I need to pass a buffer containing audio stream from C# to native dll. Buffer resides in struct. It's desirable that buffer will pass via interface & not via disk path. I've seen this method:
// native
struct MyStruct
{
short* buffer
}
void Foo(MyStruct *myStruct);
// managed
[StructLayout(LayoutKind.Sequential)]
public struct MyStruct
{
[MarshalAs(UnmanagedType.ByValArray, SizeConst=1000)]
public short[] buffer;
}
[DllImport("My.dll")]
public static extern void Foo(ref MyStruct myStruct);
First question is if this code is correct for short* buffer of size up to 1K shorts?
Second, size is unkown in advance: do I need to set maximum size in SizeConst (could be several MB)?
First of all, the two structures in your question don't match. The C# structure would match
struct MyStruct
{
short arr[1000];
};
That's what ByValArray means – an array allocated inline in the structure.
If the size is dynamic (that is only known at runtime), then you probably should not expect to get the marshaller to handle this for you. You certainly don't want to force marshalling to a constant sized buffer every time because that will be inefficient. In fact you really want to avoid copying the buffer around at all. And the p/invoke marshaler has an upper limit on the size of objects that it is prepared to marshal.
It is going to be far cleaner and efficient to pin the array manually, and pass its address. And you should also pass the length of the array so that the C++ code knows how much it is expected to read.
On the C++ side:
struct BufferStruct
{
int len;
short* arr;
};
void Foo(const BufferStruct buffer);
On the C# side:
[StructLayout(LayoutKind.Sequential)]
public struct BufferStruct
{
public int len;
public IntPtr arr;
}
[DllImport("My.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern void Foo(BufferStruct buffer);
Then you call the function like this:
short[] arr = ...;
GCHandle gch = GCHandle.Alloc(arr, GCHandleType.Pinned);
try
{
BufferStruct buffer;
buffer.len = buffer.Length;
buffer.arr = gch.AddrOfPinnedObject();
Foo(buffer);
}
finally
{
gch.Free();
}
All of this would be easier still if you did not force the array into a struct. If you passed the length and array as parameters, then the marshaller would pin the array for you and make the code even simpler.
Related
I have a library in C++ containing a library returning a Byte* :
typedef unsigned char Byte;
Byte* RotateImage90(Byte* data, int w, int h);
I'm using this library in a program in C# (Xamarin) :
[DllImport("libCpp", EntryPoint = "RotateImage90")]
public static extern IntPtr rotate90(byte[] data, int w, int h);
Byte[] test(Byte[] data, int w, int h)
{
IntPtr ptr = rotate90(data, w, h);
Byte[] img = ????;// <= function missing
return img;
}
It works good, but I don't know how to convert the pointer to a Byte array. Someone know a function to do that ?
A problem of your function interface is that the memory that the function dynamically allocates to return the rotated image byte array, must be allocated with the same memory allocator that the C# side (or whatever client code) will use to release the memory.
In other words, the module that allocates the memory and the module that frees it must use the same allocator.
When I needed to pass some array data between native code and C# code I successfully used Safe Arrays. On the C++ side, you can use ATL's CComSafeArray to simplify the safe array programming; on the other hand, C# and the CLR understand safe arrays well, so it's easy to get the array data in C# and consume it in managed code.
You can use a function like this to produce a safe array of bytes in C++ (in your case, the safe array will store the rotated image data):
extern "C" HRESULT __stdcall ProduceSafeArrayOfBytes(/* [out] */ SAFEARRAY** ppsa)
{
HRESULT hr = S_OK;
try
{
// Create the safe array to be returned to the caller
CComSafeArray<BYTE> sa( /* Element count */);
// Fill the safe array data.
// You can use a simple sa[i] syntax,
// where 'i' is a 0-based index
...
// Return the safe array to the caller (transfer ownership)
*ppsa = sa.Detach();
}
// Convert exceptions to HRESULT return codes
catch (const CAtlException& e)
{
hr = e;
}
catch (const std::exception& )
{
hr = E_FAIL;
}
return hr;
}
On the C# side, you can use this PInvoke signature:
[DllImport("NativeDll.dll", PreserveSig = false)]
public static extern void ProduceSafeArrayOfBytes(
[Out, MarshalAs(UnmanagedType.SafeArray, SafeArraySubType = VarEnum.VT_UI1)]
out byte[] result
);
The VT_UI1 enum field tells the .NET Marshaller that the safe array contains bytes.
