Question

In a C dll, I have a function like this:

char* GetSomeText(char* szInputText)
{
      char* ptrReturnValue = (char*) malloc(strlen(szInputText) * 1000); // Actually done after parsemarkup with the proper length
      init_parser(); // Allocates an internal processing buffer for ParseMarkup result, which I need to copy
      sprintf(ptrReturnValue, "%s", ParseMarkup(szInputText) );
      terminate_parser(); // Frees the internal processing buffer
      return ptrReturnValue;
}

I would like to call it from C# using P/invoke.

[DllImport("MyDll.dll")]
private static extern string GetSomeText(string strInput);

How do I properly release the allocated memory?

I am writing cross-platform code targeting both Windows and Linux.

Edit:
Like this

[DllImport("MyDll.dll")]
private static extern System.IntPtr GetSomeText(string strInput);

[DllImport("MyDll.dll")]
private static extern void FreePointer(System.IntPtr ptrInput);

IntPtr ptr = GetSomeText("SomeText");
string result = Marshal.PtrToStringAuto(ptr);
FreePointer(ptr);

Answer 1

You should marshal returned strings as IntPtr otherwise the CLR may free the memory using the wrong allocator, potentially causing heap corruption and all sorts of problems.

See this almost (but not quite) duplicate question PInvoke for C function that returns char *.

Ideally your C dll should also expose a FreeText function for you to use when you wish to free the string. This ensures that the string is deallocated in the correct way (even if the C dll changes).