From: Claudio Fiorini <claudio@...>
Date: 2009-01-23T05:10:40+09:00
Subject: [ruby-core:21526] Data_Wrap_Struct question about mark and free function

Hi all,
first of all sorry if my question doesn't deserve an answer,
i'm writing an extension to use odbtp, http://odbtp.sourceforge.net , 
and before to start i was reading the postgresql extension and i got in
Data_Set_Struct

#define Data_Set_Struct(obj, ptr) do { \
	Check_Type(obj, T_DATA); \
	DATA_PTR(obj) = ptr; \
} while(0);

so in my source i tried to use but i got this error:
error: expected expression before 'do'
why happen?

after this i switched to Data_Wrap_Struct using NULL without doing the
mark and free function. Now my question is what the big difference
with the use of the mark and free function?

Thanks 

claudio