How the attributes of "userdata" are altered with every handler function call?

Hello,

I have built a C Struc to keep track of the elements and attributes that the handler functions read. I make the explicit cast from "void* data* to my C Struc. In the first pass, 
the attributes are stored correctly in the C Struct. However, every time the handlers are called, the C Struct attributes (that were initially stored correctly) change. 

In other words, every time the handlers are called they override part of the information that has been stored (but not through the program logic). Is like the "void* user data" argument is corrupted with each handler function call.

I have been trapped with this problem for a week. Does anybody have an idea of why this is happening?

Thank you in advance for your help!
-Rodrigo