I have a c++ plus method that generates a value. i am calling this method from a c# application.
The C++ method is like this:
extern "C" REGISTRATION_API char * generate(char dIn[],char dOut[])
The generate method returns an array of chars (sOut[]=returnvalue; return sOut;)
Now I'm calling this method from my c# app:
[DllImport("mydll.dll")]
static extern string generate(string sIn, string sOut);
As you can see, the return type in c# is string. What is happening is that the returned value in c# is not correct and it is corrupted. (The value is correct inside the generate method, but whenever i call it from c# to extract it, i get some erroneous value.)
Is it OK that my method in C# has a string return value while in c++ it's a char*?
Please share your comments, it is urgent, thanks.
Unfortunately, char* can be somewhat ambiguous. (Literally, it's a pointer to a single character, where you may or may not be able to get at other characters by dereferencing the pointer like an array.) Usually, though, it's a pointer to a null-terminated string of single bytes in ANSI encoding (what Microsoft calls an LPStr). By default, P/Invoke marshals instances of System.String as the BStr type, which is like a Unicode Pascal string (with a length at the beginning). You'll need to use the MarshalAs attribute to explicitly specify how you want the strings marshaled between managed and unmanaged code.
Most likely, you'll want to declare it like so:
[DllImport("mydll.dll")]
[return: MarshalAs(UnmanagedType.LPStr)]
static extern string generate(
[MarshalAs(UnmanagedType.LPStr)] string sIn,
[MarshalAs(UnmanagedType.LPStr)] out string sOut);
However, your function signature is confusing: is it returning the string, or is it setting an output variable? You may have to adjust the P/Invoke declaration to fit your needs.
Is it OK that my method in C# has a
string return value while in c++ it's
a char*?
It depends really.
If that C++ method is allocating the memory for the string and expecting the caller to deallocate it then you have problems...big problems. You could try calling Marshal.FreeHGlobal on it, but there is no guarentee that the C++ method actually allocated the memory on the global heap. Most APIs have you feed the pointer back into one of their methods to free the memory.
It is also possible that the char * is pointing to memory which does not need to be freed. In that case using a string should be fine.
You could have the C# declaration return an IntPtr and then call one of the Marhsal.PtrToStringXXX methods.
This method cannot be called safely from a native C++ application either. It returns a pointer to an array on the stack frame. Any call to another function will overwrite that stack frame and corrupt the array.
This probably works by accident in a C++ program right now because there is no function call after obtaining the return value of this function. This kind of luck is no longer available when the P/Invoke marshaller sits in between.
You will have to redesign the C++ function. You should give it arguments that allows the caller to pass its own buffer to be filled with the result. For example:
extern "C" void __stdcall generate(const char* input, char* output, int outputSize)
Call this in C# code by passing a StringBuilder for the output argument, properly initialized with a sufficient Capacity to receive the string. The outputSize argument ensures that the C++ function can avoid writing past the end of the buffer and corrupt the garbage collected heap. Don't ignore it.
Related
I have a .NET program that uses a DLL export to get a name of a user.
public static extern string Name(byte[] buf);
This is the export, and I would really like to not change it, as a lot of code relies on it. So, I would like to know, in C++, how would I convert a char* array to the byte buffer?
I have tried this:
void Name(std::byte buf[256])
{
std::string s{ "test" };
std::byte* ptr = reinterpret_cast<std::byte*>(s.data());
buf = ptr;
return;
}
When I print out the string that I convert, I get nothing, it is empty.
Your C++ function implementation does not match the expectations of the C# function declaration.
The C# byte array is marshalled into the function as a pinned pointer to the array's raw memory. The syntax you are using in the C++ code for the parameter (std::byte buf[256]) is just syntax sugar, the compiler actually treats it as a pointer (std::byte* buf). Which is fine in this situation. However, your C++ function is not actually copying anything into the memory that the pointer is pointing at. You are simply changing the pointer itself to point at a different memory address. And the pointer itself is a local variable, so when the function exits, the pointer will no longer exist, and it won't matter what it is pointing at.
