I'm converting some code from C++ to C#, and I need some insight on how to deal with lines like these:
PDWORD rgb32_data = (PDWORD) malloc(640*480*4);
From what I understand, the PDWORD type represents an unsigned integer (32bit). What would be a good C# strategy to implement a similar structure?
EDIT: I figured out that I need to use an IntPtr to replace the PDWORD, but the IntPtr would hold a single value as opposed to many values as in an array. Any C# suggestions?
EDIT2: Uint[]s don't work for me - unfortunately, the only way I can make the program compile, with all its dependencies, is when I enter an IntPtr as a the argument, not an IntPtr[].
Thanks.
EDIT - I've marked the solution, in addition I needed to do the following:
Since I needed an IntPtr cast, I added this code
IntPtr rgb32_ptr;
GCHandle handle = GCHandle.Alloc(rgb32_data, GCHandleType.Pinned);
try
{
rgb32_ptr = handle.AddrOfPinnedObject();
}
finally
{
if (handle.IsAllocated)
{
handle.Free();
}
}
to get an IntPtr to the same.
According to MSDN, PDWORD is defined as:
typedef DWORD* PDWORD;
So it's a pointer to a DWORD, which MSDN also says is an unsigned 32-bit integer - in other words, a uint.
Note that your line of C++ code allocates memory for an array of 640 * 480 = 307,200 DWORDs.
You could translate this directly as:
uint[] rgb32_data = new uint[640 * 480];
But the code might really want to use it as just a series of bytes in memory, so you might instead want to use a byte array
byte[] rgb32_data = new byte[640 * 480 * 4];
and use the methods in the BitConverter class (especially the ToInt32() method) if you think that is closer to how the original program handles the data.
You can use unsafe pointers and Marshal.AllocHGlobal()
Program()
{
unsafe
{
IntPtr ptr = Marshal.AllocHGlobal(640 * 480 * 4);
uint* ptr2 = (uint*)ptr.ToPointer();
for (int i = 0; i < 640*480; i++)
{
ptr2[i] = 0;
}
Marshal.FreeHGlobal(ptr);
}
}
PDWORD is a pointer to a 32-bit unsigned integer. In C#, that would be ref uint32. Or IntPtr if you're on the 32-bit runtime.
It depends on how you're going to use it. If you're passing it to unmanaged code, then I'd suggest the ref UInt32. If you're using it for memory allocation (the return value from a Windows API function that allocates memory, for example), then you'd want to use IntPtr.
PDWORD is a pointer to a DWORD. There are a few options to convert the code to C# depending on how it will be used.
For the DWORD itself you can use System.UInt32 as an equivalent. Marshal.AllocHGlobal might be a possibility as a replacement, but we would need to see how it's going to be used. Are you accessing the memory yourself or maybe passing it to another unmanaged function?
Related
I'm reading all about structure marshalling between C and C# in 64bit environment without success.
What I need to do is to pass the structure
typedef struct DescStructTa
{
char* pszNa;
unsigned long ulTId;
char* pszT;
unsigned short usRId;
unsigned long ulOff;
unsigned long ulSi;
char szAcc[2];
unsigned char bySSize;
} DescStruct;
from a C# application to a C DLL not made by us calling the method
MT_DLL_DECL long GetAll(UINTPTR ulHandler, DescStruct **ppList, unsigned long *pulNumS);
After long search I wrote the following code:
[StructLayout(LayoutKind.Sequential, Pack = 8)]
public struct PlcSymbolDescStruct
{
[MarshalAs(UnmanagedType.LPStr)]
string pszNa;
UInt32 ulTId;
[MarshalAs(UnmanagedType.LPStr)]
string pszT;
[MarshalAs(UnmanagedType.U2)]
ushort usRId;
UInt32 ulOff;
UInt32 ulSi;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 2)]
string szAcc;
[MarshalAs(UnmanagedType.U1)]
byte bySSize;
}
[DllImport("DataSource.dll", SetLastError = true, CallingConvention = CallingConvention.StdCall)]
[return: MarshalAs(UnmanagedType.U8)]public static extern long GetAll(ulong ulHandler, out DescStruct [] ppList,
out uint pulNumS);
Unfortunately this is still giving me a heap corruption.
If I try to pack less than 8 it gives (as expected) an access violation, but I expected this to work and I'm not able to figure out what could be wrong.
Any suggestion?
Thanks in advance!
