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Practical Parsing for ANSI C

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A Lazy Trick

Tags are optional names you can associate to a struct or union definition in order to recall that definition later. In this example, tag_employee is a tag:

struct tag_employee 
{ 
   char     * name;
   unsigned   age;
};
/* ... */
struct tag_employee my_employee;

The peculiar thing with tags is that they live in a separate namespace from declared variables and types. Consequently, it is perfectly legal (although it may be very stupid) to use the same name both as a struct tag and as a user type name:


 1:  typedef unsigned long int ulint_t;
 2:  struct ulint_t {
 3:     char a;
 4:     float b;
 5:  } a;
 6:  void function()
 7:  {
 8:     ulint_t         my_var;
 9:     struct ulint_t  my_struct;
10:  }

Unfortunately, our current front-end fails to parse this code. In fact, it tokenizes ulint_t on line 8 as TYPE_NAME, when an IDENTIFIER is expected after the struct or union keyword, as the syntax rules from the C grammar show:


struct_or_union_specifier	
	: struct_or_union IDENTIFIER
	| struct_or_union IDENTIFIER '{' struct_declaration_list '}'
	| struct_or_union            '{' struct_declaration_list '}'
	;

To solve this, I use a simple (though not elegant) workaround. I do not make the lexer capable of distinguishing tags from identifiers and type names. Instead, I modify the grammar to make it irrelevant whether a tag is tokenized as IDENTIFIER or TYPE_NAME. To do so, in the previous rules, I add for each rule involving IDENTIFIER a copy that involves TYPE_NAME. The result is:


struct_or_union_specifier	
  : struct_or_union IDENTIFIER
  | struct_or_union IDENTIFIER      '{' struct_declaration_list '}'
  | struct_or_union TYPE_NAME
  | struct_or_union TYPE_NAME       '{' struct_declaration_list '}'
  | struct_or_union      '{' struct_declaration_list '}'   
   ;


The solution is simple and effective, it does not cause any ambiguity in the grammar, and it addresses all the cases where a tag can be used, including cast expressions such as pq = (struct tag_q *) ps;.


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