I have a C++ dll which is used to card printing( ID cards ). My implementation done using C#.Net. I used following code to call c++ dll.
[DllImport(#"J230i.dll",CallingConvention = CallingConvention.Cdecl,SetLastError=true)]
public static extern int N_PrintJobStatus(ref int[] nPrtintjobStatus);
int[] pJob = {0,0,0,0,0,0,0,0} ;
ret = N_PrintJobStatus( ref pJob);
N_PrintJobStatus method signature given as bellow
N_PrintJobStatus(int *pJobStatus )
After calling the method it gives following error
A call to PInvoke function '********!*********.frmCardPrint::N_PrintJobStatus' has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature. Check that the calling convention and parameters of the PInvoke signature match the target unmanaged signature.
How can I fix this issue
thank you .....
Your translation is incorrect. An int array, int* does not map to ref int[]. The latter would be marshalled as int**. You need instead to use int[].
[DllImport(#"J230i.dll", CallingConvention = CallingConvention.Cdecl,
SetLastError = true)]
public static extern int N_PrintJobStatus(int[] nPrtintjobStatus);
Allocate the array before calling the function. Presumably you have some way to determine how long it should be. As it stands this function looks like a buffer overrun waiting to happen. How can the function know how long the array is and so take steps to avoid writing beyond its end?
It's not clear that this is the only problem. We cannot be sure that the return type really is int. Or that the calling convention is cdecl. Or that the function really does call SetLastError.
Related
My c++ function is given below
# define MyFunction _declspec(dllexport)
extern "C" {
MyFunction int SubtractNumbers(int a, int b)
{
return a - b;
}
MyFunction const char* GetStringKey()
{
return "My String";
}
}
Calling c++ function from windows form is given below,
[DllImport(cppFunctionsDll, CallingConvention = CallingConvention.Cdecl)]
private const string DllFilePath = #"D:\\Projects\\December\\17-12-2020\\project-device-
setup\\Debug\\AccurynCPP.dll";
[DllImport(DllFilePath, CallingConvention = CallingConvention.Cdecl)]
public static extern int SubtractNumbers(int a, int b);
[DllImport(DllFilePath, CallingConvention = CallingConvention.Cdecl)]
public static extern string GetStringKey();
public void GetSubtract()
{
lblSubtract.Text= Convert.ToString(SubtractNumbers(1, 2));
}
public void GetStringFunction()
{
lblstringKey.Text= GetStringKey();
}
From the above function, GetSubtract() worked perfectly. But GetStringKey() not worked. when it reach on it's function while debugging, it automatically cancelled running mode in visual studio. How can i fix this issue.
The calling convention describes to the compiler and linker how parameters, the stack and registers are to be handled. So if these don't match things won't work or memory can be corrupted. For instance, if the caller thinks the first parameter should be passed in the register eax, but the callee thinks it's on the stack. Things will obviously not work. For more info on calling conventions Wikipedia has a good description here. The calling convention you choose isn't very important, unless the compiler forces one on you. For instance for 64 bit programs Microsoft only uses one to my knowledge. So, I suggest adding __cdecl in front of the functions you have written. And using CallingConvention.Cdecl on the DLLImport lines.
Also, all the data types must exactly match. For instance:
[DllImport(DllFilePath, CallingConvention = CallingConvention.Cdecl)]
public static extern string GetStringKey();
This says it is returning a string, however despite our using the term string to represent a series of characters followed by a null. In this case string is a formal type. Which is they type std::string. Which is not what is being returned by the C code. The C code is returning a pointer to a char.
Generally passing complex types based on a library is kinda iffy. That's because it requires both languages to use the exact data structure to represent the type. So when crossing language boundaries it's often useful to stay with your own custom structures and very primitive types.
So I recommend your C function be this instead:
__cdecl bool GetStringKey(char *buffer, int maximumLength)
{
// Write code here to copy all the characters from your "My String"
// to the buffer or use a standard library function for copying char * strings (often called C strings) when copying make sure to copy the null character at the end. It's definitely needed.
return true; // Yes it fit
}
You will need to change your DllImport to match. Once that is working you can add code to never go over the maximum buffer length (to prevent memory corruption) and maybe change the bool to the length of the string or -1 for failure.
I suggested this particular function prototype because it doesn't pass memory between languages. Instead the C code fills in a buffer from the C++ code. Which is the safest way to do something. Passing memory between unrelated modules (in this case C and C++ doesn't always work). Though I suspect in this case your C and C++ compiler and libraries are enough alike. That you could just switch to returning a std::string. As long as both sides match perfectly.
