In the past, I have passed a byte array from a C# method to an unmanaged C++ function. I am now trying to pass a pointer to a buffer of type unsigned char from a C++ method back into a C# method using reverse PInvoke, which uses a callback to get back to the C# code. I have tried several different ideas - like passing Ref Byte, Byte *, and IntPtr for the 2nd argument, but none of them seem to work. Here is my test code for using IntPtr:
C# code:
namespace TestPInvoke
{
class Program
{
static void Main(string[] args)
{
foo f = new foo();
f.DispMsg();
}
}
unsafe public class foo
{
public delegate void callback(int NumBytes, IntPtr pBuf);
public static void callee(int NumBytes, IntPtr pBuf)
{
System.Console.WriteLine("NumBytes = " + NumBytes.ToString() + ", pBuf = ");
String s = "";
Byte* p = (Byte*)pBuf.ToPointer();
for (int Loop = 0; Loop < 50; Loop++)
{
s += p++->ToString() + " ";
}
System.Console.WriteLine(s);
}
public void DispMsg()
{
caller(new callback(foo.callee));
}
[DllImport(#"C:\Users\Bob\Documents\Visual Studio 2008\Projects\AttackPoker1\Win32Client\TestPInvoke\bin\Debug\TestPInvokeDLLCPP.dll", CallingConvention = CallingConvention.StdCall)]
public static extern void caller(callback call);
}
}
C++ code:
#include <stdio.h>
#include <string.h>
typedef unsigned char Byte;
typedef void (__stdcall *callback)(const int bytesInMsg, Byte* pintBuf);
extern "C" __declspec(dllexport) void __stdcall caller(callback call)
{
// Debug Test on how to pass a pointer to a byte buffer to a C# method.
Byte* pBuf = new Byte[50];
// Initialize the buffer to something.
Byte* p = pBuf;
for (Byte Loop = 0; Loop < 50; Loop++)
*p = Loop;
// Initiate the callback into the C# code.
call(50, pBuf);
// Delete pBuf later.
}
When the C++ code calls the C# callback callee method, the bytesInMsg argument is correct. But, the returned pointer does not point to the start of the buffer. Dereferencing the pointer always seems to point to the last value in the buffer (49 or 0x31), but after looking at it in the memory window, the rest of the bytes both before and after are garbage.
Does anyone have any suggestions on how I can get this to work without marshaling a large array? What I'm hoping to do is pass a pointer to a large buffer created on the C++ side one time to a C# class that will then be able to read data from that buffer efficiently.
If this can not be done, then I will have to allocate the memory buffers from C#, pin them, and pass them into the C++ methods.
All the pinvoke is fine and works properly. You just have a silly bug in your C++ code, you are forgetting to increment the pointer so you only ever set the first element of the array. Use
*p++ = Loop;
Or the more sane version that simply indexes the array:
// Initialize the buffer to something.
for (int ix = 0; ix < 50; ++ix)
pBuf[ix] = ix;
Related
I have a SDK written in C++ which manages a device. My program controlling the device is written in C#, so naturally a CLI wrapper class does the translation between both languages. My C# project includes the wrapper as a DLL.
My issue is that the C++ SDK is using pointers to head to arrays of data. These pointers are also available in the wrapper.
Wrapper .cpp code:
Wrapper::Wrapper()
{
myData = new DataAquis(); //initiate C++ class's instance
}
int Wrapper::Start()
{
//(..)
int num = myData->Start();
ptr = (myData->img);
return num;
}
This code initializes the device and creates a pointer to a data structure (array of unsigned char).
Wrapper SDK .cpp code:
int DataAquis::Start()
{
// (...)
// Pointer from SDK
img = pBuffer;
// (...)
return FAILED(nError) ? -1 : 0;
}
Wrapper .h code:
public ref class Wrapper
{
public:
Wrapper();
// (...)
unsigned char *ptr;
private:
// (...)
};
Code C#:
public static Wrapper myDataAqui;
// (...)
private static void DataAquisition()
{
// call the start function in wrapper
myDataAqui.Start();
// Unsafe code for pointer use
unsafe
{
// point to aquired data
byte* imgptr1 = myDataAqui.ptr;
// AccesViolationException in above line.
