Include struct in the %union def with Bison/Yacc

Question

I am trying to include a struct as part of the union with Bison, but I get an error on the \'struct node args\' in %union:

parser.y:17: error: field ‘args’         


        

Answer 1

Maybe simpler (I think) - used this myself:

%union {
  char   c; 
  struct {
     char name[30];
     int  type;
  } s;
}

Then, in flex you can use "yylval.s.name" , or "yylval.s.type", etc.. while in bison, instead of $$=0, $1=bla... you can now write $<s.type>$=0 , $<s.type>1=bla...

Answer 2

Even better, use the %code directive with the "requires" option, i.e.:

%code requires {
    struct node {
        char * val;
        struct node * next;
    };
}

%union {
    char * string;
    struct node args;
}

This will include the code in the "requires" block in the tab.h file as well as the parser source file.

From the documentation: http://www.gnu.org/software/bison/manual/html_node/Decl-Summary.html#Decl-Summary

• requires
  • Purpose: This is the best place to write dependency code required for YYSTYPE and YYLTYPE. In other words, it's the best place to define types referenced in %union directives, and it's the best place to override Bison's default YYSTYPE and YYLTYPE definitions.

Answer 3

It comes down to the lame y.tab.h output you get.

You need to fix this by ensuring that "struct node" is defined before you include y.tab.h anywhere.

To do this create a file node.h with the struct definition.

Then include node.h before y.tab.h in your parser.l file, parser.y file as well as any c files you have which include y.tab.h. This is a little annoying.

Alternatively you could change "struct node args" to "struct node* args" since you would not need to know the full type until you go to use it somewhere. Not sure if this would fit with your code.

Either one should work.