In C:
extern "C" __declspec(dllexport) int CfgGetVariableString(const char *Name, char **Value)
{
char StrValue[STR_MAX];
int RetValue = GetVariableToStrValue(Name, StrValue, STR_MAX);
if (RetValue == 0)
*Value = StrValue;
return RetValue;
}
C#
[DllImport(DllName, CallingConvention = DllCallingConvention)]
private static extern int CfgGetVariableString([MarshalAs(UnmanagedType.LPStr)]string name, [MarshalAs(UnmanagedType.LPStr)]out string value);
This does not work. I can make it work by calling CoTaskMemAlloc, but then I guess I should free it with separate managed to native call?
So what is the cleanest way to do it?
I can make it work by calling CoTaskMemAlloc.
That is the only way that it can work. Because the marshaller will call CoTaskMemFree to deallocate the memory returned from the native function.
I guess I should free it with separate managed to native call?
As mentioned above, that's not necessary because the framework already does so.
What is the cleanest way to do it?
In my opinion the cleanest approach here, since you are already using a string length determined at compile time, is to have the caller allocate the memory. Change the type from char** to char*. Add an extra argument containing the length of the allocated string. Use StringBuilder on the C# side. There are countless examples of this pattern available here and indeed in other places so I don't feel any need to add to the canon.
I am trying to call an existing C dll from a C# application, using System.Runtime.InteropServices, and am having difficulty matching the signatures between the PInvoke function and the Target function.
The target function is
__declspec(dllexport) DWORD GetSomeString(char* strOut);
And my PInvoke function is
[DllImport("Existing.dll")]
public static extern uint GetSomeString([MarshalAs(UnmanagedType.LPWStr)]
string strDisplay);
I make the function call
string tempStr = "My Output String";
uint retVal = GetSomeString(tempStr);
But I get the message
Managed Debugging Assistant 'PInvokeStackImbalance' has detected a problem...
...A call to PInvoke function 'GetSomeString' has unbalanced the stack. This is likely because the managed PInvoke signature does not match the unmanaged target signature. Check that the calling convention and parameters of the PInvoke signature match the target unmanaged signature.
I have also tried implementing the PInvoke function as
[DllImport("Existing.dll")]
public static extern uint GetSomeString([MarshalAs(UnmanagedType.LPWStr)]
StringBuilder strDisplay);
But to no avail.
Does anyone out there have an idea what I could be doing wrong?
Please let me know if further info is required or my question is unclear.
Thanks in advance.
You need to specify the calling convention. By default, PInvoke uses StdCall, but your method is (likely) Cdecl.
[DllImport("Existing.dll", CallingConvention=CallingConvention.Cdecl)]
In addition to the incorrect calling convention mentioned by Reed Copsey, the matching type for char* is UnmanagedType.LPStr.
I´m trying to call a method that is in a C++ dll declarated as __declspec(dllexport) to use in C#, but I don´t know how to return a string value from C++ and how to declare the signature using DllImport in C#.
C++ code "VNVAPI.dll"
__declspec(dllexport) char * GetGpuName(int phyGPUid)
{
CNvidia * pInstance = CNvidia::GetInstance();
char szName[512]={0};
pInstance->GetGpuName(phyGPUid,szName,512);
return szName;
}
C# method signature:
[DllImport("VNVAPI.dll")]
public static extern char GetGpuName(int phyGPUid);
Error generated:
A call to PInvoke function
'Core!Core.Hardware.IO.NVAPI::GetGpuName'
has unbalanced the stack. This is
likely because the managed PInvoke
signature does not match the unmanaged
target signature. Check that the
calling convention and parameters of
the PInvoke signature match the target
unmanaged signature.
Thanks.
As has been pointed out by others you need to specify the C calling convention in your P/Invoke and also use string on the managed side to marshal the null terminated char*.
However you should rejig the C++ routine to take a char* as an input parameter, together with a buffer length parameter. You then write into this buffer in the native code. This avoids the current problem that the data, as you presently have the code, is returned from the stack which, of course, is unwound as the function returns.
The suggestion to use static will make this memory global and so avoid stack unwind problems, at the expense of thread safety. Yes it will likely work for this use case but its a bad habit to get into.
The error message might be confusing.
Without going into detail, try this:
static char szName[512]={0};
If you still get the error, the you need to specify the calling convention in the DllImport attribute.
Edit:
Also make the return type string for C# method declaration.
The error message suggests checking the calling convention. I suspect that the function is using C calling convention, which would explain the unbalanced stack. I would suggest that you specify the calling convention (as #leppi suggested) in your DllImport attribute. Like this:
[DllImport("VNVAPI.dll", CallingConvention=CallingConvention.Cdecl)]
According to the Strings samples, the return value should be string:
static extern string GetGpuName(int phyGPUid);
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.