I’m not sure you need Pack = 8. The default packing appears to be more or less compatible between C# and C++.
The szAcc field probably uses Unicode. To switch, specify CharSet=CharSet.Ansi in your [StructLayout]
Finally, if your C API has that double pointer to the array of them, you have to allocate that memory in a special way, either CoTaskMemAlloc or LocalAlloc, forgot which one.
It’s easier, also more efficient, to implement 2 functions instead. One to obtain the required size of the array. Another one to write them into caller-allocated buffer, a single pointer in C++ as opposed to double pointer, and UnmanagedType.LPArray in C#.
Update
allocation should not be necessary since the documentation states that the call returns a pointer to it's internal data structure
The runtime doesn’t know what was written in that documentation, attempts to free anyway, and crashes. You can use out IntPtr in C# API, and marshal manually.
If you have modern .NET, use unsafe, cast the IntPtr to PlcSymbolDescStruct* raw pointer, and construct ReadOnlySpan<PlcSymbolDescStruct> from that raw pointer.
if you’re stuck with old desktop .NET, call Marshal.PtrToStructure<PlcSymbolDescStruct> in a loop, incrementing the pointer by Marshal.SizeOf<PlcSymbolDescStruct>() bytes between loop iterations. No need to use unsafe code in this case.
I have a C DLL which does some processing on an image and returns the result. So I am trying to pass an image over from the .NET side, but right now I am stuck, and I'm not sure whether that's on the types, the intricacies of marshaling, or syntax. Since I am a noob it could be all three.
I can call other functions in the DLL in question, so at least some of the foundations are in place. For example, when I call:
IntPtr versionIntPtr = GetDLLVersionNumber();
string version = Marshal.PtrToStringAnsi(versionIntPtr);
Console.WriteLine("DLL version number reported as: " + version);
and this works absolutely fine, printing the current version of the DLL to the console.
When I look at C code sample which uses the same function, it appears to use the function as follows:
unsigned char* ExtractImageInfo(const char* pixels, size_t width, size_t height)
(I also have a header file where the same function is mentioned as follows:
... ExtractImageInformation(struct ImageData image, void* imageInformation);
but I don't really know what this means.)
So my attempt to use this function goes as follows:
First I specify the interface to the DLL:
[DllImport("C:\\MyDLL", EntryPoint = "ExtractImageInfo", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
public static extern IntPtr ExtractImageInfo(IntPtr image, UInt32 imageWidth, UInt32 imageHeight);
Then, I get an image of the right type and get its dimensions:
Bitmap bitmap1 = (Bitmap)Image.FromFile("C:\\Images\\myImage.bmp");
UInt32 _imageWidth = Convert.ToUInt32(bitmap1.Width);
UInt32 _imageHeight = Convert.ToUInt32(bitmap1.Height);
Then I get a pointer to the image (since I think that's what I need to pass):
IntPtr bitmap1Ptr = bitmap1.GetHbitmap();
And then I call the function...
IntPtr myProcessedImage;
myProcessedImage = ExtractImageInfo(bitmap1Ptr, _imageWidth, _imageHeight);
But it doesn't work - I don't get back a pointer to my processed data. A big part of what I'm wondering here is whether I am passing the image in the right way, and whether the syntax for my interface to the native function is right. I think probably the answer to both could be NO!
I've only had a few days of reading and experimenting with PInvoke, so if anyone can point out to me the error of my ways I will be eternally grateful :-)
unsigned char* ExtractImageInfo(const char* pixels, size_t width, size_t height)
The name of the argument strongly suggests it wants a pointer to the raw pixel data. You can get one from Bitmap.LockBits(), BitmapData.Scan0 property. Don't call UnlockBits() until the function returns.
The return type of the function is a significant memory management problem. Whatever buffer pointer is returned is almost certainly going to have to be released. You cannot do so from managed code, you don't have access to the allocator used by this C code to call the proper version of free(). Call this function a million times and double-check that you don't have an out-of-control memory leak. If you do then you can't pinvoke it, a C++/CLI wrapper is required although the odds that it works correctly are fairly slim as well. This is a poorly designed function that's hard to use from any code, including C.
