Tracing YACC Parsers

In order to trace the steps of a Yacc-generated parses, you should:

1. Declare an external variable yydebug and assign a non-0 value to it (most likely, in your main function:
   
   extern int yydebug;
   ...
   yydebug = 1;
   

   As an example, consider this main.c file with debugging-triggering code included. Note that the declaration and assignment of yydebug are included only if the YYDEBUG macro has been defined.
   

2. Compile your parser with the YYDEBUG macro turned on. For example:
   
   gcc -DYYDEBUG lex.yy.c y.tab.c main.c -ll
   
   
3. Run the parser on a test file and trace the output, comparing it with the content of the y.output file created for your Yacc specification file.
   An example of a trace output for the expressions grammar and the input 3+4*5= is shown here (using the corresponding y.output file).

For additional information, read the section on debugging parsers in the on-line Bison manual.