parser/lex yyunput() problem

[Bansidhar Arvind Deshpande - CTD, Chennai.]

Hi 

I want to know, how does the yyunput() function internally works (preferably
I want source code 
of yyunput(). ) 

Is there any way to make lexer to read the previous token one more time ? I
am using yyunput()
but it has some side effect with respect to my source code. 

Thanks

Bansidhar