Handling #define macros in yacc/bison lex

Question

How would I implement #define macros with yacc/bison?

I was thinking all define characters must match a regular variable. Variables are defined as [a-zA-Z_][a-zA-Z0-9_]* so I figure I can put a check there to see if the variable is a define'd worked or not. Then replace the text with what it should be.

How can i do that? Right now I want to completely ignore the word BAD as if I defined it as #define BAD in C. Below is the code for that lex rule but I am doing it wrong. Also lex complains about "BA" being in the stream. I know the below is completely wrong and illogical so how do I ignore BAD and then how do I replace it with something like float

    if(strcmp(yytext, "BAD")==0) {
        int i, l = strlen(yytext);
        for(i=0; i<l; i++) { REJECT }
        return;
    }
    return VAR; }

I know the major steps are 1) define the define, 2) detect it in source 3) make lex forget the macro characters 4) insert the new correct characters.

Answer 1

Put a rule in lex to find the define. Then use unput to insert the replacement text. Note the text is to be inserted backwards

[a-zA-Z0-9_]* {
        if(strcmp(yytext, "HARDCODED_DEFINE")==0) {
            const char s[]="int replacement_text";
            int z;
            for(z=strlen(s)-1; z>=0; z--)
                unput(s[z]);
        }
        else
            return VAR_TOK; 
        }