[Mono-devel-list] Interop with unmanaged code without copying or memory allocation?

Karl Waclawek karl at waclawek.net
Sun Jan 11 20:53:07 EST 2004

I am fairly new to C#/.NET/Mono, so this may be a trivial problem
(however, I did read http://www.jprl.com/~jon/interop.html):

My task is to write a C# wrapper for the Expat XML parser,
which is available as a dynamic library (.dll, .so).
I want this wrapper to have as little overhead as possible,
as the main "raison d'etre" for Expat is its speed.

The Expat API is mostly character pointer based, two of the
most commonly used call-back functions have this signature
(i.e. call-back from Dll back to application):

/* atts is array of name/value pairs, terminated by 0;
   names and values are 0 terminated.
typedef void (XMLCALL *XML_StartElementHandler) (void *userData,
                                                 const XML_Char *name,
                                                 const XML_Char **atts);

/* s is not 0 terminated. */
typedef void (XMLCALL *XML_CharacterDataHandler) (void *userData,
                                                  const XML_Char *s, 
                                                  int len);

where XML_Char is a two byte entity (ushort or wchar_t).

What I would like to do is wrap these call-backs in a way
that the actual call-backs look like this in C#:

void StartElement(string Name, IAttributes atts);
void Characters(char[] ch, int start, int length);

(The wrapper routines themselves will have to be delegates, 
as far as I could tell).
Ignoring the "IAttributes atts" argument, I have two problems:

1) I would like to "intern" the first occurrence of a given name passed to
   XML_StartElementHandler() and let subsequent occurrences of the same name
   pass the same interned string object to the StartElement function in C#.
However, it seems I cannot look up an instance in the string pool
passing some array or pointer, I have to allocate a new string instance
based on the contents of name just to call String.Intern. This
defeats the purpose.

2) It would be nice if I could somehow cast the "const XML_Char *s"
   argument passed to XML_CharacterDataHandler() to an array.
It seems that the best I can do in C# is to re-use the same array
instance (for the ch argument) and copy the characters from s into it.

So, for case 1) I have the problem of new string allocation
for each call, and for case 2) I have the problem of copying.

I guess going from unmanaged to managed will always require
copying, so I won't be able to improve on 2), but case 1) really
bothers me as it should be possible to look up an interned string
using something else but another string.

Any advice?


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