need to make sure that your delegate isn't garbage collected until after the managed code is done calling it by either keeping a reference to it in your code, or pinning it.
See Marshal.GetFunctionPointerForDelegate, which will give you a function pointer for calling managed (i.e. C# code) from unmanaged code.
Have a look at this, Marshal.GetDelegateForFunctionPointer?
The exception:
Managed Debugging Assistant 'PInvokeStackImbalance' has detected a problem in 'D:\UI_still_in_progress\user_interface\bin\Debug\WindowsFormsApplication1.vshost.exe'.
Basically my application has a background real time process in c++ which communicates with a micro-processor. Ive set up a c++ dll file which allows me to pass data from the c++ application and c# UI in real time.
My problem is that I have to create a finite state machine to make sure the physical device and c++ app are all doing what they should be based on whats going on in the UI. so my goal is to pass simple integers representing the states from c# to c++.
The real time data is passing from c++ to c# with no issues using the following implementation:
c++ .h file snippet:
extern "C" __declspec (dllexport) int grabx();
extern "C" __declspec (dllexport) void setFSMstate(int s);
C++ .cpp file snippet:
int grabx()
{
return x0;
}
void setFSMstate(int s)
{
state = s;
}
c# class
public class MyDllMethods {
[DllImport("dllTESTER.dll")]
public static extern int grabx();
[DllImport("dllTESTER.dll")]
public static extern void setFSMstate(int s);
}
c# method calls
xx = MyDllMethods.grabx();
MyDllMethods.setFSMstate(2);
The grabx() function works fine and i can read the x value from the device in real time. When my UI moves to a new screen i attempt to change the state in every application by passing a 2 into setFSMstate(). When this method is called the exception is thrown.
I am quite new to P/Invoke so if there is anything I missed please help out. I may have marshaled it wrong but im confident that type int is the same in c# and c++?
The problem you're running into is a calling convention mismatch. Calling conventions specify how parameters are passed to functions, and how their return values are passed back to the caller. Your C++ functions are being declared with an implicit calling convention of __cdecl, but P/Invoke's default calling convention is __stdcall.
To fix this, you just have to change your P/Invoke configuration:
[DllImport("dllTESTER.dll", CallingConvention = CallingConvention.Cdecl)]
(The reason you didn't have any problems with your first method is that it doesn't have any parameters—__cdecl and __stdcall pass parameters differently, but they both specify that integer return values are stored in the EAX register.)
I have a weird problem I have C++ DLL that I am importing in C# library using DLL import. If I specify entry point, everything works as expected, here is the example:
internal static class UnsafeMethods
{
[DllImport("GoodSchool.dll", EntryPoint = #"?AddNum##YAHHH#Z")]
public static extern int AddNum(int num1, int num2);
}
class Program
{
static void Main(string[] args)
{
Console.WriteLine(UnsafeMethods.AddNum(4,5));
}
}
However, if I use a simplfied import like here:
[DllImport("GoodSchool.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern int AddNum(int num1, int num2);
I get familiar error message:
Unhandled Exception: System.EntryPointNotFoundException: Unable to find an entry point named 'AddNum' in DLL 'GoodSchool.dll'
I used depends to verify that method is properly exposed, and I decoded notation to verify parameters and naming convention - all seems good.
Function signature in C++ is very simple:
__declspec(dllexport) int AddNum(int num1, int num2);
Any suggestions how I can call this method in C# without providing decorated name as EntryPoint? What do I do wrong? I do not want to use "C" export, as my understanding is that decorated function name is perfectly fine to be used with DllImport.
C++ mangles function names to account for function name overloading. After all, if the DLL had
__declspec(dllexport) int AddNum(int num1);
__declspec(dllexport) int AddNum(int num1, int num2);
which would AddNum refer to?
The symbol ?AddNum##YAHHH#Z is the mangled (a.k.a. decorated) name exposed in the unmanaged DLL.
https://en.wikipedia.org/wiki/Name_mangling#Name_mangling_in_C.2B.2B
Your DLL does not export something called AddNum.
The decorated function name is fine to use with DllImport, as you can see by the fact that you're doing it. But that requires you to specify the decorated function name in the import. The undecorated name does not exist as far as the linker (static or dynamic) is concerned -- AddNum is simply not a symbol that your library exposes.