I am trying to import a function from a c dll into C#. The c function looks like this
unsigned short write_buffer( unsigned short device_number, unsigned short word_count, unsigned long buffer_link, unsigned short* buffer)
my attempt at a C# import looks like this
[DllImport("sslib32.dll", CharSet = CharSet.Ansi, SetLastError = true)]
private static extern ushort write_buffer(ushort deviceNumber, ushort wordCount, UInt32 bufferLink, IntPtr buffer)
In C# i have a Dictionary of messages that i would like to pass to this function. The Dictionary looks like this:
Dictionary<string, List<ushort>> msgs
I am a bit confused how to make a make a proper call to pass msgs as the buffer. deviceNumber is 2, wordCount is 32, and buffLink is 0. So i know the call should look something like this
write_buffer(2,32,0, msgs[key]);
Obviously i am getting an invalid argument for the IntPtr. What is the proper way to make this call?
It is quite unclear what buffer should contain and in which direction its data flows. Your dictionary suggests it should be an array. Just declare it that way:
private static extern ushort write_buffer(.., ushort[] buffer);
And use msgs[key].ToArray() in the call.
Using constants in the write_buffer() call does not make that a likely scenario though, there ought to be msgs[key].Count in there somewhere.
You can generate P/Invoke signatures using the P/Invoke Interop Assistant tool that is referenced here.
In the January 2008 issue of the MSDN
Magazine, Yi Zhang and Xiaoying Guo
have published a CLR Inside Out column
on marshaling between managed and
unmanaged code. In that column, they
introduce the P/Invoke Interop
Assistant, an automatic GUI and
command-line utility for converting
between managed and unmanaged
signatures (in both directions). This
conversion, of course, is not limited
just to Windows signatures; give the
tool a snippet of your own C header
files and it will dutifully convert
them to pretty-printed C#
[DllImport]'s.
If you don't mind marking the code unsafe, you can simply do:
fixed(ushort* pData = msgs[key])
{
write_buffer(2,32,0, pData);
}
And declare your DllImport to take ushort* as the last argument.
The alternative is to use Marshal.StructureToPtr to get the array marshalled into fixed memory. This requires you to allocate memory first using AllocHGlobal, and freeing using FreeHGlobal.
In the Compact Framework 3.5, I am attempting to call an ActiveX object that has an IDL function signature:
HRESULT MyFunc([out] SAFEARRAY(float) *var)
The Interop generation creates the msil
[out] class [mscorlib]System.Array& marshal( safearray float32)
Which seems reasonable enough, but I keep getting a "NotSupportedException". According to an article entitled "Interop: Common issues and debugging techniques" (I can't post more than one hyperlink, it's the first google result for that phrase), in the first bullet point under the "Marshaling" heading, the compact framework doesn't properly marshal SAFEARRAYs.
I have attempted to get around this problem, by manipulating the answer described in this MSDN forum post (Last entry describes his method): http://social.msdn.microsoft.com/forums/en-US/clr/thread/6641abfc-3a9c-4976-a523-43890b2b79a2/
So, I have created the following definition:
[StructLayout(LayoutKind.Sequential)]
struct SafeArray
{
public ushort dimensions; // Count of dimensions in the SAFEARRAY
public ushort features; // Flags to describe SAFEARRAY usage
public uint elementSize; // Size of an array element
public uint locks; // Number of times locked without unlocking
public IntPtr dataPtr; // Pointer to the array data
public uint elementCount; // Element count for first (only) dimension
public int lowerBound; // Lower bound for first (only) dimension
}
And redefined the IDL for the function signature to:
HRESULT MyFunc([out] long *var)
And then issuing the following code:
IntPtr safeArrayPtr = Marshal.AllocCoTaskMem(Marshal.SizeOf(typeof(SafeArray)));
SafeArray safeArray;
safeArray.dimensions = 1;
safeArray.features = 0;
safeArray.elementSize = (uint)(Marshal.SizeOf(typeof(float)));
safeArray.locks = 0;
safeArray.elementCount = 6;
safeArray.lowerBound = 0;
safeArray.dataPtr = Marshal.AllocCoTaskMem((int)(safeArray.elementCount * safeArray.elementSize));
Marshal.StructureToPtr(safeArray, safeArrayPtr, false);
int iTmp = safeArrayPtr.ToInt32();
MyFunc(out iTmp)
While the code appears to succeed, when I try to read back the data values, using the Marshal.Copy(dataPtr, myFloatArr, false) function, I am getting all 0's for the data, which tells me that the pointer that the ActiveX DLL is getting is probably totally bogus and it's off writing into oblivion.