// Data processing
for (y = 0; y < 256; y++)
{
for (x = 0; x < 320; x++)
{
int temp = x * 256 + 255 - y;
Spectrum1.X_Data_brut[bufferIndex][temp] = (UInt16)(*imgptr1++ + *imgptr1++ * 256);
aquirData[temp] = Spectrum1.X_Data_brut[bufferIndex][temp];
}
}
// (...)
}
}
As shown, an AccessViolationException is triggered at the line where I cast the Wrapper pointer to a local byte pointer.
If I put a breakpoint on that line, I can see that the Wrapper pointer correctly points to a memory address, but says that it is unable to read memory, so the pointed data is never gathered in C#.
I have read that the C# equivalent of an unsigned char in C++ is a byte, so normally I should read the same amount of data and never go outside the boundaries of my data structure.
Additionnal information that could be useful:
This project has been copied from another PC and the same code is functional on that PC.
Both PC have same Visual Studio, same .Net version, same SDK, both 64 bit compiled. Only Windows versions differ (working on Windows 8 and not working on Windows 7).
I unsuccessfully tried using Marshal functions.
Do you have any ideas how to fix this ?
I'm not sure why you're getting an exception but I'd marshal it into a CLR array on the C++/CLI side so no unsafe code is needed on the C# side.
C++/CLI:
#include <vcclr.h>
#include <algorithm>
#pragma unmanaged
const int data_size = 100;
unsigned char * foo()
{
auto p = new unsigned char[data_size];
for (int i = 0; i < data_size; ++i)
p[i] = (unsigned char)i;
return p;
}
#pragma managed
public ref class Wrapper
{
public:
array<unsigned char>^ data;
void Start()
{
auto p = foo();
data = gcnew array<unsigned char>(data_size);
pin_ptr<unsigned char> data_pin = &data[0];
std::copy(p, p+data_size, (unsigned char *)data_pin);
}
};
C#:
class Program
{
static void Main(string[] args)
{
var wrapper = new Wrapper();
wrapper.Start();
System.Console.WriteLine($"{wrapper.data[15]}");
}
}
This will contain any possible problems close to the source and make debugging a lot less confusing. If it crashes in std::copy then you're just using your C++ object wrong.
The question in short words is :
How to free memory returned from Native DLL as ItrPtr in managed code?
Details :
Assume we have simple function takes two parameters as OUTPUT, The first one is Reference Pointer to byte array and the second one is Reference Int .
The function will allocate amount of bytes based on some rules and return the pointer of memory and the size of bytes and the return value (1 for success and 0 for fail) .
The code below works fine and I can get the byte array correctly and the count of bytes and the return value, but when I try to free the memory using the pointer (IntPtr) I get exception :
Windows has triggered a breakpoint in TestCppDllCall.exe.
This may be due to a corruption of the heap, which indicates a bug in TestCppDllCall.exe or any of the DLLs it has loaded.
This may also be due to the user pressing F12 while TestCppDllCall.exe has focus.
The output window may have more diagnostic information.
To make things clear :
The next C# code work correctly with other DLL function have the same signature and freeing the memory works without any problem .
Any modification in (C) code accepted if you need to change allocation memory method or adding any other code .
All the functionality I need is Native DLL function accept Two Parameter by reference (Byte array and int , In c# [IntPtr of byte array and int]) fill them with some values based on some rules and return the function result (Success or Fail) .
