Reentrant Bison and Flex-2.5.4

Hi,
I'm trying to make a reentrant decoder for SIP messages 
using flex and bison on Linux/Windows. I went through the 
manuals for both bison and flex. Bison manual discusses the 
pure-parser, that is, a reentrant parser and suggests ways 
of doing it. I'm able to obtain a reentrant parser in which 
the global communication variables (yylval and yylloc) are 
local to yyparse.
But when I'm trying to compile the generated C code from 
flex with the generated reentrant C code of bison, I'm 
getting compilation errors saying 
"yylval undeclared in lex.yy.c" and "yylloc undeclared in 
lex.yy.c".

I'm using Flex-2.5.4 which doesn't generate a reentrant 
scanner. I know that Flex-2.5.31 supports reentrant scanners 
but how do I compile with the bison?
Also, is there any way of compiling Flex-2.5.4 with the 
existing reentrant bison?

Looking forward to your suggestions.

Thanks,
Avishek.
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