Any suggestions as to what I may have messed up in these definitions, or suggestions for other ways of approaching this problem?
Thanks In Advance...
Well, I've solved this one.
Hopefully, my answer will help others who encounter the same problem. The problem that I was running into was that the [out] tag in a COM tlb declaration means that whatever I pass in will be overwritten with the object created inside the COM library. A rather complicated version of the classic (and very elementary problem) "Pass By Reference vs Pass By Value"
So, the correct marshaling is to use the definition of SafeArray that I posted above.
Don't touch the IDL signature itself - that's not a very clean way of doing it. Instead, use ildasm on the generated Interop library to modify the il from:
[out] class [mscorlib]System.Array& marshal( safearray float32)
to
[out] native int&
and then reassemble with ilasm which will produce a C# function signature
void MyFunc(out IntPtr var)
The calling code then becomes:
IntPtr ip;
SafeArray safeArray;
float []arrFloats;
MyFunc(out ip);
//Marshal the structure itself
Marshal.StructureToPtr(safeArray, ip, false);
//Marshal the data over to .NET
float []arrFloats = new float[safeArray.elementCount];
Marshal.Copy(safeArray.dataPtr, arrFloats, 0, (int)safeArray.elementCount);
Finally, we need to free the memory (remember, we changed the function signature so we're not giving .NET enough information to actually free the memory on its own.
//Don't forget to free both the structure and the object
Marshal.FreeCoTaskMem(safeArray.dataPtr);
Marshal.FreeCoTaskMem(ip);
I have two unmanaged pointers in the form of IntPtr and want to copy data between them. How can I do this? I know the method Marshal.Copy, but it can only copy between unmanaged and managed.
And the second part: Is copying unmanaged data from C# slower than doing it in unmanaged C/C++ using memcpy?
Edit: I would be especially interested in a platform independet implementation.
You can use the win32 memcpy function via P-Invoke.
[DllImport("msvcrt.dll", SetLastError = false)]
static extern IntPtr memcpy(IntPtr dest, IntPtr src, int count);
Apart from the (slight) overhead calling a win32 function from managed code, the actual copy performance should be the same as C/C++ code that uses the same function.
Don't forget that you can also use an unsafe block (and compiler option) and simply copy the data one byte/int/long at a time:
unsafe
{
// srcPtr and destPtr are IntPtr's pointing to valid memory locations
// size is the number of long (normally 4 bytes) to copy
long* src = (long*)srcPtr;
long* dest = (long*)destPtr;
for (int i = 0; i < size / sizeof(long); i++)
{
dest[i] = src[i];
}
}
This removes the platform dependency, but you need to be very careful with the bounds checking and pointer arithmetic.
Try System.Buffer.MemoryCopy, see the bottom of the page for supported target frameworks.
I believe that the main difference between this and the other solutions that use P/Invoke is that this method avoids the P/Invoke for smaller sizes and just does the copying directly.
Here's the guts of the implementation in .NET Core (latest as of 2020-09-04).
Without making comments on performance, purely because I have not tested it. You can achieve the same performance as unmanaged copy by using either CopyMemory or MoveMemory from Kernel32 via interop.
Here is the declaration for CopyMemory
[DllImport("kernel32.dll")]
static extern void CopyMemory(IntPtr destination, IntPtr source, uint length);
You could look at System.Runtime.CompilerServices.Unsafe.CopyBlock
It seems to allow you to copy bytes from the source address (designated by a void*) to the destination address (designated by a void*).
It also overriden to support ref byte as the source and destination.
[edit]
Disappointingly it appears not to be implemented in Mono
[edit] For those who are interested in this and using Unity, you should instead look to Unity's UnsafeUtility.MemCpy
CopyMemory aka RtlCopyMemory aka memcpy() will be just as fast whether called from C# or C (other than the tiny overhead of PInvoking the method itself).
Something to keep in mind, though, is that CopyMemory should only be used when you're sure that the source and destination ranges do not overlap. If they do overlap, you need to use MoveMemory instead, which will be slower.
Here is a declaration for CopyMeSomeMemory, showing how many different ways you can do the same thing in .Net:
[DllImport("kernel32.dll", EntryPoint = "RtlCopyMemory")]
public static extern void CopyMeSomeMemory(IntPtr Destination,
IntPtr Source, uint Length);
For the record, I think Buffer.BlockCopy in .Net just wraps one of these functions, too.