C# Free IntPtr from C malloc

Question

I have an unmanaged DLL on C with func:

char* My_Func(char* data, int input_length, int output_length);

In this func i have

result = (char*)malloc(output_lenght);
strcpy(result,test_char);
return(result);

In C# i import it

[DllImport(@"libsmev.DLL", CharSet = CharSet.Ansi)]
public static extern IntPtr My_Func([MarshalAs(UnmanagedType.LPStr)]string data, int input_length, out int output_length);

And call it

IntPtr result = My_Func (n1, n1.Lenght,n2);

How to free char* or IntPtr?

Marshal.FreeHGlobal(IntPtr) and Marshal.FreeCoTaskMem(IntPtr) doesn`t work.

Answer 1

In C, something like this:

void Free_Obj(char* obj) {
    free(obj);
}

In C#, something like this:

[DllImport("libsmev.DLL")]
public static extern void Free_Obj(IntPtr obj);

And call it:

IntPtr result = My_Func (n1, n1.Lenght,n2);
Free_Obj(result);

Answer 2

You need to export a function that can free the pointer using free. As you have noticed Marshal.FreeHGlobal and Marshal.FreeCoTaskMem doesn't work because these functions expect pointer that have allocated using a different allocation function.

If you create an API that allocates memory and returns a pointer to this memory you have to also add a function in the API to free the memory. Otherwise your API will not be available outside the library that was used to allocate the memory (in this case the C standard library).

Answer 3

Instread of creating a custom free function in your DLL, you could also use CoTaskMemAlloc instead of malloc to allocate the memory.

You can then use Marshal.FreeCoTaskMem to free it.