Also, the C# declaration is expecting the function to return something that can be marshalled to a .NET string, but the C++ function is not actually return'ing anything at all. The default marshalling behavior for a string return value is as UnmanagedType.LPStr, so the C++ function needs to return a pointer to a null-terminated char* string. The memory for that string must be allocated with CoTaskMemAlloc(), as the marshaller will take ownership of the memory and free it with CoTaskMemFree() after converting the char data to a string.
Also, the C++ function has no calling convention defined, so it is going to use the compiler's default (which is usually __cdecl, unless you change your compiler's settings). However, the default calling convention that .NET's DllImport expects the C++ function to use is __stdcall instead (for compatibility with Win32 APIs). This mismatch won't matter in 64bit, but it matters alot in 32bit.
Without changing your C# declaration, try this on the C++ side:
char* __stdcall Name(std::byte buf[256])
{
std::string s{ "test" };
size_t size = s.size() + 1;
memcpy(buf, s.c_str(), size);
void *ptr = CoTaskMemAlloc(size);
memcpy(ptr, s.c_str(), size);
return ptr;
}
That being said, it is weird to have a function that returns a string in both a byte array output parameter and in a return value. Are you sure the byte array is not meant to be used as an input parameter instead, where the function parses its content to extract amd a string? That would make more sense. It would really help if you would update your question to show how your .NET code is actually calling the C++ function and using the byte array.
When you write
void Name(std::byte buf[256])
that declares buf as pointer to 256 bytes, not an array. So when you later write
buf = ptr;
all you are doing is changing the local variable buf to now point at ptr. And at the end of the function the local variable dies.
Use std::array<std::byte, 256>& and copy the string contents into that. Or even better return a freshly made array instead of an in/out parameter.
I am writing a C# code, and there is a code that needs calling an unmanaged C++ library.
The signature in the library's header is like this
bool GetValueFromFile(char* sPathToFile, char* &sResult);
What signature should I translate this in C#? I tried:
bool GetValueFromFile(string filePath, ref string result)
But it does not work. There is no exception and the return value is true. But the string result stays null. It is the same for out string result or StringBuilder result.
I use Marshal.GetDelegateForFunctionPointer to get the function pointer as delegate.
You can handle a reference to a pointer pretty much like a pointer to a pointer, at least as far as P/Invoke is concerned.
I think you will probably need to use an IntPtr for the sResult parameter, along with either Marshal.PtrToStringAnsi() or Marshal.PtrToStringAuto(), but it's a bit difficult to say without knowing whether the C/C++ function allocates the string memory or not.
If it works, you will probably still need to free the memory (after getting the string) using Marshal.FreeCoTaskMem() or Marshal.FreeHGlobal(), but again this is impossible to know for sure without knowing what the C/C++ function does.
NOTE: If using an IntPtr to get an output value, you will need to use out result or ref result.
You'll need to pass a pointer by reference. Assuming that sResult is passed from native to managed, i.e. that it has out semantics, here's the signature:
bool GetValueFromFile(string filePath, out IntPtr result);
Once you've called this you will need to convert it to a string:
IntPtr resultPtr;
if (GetValueFromFile(filePath, out resultPtr))
string result = Marshal.PtrToStringAnsi(resultPtr);
It's not clear who is responsible for freeing the memory that the native code allocates. Presumably that is documented somewhere and you already know how to handle that issue.
Assume this C function:
void do_something(const char* str)
It stores the string somewhere for later reference.
Furthermore, I have this signature in C# to call this function:
[DllImport("NativeLib")]
static extern void do_something(string str);
Now, what do I need to do when passing a string to this method:
Do I need to pin the string (with GCHandle.Alloc()) (or is the marshaller creating a copy)?
If I do need to pin it, do I pass the "original" string (i.e. the string I passed to GCHandle.Alloc())? Or do I need to pass the return value of GCHandle.AddrOfPinnedObject()?
Is string the correct data type (for do_something) in this case? Or should I use IntPtr instead?
Storing pointers through an interop boundary is a Really Bad Idea, do everything you can to modify the C code to make a copy of the string instead.