CppDll.h
#ifdef CPPDLL_EXPORTS
#define CPPDLL_API __declspec(dllexport)
#else
#define CPPDLL_API __declspec(dllimport)
#endif
extern "C" CPPDLL_API int writeToBuffer(unsigned char *&myBuffer, int& mySize);
CppDll.cpp
#include "stdafx.h"
#include "CppDll.h"
extern "C" CPPDLL_API int writeToBuffer(unsigned char*& myBuffer, int& mySize)
{
mySize = 26;
unsigned char* pTemp = new unsigned char[26];
for(int i = 0; i < 26; i++)
{
pTemp[i] = 65 + i;
}
myBuffer = pTemp;
return 1;
}
C# code :
using System;
using System.Text;
using System.Runtime.InteropServices;
namespace TestCppDllCall
{
class Program
{
const string KERNEL32 = #"kernel32.dll";
const string _dllLocation = #"D:\CppDll\Bin\CppDll.dll";
const string funEntryPoint = #"writeToBuffer";
[DllImport(KERNEL32, SetLastError = true)]
public static extern IntPtr GetProcessHeap();
[DllImport(KERNEL32, SetLastError = true)]
public static extern bool HeapFree(IntPtr hHeap, uint dwFlags, IntPtr lpMem);
[DllImport(_dllLocation, EntryPoint = funEntryPoint, CallingConvention = CallingConvention.Cdecl)]
public static extern int writeToBuffer(out IntPtr myBuffer, out int mySize);
static void Main(string[] args)
{
IntPtr byteArrayPointer = IntPtr.Zero;
int arraySize;
try
{
int retValue = writeToBuffer(out byteArrayPointer, out arraySize);
if (retValue == 1 && byteArrayPointer != IntPtr.Zero)
{
byte[] byteArrayBuffer = new byte[arraySize];
Marshal.Copy(byteArrayPointer, byteArrayBuffer, 0, byteArrayBuffer.Length);
string strMyBuffer = Encoding.Default.GetString(byteArrayBuffer);
Console.WriteLine("Return Value : {0}\r\nArray Size : {1}\r\nReturn String : {2}",
retValue, arraySize, strMyBuffer);
}
}
catch (Exception ex)
{
Console.WriteLine("Error calling DLL \r\n {0}", ex.Message);
}
finally
{
if (byteArrayPointer != IntPtr.Zero)
HeapFree(GetProcessHeap(), 0, byteArrayPointer);
}
Console.ReadKey();
}
}
}
When I debug this code i set break point in the line (return 1) and the value of the buffer was :
myBuffer = 0x031b4fc0 "ABCDEFGHIJKLMNOPQRSTUVWXYZ««««««««î"
And I got the same value in C# code when the function call return and the value was :
52121536
The result I Got the correct Memory pointer and i am able to get the byte array value , how to free these memory blocks with this pointer in C# ?
Please let me know if there anything is not clear or if there any typo, I am not native English speaker .
Short answer: you should add a separate method in the DLL that frees the memory for you.
Long answer: there are different ways in which the memory can be allocated inside your DLL implementation. The way you free the memory must match the way in which you have allocated the memory. For example, memory allocated with new[] (with square brackets) needs to be freed with delete[] (as opposed to delete or free). C# does not provide a mechanism for you to do it; you need to send the pointer back to C++.
extern "C" CPPDLL_API void freeBuffer(unsigned char* myBuffer) {
delete[] myBuffer;
}
If you are allocating your own memory in native code, use CoTaskMemAlloc and you can free the pointer in managed code with Marshal.FreeCoTaskMem. CoTaskMemAlloc is described as "the only way to share memory in a COM-based application" (see http://msdn.microsoft.com/en-us/library/windows/desktop/aa366533(v=vs.85).aspx )
if you need to use the memory allocated with CoTaskMemAlloc with a native C++ object, you can use placement new to initialize the memory as if the new operator was used. For example:
void * p = CoTaskMemAlloc(sizeof(MyType));
MyType * pMyType = new (p) MyType;
This doesn't allocate memory with new just calls the constructor on the pre-allocated memory.
Calling Marshal.FreeCoTaskMem does not call the destructor of the type (which isn't needed if you just need to free memory); if you need to do more than free memory by calling the destructor you'll have to provide a native method that does that and P/Invoke it. Passing native class instances to managed code is not supported anyway.
If you need to allocate memory with some other API, you'll need to expose it in managed code through P/Invoke in order to free it in managed code.
HeapFree(GetProcessHeap(), 0, byteArrayPointer);
No, that can't work. The heap handle is wrong, the CRT creates its own heap with HeapCreate(). It's buried in the CRT data, you can't get to it. You could technically find the handle back from GetProcessHeaps(), except you don't know which one it is.