You can get the array data in C# with simple code like this:
byte[] data;
ProduceSafeArrayOfBytes(out data);
As you can see, in your C# code you deal with a simple byte[] array; all the proper data marshalling (including freeing memory) happens automatically under the hood.
You can modify the aforementioned skeleton code, adding the other function parameters, like your image width and height.
As an alternative, another option would be developing a tiny bridging layer using C++/CLI, to convert from raw C-style arrays to .NET managed arrays.
Anyway, the note on your DLL function interface to use a common memory allocator for allocating and releasing the array memory is still valid.
As a third option, if you can modify your DLL function interface, you can require the caller to allocate an array and pass it to the DLL function. The function will write the result data in this caller-allocated array.
This would simplify the memory management, as you give the DLL function a block of memory that is already allocated by the caller. The caller will have the responsibility for both allocating and releasing that memory.
So, your DLL function would look like this:
extern "C" void __cdecl RotateImage90(Byte* result, Byte* data, int width, int height);
The result array is allocated by the caller, who is also responsible to free it.
The function just writes its output in this caller-allocated array.
PInvoke would look like this:
[DllImport("NativeDll.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern void RotateImage90(byte[] result, byte[] data, int width, int height);
I have a Unity5 program that is using a common structure (smMsg) to send data from a C++ DLL into C#.
The structure contains an array:
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 16)]
public float[] mtx;
This array is to be considered as a 4x4 Matrix, hence the constant size of 16.
The program works, receiving data into the C# corresponding structure; however, it seems that from my array I am missing the first element (mtx[0]) each time I run the program. It seems every other element has shifted to the left with an extra 0 for the last element, maintaining the sequential order they are to be in.
I considered it to be because of the UnmanagedType I was using, however other sources tell me that UnmanagedType.ByValArray is the correct type.
Would anyone have a direction or lead I can follow to help solve this problem?
Process of Copying Data
C#:
// DLL Import
[DllImport(DLL_NAME, EntryPoint = "smCopy")]
private static extern void smCopyData(IntPtr dest, IntPtr len); //const VOID *dest, SIZE_T len);
// Copying data logic
{
// allocate intptr to buffer
var len = Marshal.SizeOf(typeof(smMsg));
IntPtr msg_intptr = Marshal.AllocHGlobal(len);
try
{
// Copy data
smCopyData(msg_intptr, (IntPtr)(len));
// Set POINTER data to struct
return_msg = (smMsg)Marshal.PtrToStructure(msg_intptr, typeof(smMsg));
}
finally
{
// free unmanaged memory!
Marshal.FreeHGlobal(msg_intptr);
}
}
C++
// Function called via DLL
void DLL_API smCopy(const VOID *dest, SIZE_T len)
{
CopyMemory((PVOID)(dest), (PVOID)(mapBuffer), len);
}
Struct Definition
The main piece of data I am interested in is float[] mtx
[StructLayout(LayoutKind.Sequential)]
public struct smMsg
{
public smHeader header;
public smData data;
}
[StructLayout(LayoutKind.Sequential)]
public struct smData
{
// Event ID.
public int evtId;
public int status;
// Floating point values. Actual data to be transmitted.
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 16)]
public float[] mtx;
}
[StructLayout(LayoutKind.Sequential)]
public struct smHeader
{
public ushort chkSum;
public char numWords;
public char msgType;
}
UPDATE 2/29
Thanks to #Zastai, I ended up able to recover the missing element. As it turns out, I wanted to be using "byte" as the data type instead of char, as char is a C# unicode type (short).
What I ended up doing is changing my smHeader as:
[StructLayout(LayoutKind.Sequential)]
public struct smHeader
{
public ushort chkSum;
public byte numWords;
public byte msgType;
}
.. which in turn lowered the smHeader size from 6 to 4, setting the smMsg struct size equal in terms of both C# & C++.
Thanks to #Zastai for helping out a ton.
