CODEWITHSUNDEEP
c#c++memory-management
user2428614
user2428614

Reputation: 1

Receiving char* from C++ method, converting to managed string, then deallocating memory

I have a third party unmanaged C++ dll that I need to call from C#. The C++ function returns a char*. I've figured out how to convert that to a managed string in C#. But I don't know if I need to deallocate the memory. The following code works except that Marshal.FreeHGlobal(p) throws "The handle is invalid.". So do I need to deallocate the memory, and if so, how?

[DllImport("abc.dll", EntryPoint = "?GetVersion@ABC@@QEAAPEADXZ", ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
private static extern unsafe char* GetVersionRaw();


public static unsafe string GetVersion()
{
    char* x = Abc.GetVersionRaw(); // call the function in the unmanaged DLL
    IntPtr p = new IntPtr(x);
    string s = Marshal.PtrToStringAnsi(p);
    Marshal.FreeHGlobal(p);
    return s;
}

Upvotes: 0

Views: 249

Answers (1)

nobody
nobody

Reputation: 20174

It is impossible to say in general how to free an arbitrary pointer returned from a C++ function. There are many ways the pointer could have been produced (new, malloc, VirtualAlloc when you're on Windows, etc). There is no way to determine by inspection which one the pointer came from, and using the wrong deallocation function is an error (which may crash your process). Also, even if you know which function to call, your code may be linked against a different version of the language runtime library - so you might be correctly calling free or delete, but an incompatible version.

Normal procedure is for libraries that perform allocation to provide a corresponding deallocation function that frees the allocated memory appropriately. If the library you're using doesn't have this, you'll just have to try what they've told you and follow up if it doesn't work.

Upvotes: 2

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