ucpp is a C preprocessor and lexer. It has the following goals: 

   - Strict conformance to the C99 standard (with switches to
     disable some extensions and mimic C90 behaviour) 
   - Low memory consumption 
   - Possibility to use the code as a lexer (that outputs tokens
     directly) 

WWW: http://www.di.ens.fr/~pornin/ucpp/