Turns out that when using char in C++ for a struct, it's C# counterpart is a byte. This resolved the difference in structure size between C# & C++. Because of this error, the smMsg struct in C# allocated more memory than it needed.
The question is updated with the new smHeader definition.
I'm calling the IDiaSourceFile::get_checksum member function, which has the following auto-generated .NET signature:
void get_checksum(uint cbData, out uint pcbData, byte[] pbData);
Note that the parameter pbData is an out parameter. Do I need to worry about pinning the array passed in here when someone calls get_checksum?
(Background:
I inherited some code that looks like this:
unsafe
{
fixed (byte* p = scratch_hash)
{
sourceFile.get_checksum(c, out c, scratch_hash);
}
}
which pins scratch_hash as p and then never actually uses p. I've not seen anything like this before and given the state of the surrounding code I suspect the pin is completely unnecessary here
)
With the current definition:
void get_checksum(uint cbData, out uint pcbData, byte[] pbData);
You don't need to pin pbData. However, you'd need to pre-allocate an array for the returned the data, but you don't know the size in advance. The method will fail if the array size you pass via cbData is not large enough, and your current method signature doesn't allow to find out the buffer size.
The original C++ declaration does allow that:
HRESULT get_checksum (
DWORD cbData,
DWORD* pcbData,
BYTE data[]
);
data [in, out] A buffer that is filled with the checksum bytes. If
this parameter is NULL, then pcbData returns the number of bytes
required.
So, a more efficient approach might be to declare and use it like this:
void get_checksum(
uint cbData,
out uint pcbData,
IntPtr data);
// get size
uint size;
obj.get_checksum(0, out size, IntPtr.Zero);
// get data
var buff = new byte[size];
unsafe
{
fixed (byte* p = buff)
{
uint cbData;
obj.get_checksum(size, out cbData, (IntPtr)p);
if (size != cbData)
throw new InvalidOperationException("cbData");
}
}
If you don't want (or can't) use unsafe code, here's an alternative:
// get size
uint size;
obj.get_checksum(0, out size, IntPtr.Zero);
// get the data
byte[] buff;
var p = Marshal.AllocHGlobal((int)size);
try
{
uint cbData;
obj.get_checksum(size, out cbData, p);
if (size < cbData)
throw new InvalidOperationException("cbData");
buff = new byte[cbData];
Marshal.Copy(p, buff, 0, (int)cbData);
}
finally
{
Marshal.FreeHGlobal(p);
}
No, you don't have to do it in this case - and you can't, even.
However, you might want to change that signature if you run into performance issues, because the marshaller does a lot of work to pass the byte array to the native method - it has to allocate new memory, copy the managed byte array to that, and then deallocate the memory again.
This means that the code you posted is complete nonsense, because while it does fix the scratch_hash array, it doesn't use that fixed pointer anyway.
If you're only calling the method once in a while and the byte array is relatively small, this can be safely ignored. However, if you do find that this causes unnecessary strain on your application, fixing could help - but you'd have to change the signature:
unsafe void get_checksum(uint cbData, out uint pcbData, byte *pbData);
Then you can use this:
fixed (byte *p = &scratch_hash[0])
sourceFile.get_checksum(c, out c, p);
All this said, note that fixing a managed object might cause issues of its own (for example, it pretty much kills any chance of heap compaction if your object is on top of the heap). Fixing should be done sparingly, and only for extremely short times - the only other reasonable alternative is to allocate the objects early, and keeping them together (for example, when using asynchronous I/O, you need a fixed reference, usually for as long as the application runs - so just allocate the buffers as early as possible, and you'll be mostly fine).
Also, I'm still finding that signature a bit wild. Could you share the header file's definition of the signature?
I have searched for days and have tried everything I could find, but still cannot get this to work.
Details:
I have a 3rd party stock trading app that is calling into my unmanaged dll. It is supplying data that the dll processes/filters and then saves into a global ring buffer. The ring buffer is an array of structures, 100 long. All of this runs in the stock trading apps process.
I also have a managed C# app calling into the same dll in a different process that needs to get the info in the global ring buffer as quickly and efficiently as possible. Everything works except that I can only get data for the first structure in the array. Also after the call to the dll from C# the C# code no longer knows that arrayMD is an array of structs, it shows up in the debugger as a simple structure. Could it be the memcpy in the dll causing the problem? I’ve tried all kinds of combinations with [In, Out], IntPtr, and Marchal.PtrToStructure combinations. I am greatly fubar. Any help would be greatly appreciated.