Pinning the string as suggested in the upvoted answer isn't going to work, the pinvoke marshaller must convert the string to char* and releases that converted string after making the native call, invalidating the pointer. You must marshal the string yourself. Declare the argument as IntPtr and use Marshal.StringToHGlobalAnsi() to create the pointer value.
Or use Marshal.StringToCoTaskMemAnsi(), it allocates from the COM heap. Which is your real problem, there is no great scenario to release the string. The C code cannot release the string because it doesn't know how it was allocated. Your C# code cannot release the string because it doesn't know when the C code is done with the pointer. This is why copying the string is so important. You can live with the memory leak if you make this call only a few times.
Given that the unmanaged code will be storing a pointer rather than copying the string, I recommend that you use GCHandle.Alloc() to manually create a copy of the string and pass it as an IntPtr.
You will then be able to manage the lifetime of the passed block of memory, only releasing it (via the GCHandle) when you are done with it.
I am new to the world of interacting with unmanaged libraries. I have an unmanaged C function that modifies a string by reference within the function. I'm having trouble passing a string from C# and getting it modified by the C function.
Here's the C function:
__declspec(dllexport) void __stdcall Test(char* name)
{
*name = "Bar";
}
This is the C# DLL import code:
[DllImport(#"C:/blah/mylibrary.dll")]
public extern static string Test(string name);
This is the code I'm using to call the function:
string s = "foo";
Test(s);
//I want s to be "Bar" after the above line
I have tried using "ref" and "out" on the string parameter, and tried Marshalling as an LPStr. Depending on what I try, I either get an error like
"The pointer passed in as a String must not be in the bottom 64K of the process's address space."
or
"Attempted to read or write protected memory. This is often an indication that other memory is corrupt."
I'm sure I'm just doing something stupid with my pointers. Can someone help me determine the appropriate C# code to get "s" to equal "bar"?
Thank you
Your C Test function doesn't do anything like you said it does. All it does it takes a local variable (name) and assigns it to a fixed string. To do what you said it does it would had to do a copy operation into the address pointed to by name:
__declspec(dllexport) void __stdcall Test(char* name)
{
strcpy(name, "Bar");
}
Of course, such an operation is a disaster in waiting since you have incorrect function signature (buffer lengths are not specified).
Considering that the C function is as above, then you should follow the rules specified at Default Marshaling for Strings:
In some circumstances, a fixed-length
character buffer must be passed into
unmanaged code to be manipulated.
Simply passing a string does not work
in this case because the callee cannot
modify the contents of the passed
buffer. Even if the string is passed
by reference, there is no way to
initialize the buffer to a given size.
The solution is to pass a
StringBuilder buffer as the argument
instead of a string. A StringBuilder
can be dereferenced and modified by
the callee, provided it does not
exceed the capacity of the
StringBuilder. It can also be
initialized to a fixed length. For
example, if you initialize a
StringBuilder buffer to a capacity of
N, the marshaler provides a buffer of
size (N+1) characters. The +1 accounts
for the fact that the unmanaged string
has a null terminator while
StringBuilder does not.
So your DLL should be like this:
[DllImport(#"C:/blah/mylibrary.dll")]
public extern static string Test(StringBuilder name);
and call it by passing a properly sized StringBuilder:
StringBuilder foo = new StringBuilder(256);
Test(foo);
Some sanity would be added to the C interface if you add a length parameter.
I have tried to do this in many ways, but none is working. Does anyone have a correct example for this? I just want to move the wchar_t* value from a function to the C# level.
This isn't as difficult as you think it is... What is wchar_t*? What value does that type typically represent? A string. It's the equivalent to the LPWSTR type defined in windows.h.
So, you marshal it as a string type. However, since it's an out parameter (or a return value), you'll need to use the StringBuilder class on the C# end, rather than the string type.
The P/Invoke syntax would look something like this:
[DllImport("MyLib.dll")]
public static extern void MyFunction(StringBuilder str);
And to use it, you first declare an instance of the StringBuiler class with the appropriate capacity, and then call the function:
StringBuilder myString = new StringBuilder(255);
MyFunction(myString);
Remember that the unmanaged C++ code must free the string in order to prevent a memory leak. It's the only one with access to the unmanaged memory area where the string was allocated.