If you instead want to do what you're asking:
call this method in C# without providing decorated name as EntryPoint?
then you cannot allow C++ to mangle the name in the first place. You can either specify the decorated name in DllImport or use extern "C" linkage on the C++ code. You must pick one or the other.
in my next project I want to implement a GUI for already existing code in C++.
My plan is to wrap the C++ part in a DLL and to implement the GUI in C#. My problem is that I don't know how to implement a callback from the unmanaged DLL into the manged C# code. I've already done some development in C# but the interfacing between managed and unmanaged code is new to me. Can anybody give me some hints or reading tips or a simple example to start from? Unfortunatly I could not find anything helpful.
You don't need to use Marshal.GetFunctionPointerForDelegate(), the P/Invoke marshaller does it automatically. You'll need to declare a delegate on the C# side whose signature is compatible with the function pointer declaration on the C++ side. For example:
using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
class UnManagedInterop {
private delegate int Callback(string text);
private Callback mInstance; // Ensure it doesn't get garbage collected
public UnManagedInterop() {
mInstance = new Callback(Handler);
SetCallback(mInstance);
}
public void Test() {
TestCallback();
}
private int Handler(string text) {
// Do something...
Console.WriteLine(text);
return 42;
}
[DllImport("cpptemp1.dll")]
private static extern void SetCallback(Callback fn);
[DllImport("cpptemp1.dll")]
private static extern void TestCallback();
}
And the corresponding C++ code used to create the unmanaged DLL:
#include "stdafx.h"
typedef int (__stdcall * Callback)(const char* text);
Callback Handler = 0;
extern "C" __declspec(dllexport)
void __stdcall SetCallback(Callback handler) {
Handler = handler;
}
extern "C" __declspec(dllexport)
void __stdcall TestCallback() {
int retval = Handler("hello world");
}
That's enough to get you started with it. There are a million details that can get you into trouble, you are bound to run into some of them. The much more productive way to get this kind of code going is writing a wrapper in the C++/CLI language. That also lets you wrap a C++ class, something you can't do with P/Invoke. A decent tutorial is available here.
P/Invoke can handle marshaling a managed delegate to a function pointer. So if you expose API's that register a call back function from your DLL and in C# pass a delegate to that function.
There is an example on MSDN of doing this with the EnumWindows function. In that article be careful to pay attention to the line in point 4 that states:
If, however, the callback function can
be invoked after the call returns, the
managed caller must take steps to
ensure that the delegate remains
uncollected until the callback
function finishes. For detailed
information about preventing garbage
collection, see Interop Marshaling
with Platform Invoke.
What that is saying is that you need to make sure that your delegate isn't garbage collected until after the managed code is done calling it by either keeping a reference to it in your code, or pinning it.
See Marshal.GetFunctionPointerForDelegate, which will give you a function pointer for calling managed (i.e. C# code) from unmanaged code.
Have a look at this, Marshal.GetDelegateForFunctionPointer?
I have following method in my Borland C++ code,
static bool UploadBitstream(void)
{
//Some code Implementation
}
And I'm trying to convert it to DLL and access it in C#.
What are the steps I need to follow to Convert the code DLL
and then use it in C# ??
First, you have to make sure that the methods are defined extern. Then you need to declare the method stdcall or pascal calling convention, and mark them dllexport. See code listing below (this is ancient memory for me, so pardon if I am a bit off on modern Borland C++ compilers).
// code.h
extern "C" {
#define FUNCTION __declspec(dllexport)
FUNCTION int __stdcall SomeFunction(int Value);
In the main
#include "code.h"
FUNCTION int __stdcall SomeFunction(int timesThree)
{
return timesThree * 3;
}
Once you've compiled your DLL all you should need to do in .NET to get access to it is use the DLLImport property.
public class stuff
{
[DLLImport("somedll.dll")]
public static extern void UploadBitstream();
}
If you need pointers or anything like that it gets more complicated but for void functions it's that simple.
It should be noted that once you invoke that function the dll will be loaded by your program and won't be released until your program is closed. You can dynamically load dlls but that is much more complicated. I can explain it here if you have a need.
Watch your terminology.
In your example UploadBitstream is function not a method.
If it is indeed a method in a class then it is very difficult to use classes from Borland compiled DLLs.
If your code is actually C not C++ you should be able to create a compatible DLL
for your simple C style functions with C++ Builder.
See the following SO question:
Use a dll from a c++ program. (borland c++ builder and in general)
where I list various compiler settings which will also apply to creating compatible DLLs.