Thanks
Here is what I am attempting.
On the dll side:
struct stMD
{
float Price;
unsigned int PriceDir;
unsigned int PriceDirCnt;
};
// Global memory
#pragma data_seg (".IPC")
bool NewPoint = false; // Flag used to signal a new point.
static stMD aryMD [100] = {{0}};
#pragma data_seg()
void __stdcall RetrieveMD (stMD *LatestMD [])
{
memcpy(*LatestMD, aryMD, sizeof(aryMD));
}
On the C# side:
[StructLayout(LayoutKind.Sequential)]
public struct stMD
{
public float Price;
public uint PriceDir;
public uint PriceDirCnt;
};
public static stMD[] arrayMD = new stMD[100];
[DllImport(#"Market.dll")]
public static extern void RetrieveMD(ref stMD[] arrayMD);
RetrieveMD(ref arrayMD);
The problem is the definition of your DLL entry point:
void __stdcall RetrieveMD (stMDP *LatestMD [])
You don't specify the size of the array, so how is C# supposed to know how many elements were copied into it? This is a problem in other languages too. Your implementation simply assumes that the provided memory is large enough to contain aryMD. But what if it's not? You've just created a buffer overrun.
If you want the caller to allocate the array, then the caller must also pass in the number of elements that the array contains.
Edit
The C++ declaration should look something like this:
// On input, length should be the number of elements in the LatestMD array.
// On output, length should be the number of valid records copied into the array.
void __stdcall RetrieveMD( stMDP * LatestMD, int * length );
The C# declaration would then look something like this:
[DllImport(#"Market.dll")]
public static extern void RetrieveMD(
[In, Out, MarshalAs(UnmanagedType.LPArray, SizeParamIndex=1)] ref stMD[] arrayMD,
[In, Out] ref int length);
Your problem, I think, is that you are trying to pass an array by reference from C# into C, and you almost never need to do that. In your case, all you want to happen is for C# to allocate memory for 100 of your structures, then pass that memory to C to be filled in. In your case, you're passing a pointer to an array of structures, which is an extra level of indirection that you don't really need.
You rarely see C functions that take a fixed sized array; instead, your function would typically be defined in C to accept a pointer and a length, e.g. something like:
void __stdcall RetrieveMD (stMDP *LatestMD, int size)
{
int count = 0;
if (size < sizeof(aryMD))
count = size;
else
count = sizeof(aryMD);
memcpy(LatestMD, aryMD, count);
}
To call this method from C#, you need to do two things. First, you need to allocate an array of the appropriate size to pass in. Second, you need to tell the marshaling code (that does the C# -> C copying) how to figure out how much data to be marshaled, via the [MarshalAs] attribute:
[DllImport(#"Market.dll")]
public static extern void RetrieveMD (
[MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 1)] stMDP[] arrayMD,
int length
);
var x = new stMDP[100];
RetrieveMD(x, 100);
I got it working. Was I ever tring to make this harder than it is.
I re-read chapter 2 of ".NET 2.0 Interoperability Recipes: A Problem-Solution Approach".
Here's what works.
On C++ side I removed the pointers
struct stMD
{
float Price;
unsigned int PriceDir;
unsigned int PriceDirCnt;
};
// Global memory
#pragma data_seg (".IPC")
bool NewPoint = false; // Flag used to signal a new point.
static stMD aryMD [100] = {{0}};
#pragma data_seg()
void __stdcall RetrieveMD (stMD LatestMD [100])
{
memcpy(LatestMD, aryMD, sizeof(aryMD));
}
On the C# side I removed both (ref)s and added [In, Out]
[StructLayout(LayoutKind.Sequential)]
public struct stMD
{
public float Price;
public uint PriceDir;
public uint PriceDirCnt;
};
public static stMD[] arrayMD = new stMD[100];
[DllImport(#"Market.dll")]
public static extern void RetrieveMD([In, Out] stMD[] arrayMD);
RetrieveMD(arrayMD);
Thanks to everyone who offered help.
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);
}
}