The pointer can be wrong too, the CRT may have added some extra info from the pointer returned by HeapAlloc() to store debugging data.
You'll need to export a function that calls delete[] to release the buffer. Or write a C++/CLI wrapper so you can use delete[] in the wrapper. With the extra requirement that the C++ code and the wrapper use the exact same version of the CRT DLL (/MD required). This almost always requires that you can recompile the C++ code.
I have a native method that has to deliver a byte array to a .NET wrapper. The natove method looks like:
__declspec(dllexport) int WaitForData(unsigned char* pBuffer)
{
return GetData(pBuffer);
}
GetData allocates a memory region using malloc and copies some data (a byte stream) into it. This byte stream was received via a socket connection. The return value is the length of pBuffer.
This method has to be called from .NET. The import declaration looks as follows:
[DllImport("CommunicationProxy.dll")]
public static extern int WaitForData(IntPtr buffer);
[EDIT]
The the P/Invoke Interop Assistant, that dasblinkenlight advised, translates the prototype to the following import signature:
public static extern int WaitForData(System.IntPtr pBuffer)
The result is the same: ptr is 0 after calling the method.
[/EDIT]
Atfer the method was called, the result is extracted:
IntPtr ptr = new IntPtr();
int length = Wrapper.WaitForData(ref ptr);
byte[] buffer = new byte[length];
for(int i = 0;i<length;i++)
{
buffer[i] = System.Runtime.InteropServices.Marshal.ReadByte(ptr, i);
}
Wrapper.FreeMemory(ptr);
The problem is, that the managed variable ptr doesn't contain the value that the native varible pBuffer contains. ptr is always 0 when Wrapper.WaitForData returns although pBuffer pointed to an allocated memory area.
Is there a mistake in the prototype? How does a pointer to a byte array need to be marshalled?
you need to pass a reference to a pointer or 'double pointer' like that
__declspec(dllexport) int WaitForData(unsigned char** pBuffer)
and then change the value of the pointer(because it's passed by value)
*pBuffer = 'something'
other option - return the pointer(then you'll have to handle the int/length some other way)
btw that's why your automatically generated prototype looks like this(doesn't have out, ref modifiers)
I would like a clean way to increase the size of a StringBuilder() as required for population by native code, the callback method below seems clean, but somehow we get a copy of the buffer instead of the actual buffer - I'm interested in explanations and solutions (preferably sticking to the callback type allocation as it would be nice and clean if only it could be made work).
using System;
using System.Runtime.InteropServices;
using System.Text;
namespace csharpapp
{
internal class Program
{
private static void Main(string[] args)
{
var buffer = new StringBuilder(12);
// straightforward, we can write to the buffer but unfortunately
// cannot adjust its size to whatever is required
Native.works(buffer, buffer.Capacity);
Console.WriteLine(buffer);
// try to allocate the size of the buffer in a callback - but now
// it seems only a copy of the buffer is passed to native code
Native.foo(size =>
{
buffer.Capacity = size;
buffer.Replace("works", "callback");
return buffer;
});
string s = buffer.ToString();
Console.WriteLine(s);
}
}
internal class Native
{
public delegate StringBuilder AllocateBufferDelegate(int bufsize);
[DllImport("w32.dll", CharSet = CharSet.Ansi)]
public static extern long foo(AllocateBufferDelegate callback);
[DllImport("w32.dll", CharSet = CharSet.Ansi)]
public static extern void works(StringBuilder buf, int bufsize);
}
}
native header
#ifdef W32_EXPORTS
#define W32_API __declspec(dllexport)
#else
#define W32_API __declspec(dllimport)
#endif
typedef char*(__stdcall *FnAllocStringBuilder)(int);
extern "C" W32_API long foo(FnAllocStringBuilder fpAllocate);
extern "C" W32_API void works(char *buf, int bufsize);
native code
#include "stdafx.h"
#include "w32.h"
#include <stdlib.h>
extern "C" W32_API long foo(FnAllocStringBuilder fpAllocate)
{
char *src = "foo X";
int len = strlen(src) + 1;
char *buf = fpAllocate(len);
return strcpy_s(buf,len,src);
}
extern "C" W32_API void works(char *buf, int bufsize)
{
strcpy_s(buf,bufsize,"works");
}
I have a theory for why this happens. I suspect that the marshalling of StringBuilder involves making a copy of the data, passing it to the P/Invoke call, and then copying back into the StringBuilder. I couldn't actually verify this though.
