[Swig-devel] Re: SWIG C# handling isn't zero-byte safe

[Olly B. <ol...@su...>]

On 2006-05-09, William S Fulton <ws...@fu...> wrote:
> Olly Betts wrote:
>> Both C# and C++ are happy to handle string objects containing zero
>> bytes, but SWIG generated C# bindings truncate strings at a zero byte.
>> Looking at the generated code it just uses c_str() of the C++ string
>> so this isn't altogether suprising.
>> 
>> Is it possible to fix this?  It looks like there's some implicit
>> pinvoke magic which handles mapping a const char * to/from a C# string
>> so I've nothing specific to google for (a few general searches haven't
>> turned up anything useful.)
>
> Maybe I've been missing something all my years doing C, but if there is 
> a '\0' in a char *, and it does not terminate the string, then it is not 
> a string, rather an array of bytes.

In C, yes, but I'm talking about C++ string objects (i.e the std::string
class) which can contain zero bytes.  To preserve them you need to use
the data() and size() (or length()) methods to get a pointer and length,
instead of c_str() which ensures a zero-terminated string.

So for example, PHP's typemaps have:

    %typemap(out) string {
	ZVAL_STRINGL($result,const_cast<char*>($1.data()),$1.length(),1);
    }

> strncpy might be able to copy these 'strings' with '\0', but any
> string handling functions such as printf will truncate the string.

It's true that you need to take care (or perhaps better avoid) passing
C++ strings to C functions if you want the zero bytes to be handled, but
you if you stick to C++ stuff it all works.  For example use:

  cout << str << endl;

instead of:

  printf("%s\n", str.c_str());

> As for marshalling C char* <-> C# string, the pinvoke marshaller expects 
> a null terminated string as that is the way strings work. You'd probably 
> have to marshall as a C# array if you want to keep the NULLs.

Can you then unmarshall that as a C# string?

I realise this is a bit of a corner case, but it's a bit sad that both
C++ and C# have fixed C's inability to handle strings with zero bytes
but you don't appear to be able to move such strings between the two!

Cheers,
    Olly