The only alternative to this would require the StringBuilder to be flattened first (it is internally a linked list of char[]'s), and the char[] pinned, and even then this would only work for marshalling to pointer-to-Unicode-chars strings, but not to ANSI or COM strings.
Thus, when you pass a StringBuilder as an argument, there's an obvious place for .NET to copy any changes back: right after the P/Invoke returns.
The same isn't true for when you pass a delegate returning a StringBuilder. In this case .NET needs to create a wrapper which converts an int => StringBuilder function into an int => char* function. This wrapper will create the char* buffer and populate it, but obviously can't copy any changes back yet. It also can't do this after the function that takes the delegate returns: it's still too early!
In fact, there is no obvious place at all where the reverse copy could occur.
So my guess is that this is what happens: when marshalling a StringBuilder-returning delegate, .NET can only perform a one-way conversion, hence any changes you make aren't reflected in the StringBuilder. This is slightly better than being completely unable to marshal such delegates.
As for solutions: I would recommend first asking the native code how large the buffer needs to be, and then passing a buffer of the appropriate size in a second call. Or, if you need better performance, guess a large enough buffer, but allow the native method to communicate that more space is required. This way most calls would involve only one P/Invoke transition.
This can be wrapped into a more convenient function that you can just call from the managed world without worrying about buffers.
In addition to the input provided by romkyns I will share the minimal changes solution I came up with. If anyone uses this be careful of your encodings!
the principal modification is:
private static void Main(string[] args)
{
byte[] bytes = null;
var gcHandle = new GCHandle();
Native.foo(size =>
{
bytes = new byte[size];
gcHandle = GCHandle.Alloc(bytes,GCHandleType.Pinned);
return gcHandle.AddrOfPinnedObject();
});
if(gcHandle.IsAllocated)
gcHandle.Free();
string s = ASCIIEncoding.ASCII.GetString(bytes);
Console.WriteLine(s);
}
with the delegate signature changeing to:
public delegate IntPtr AllocateBufferDelegate(int bufsize);
I need to pass a pointer to a structure to my DLL, any ideas how would I go about doing that?
In my C DLL:
typedef struct
{
int length;
unsigned char *value;
} Sample;
__declspec(dllexport) void __stdcall helloWorld( Sample *sample );
In my C# code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;
namespace CSharpConsole
{
class Program
{
[StructLayout(LayoutKind.Sequential, Pack = 1)]
private struct Sample
{
public Int32 length;
// What Should I Declare Here?
}
[DllImport("C:\\CTestDLL.dll")]
private static extern void helloWorld( Sample sample ); // How would I make this a pointer?
void HelloWorld()
{
Sample sample = new Sample();
sample .length = 20;
// How can I fill up the values of value?
helloWorld( sample ); // How should I pass it inside here
return;
}
static void Main(string[] args)
{
Program program = new Program();
program.HelloWorld();
}
}
}
To pass a pointer to a value type into a P/Invoke function just declare the parameter as a ref or out. This implicitly takes a reference to the parameter and passes that:
[DllImport("C:\\CTestDLL.dll")]
private static extern void helloWorld(ref Sample sample);
Since your structure has an array in it, you'll have to take care to declare it properly for this to work. I strongly recommend, if possible, that you turn it into a fixed-length array, as it will make the marshaler much, much happier:
typedef struct
{
int length;
unsigned char value[MAX_LENGTH];
} Sample;
This becomes:
public struct Sample
{
int length;
[MarshalAs(UnmanagedType.LPArray, SizeConst = MAX_LENGTH)] byte[] value;
}
If that is not possible, then the runtime has no way of knowing how much data to marshal back; in that case, you probably will have to resort to manual marshaling of your data:
public struct Sample
{
int length;
IntPtr value;
}
var sample = new Sample();
helloWorld(ref sample);
byte[] value = new byte[sample.length];
Marshal.Copy(sample.value, value, 0, sample.Length);
However, based on your comment to another answer, it looks like you just need to get a block of bytes out of the C DLL into C#. For that you don't really need a structure at all, and eliminating it would simplify things a lot. If you just want to pass in an array and have it filled in and returned to you, try something like this:
(This assumes you have control over both C and C# code bases; if not then the ref suggestion is the way to accomplish what you need.)
// In C# code:
[DllImport(#"C:\CTestDll.dll")]
private static extern void helloWorld(
int length,
[MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 0)] byte[] buffer);
byte[] buffer = new byte[1024 * 8];
helloWorld(1024 * 8, buffer);
// In C:
__declspec(dllexport) void __stdcall helloWorld(int, unsigned char *);
void helloWorld(int cb, unsigned char *buf)
{
memcpy(buf, DATASRC, cb);
}
In C, an unsigned char is, by definition, the same size as a C# byte - 8 bits, no sign. In C#, a char is actually two bytes. The runtime will automatically "convert" an unsigned char * to a byte[] for you. (There's not really a conversion at all; they are just different names for the same type.)
Default Marshaling for Value Types - gives some information on marshalling structs.
Calling Win32 DLLs in C# with P/Invoke - a little over half way down the page there's a table showing the type conversions between the standard unmanaged and managed types.
There are a few ways to pass around and convert the types.
As Michael Edenfield pointed out you can generally just use the ref keyword to indicate that a parameter should by passed by reference which is basically a pointer.
However sometimes things don't cooperate, particularly when it comes to strings or complex data types so this is where the IntPtr type comes in.
You have a couple of options for using IntPtrs.
You can create an IntPtr to a block of unmanaged memory of a specified size using:
IntPtr pointer = Marshal.AllocHGlobal(sizeOfBufferInBytes);
//Do stuff with the pointer
Marshal.FreeHGlobal(pointer); //Don't forget to release the memory
This is obviously a bit dangerous because you're manually allocating unmanaged memory.
You'll need to Marshal the data from the buffer back into a managed type, using something like Marshal.Copy(), Marshal.PtrToStructure(), Buffer.BlockCopy(), etc.
Alternatively you can create a managed object and pin it in memory while you need to, get a pointer to it and pass that to your method.
MyObject instance = new MyObject();
GCHandle gch = GCHandle.Alloc(instance, GCHandleType.Pinned);
importedMethod(gch.AddrOfPinnedObject()); //AddrOfPinnedObject() gives you an IntPtr
gch.Free(); //Release the pinned memory so the garbage collector can deal with it
This avoids the necessity for manually marshalling back to the correct data type but this is not always an option depending on the type of MyObject and whether it's blittable.
This works for me:
DLL:
typedef struct
{
int length;
unsigned char *value;
} Sample;
extern "C" __declspec(dllexport) void __stdcall helloWorld( Sample *sample )
{
MessageBoxA(NULL, (LPCSTR) sample->value, (LPCSTR) sample->value, 0);
}
C#:
class Program
{
[StructLayout(LayoutKind.Sequential, Pack = 1)]
private class Sample
{
public Int32 length;
public String value;
}
[DllImport("C:\\Users\\Kep\\Documents\\Visual Studio 2010\\Projects\\SODLL\\Debug\\DLL.dll")]
private static extern void helloWorld(Sample sample);
static void Main(string[] args)
{
Sample s = new Sample();
s.length = 10;
s.value = "Huhu";
helloWorld(s);
}
}
Important thing is to mark it as a class, not a struct in C#.
With this, you could also use constructors etc. in C#:
class Program
{
[StructLayout(LayoutKind.Sequential, Pack = 1)]
private class Sample
{
public Int32 length;
public String value;
public Sample(String s)
{
length = s.Length;
value = s;
}
}
[DllImport("C:\\Users\\Kep\\Documents\\Visual Studio 2010\\Projects\\SODLL\\Debug\\DLL.dll")]
private static extern void helloWorld(Sample sample);
static void Main(string[] args)
{
Sample s = new Sample("Huhu");
helloWorld(s);
}
}