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Editorial Team
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Asked: 2026-05-23T08:37:16+00:00

So, copying the code from Introduction to Compiler Construction in Unix I wind up

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So, copying the code from Introduction to Compiler Construction in Unix I wind up with the snippet:

/* samplec.l snipped */
static struct rwtable {               /* reserved word table */
    char *rw_name;                    /* representation */
    int rw_yylex;                     /* yylex() value */
    } rwtable[] = {                   /* sorted */
        "break",                          token(BREAK),
        "continue",                       token(CONTINUE),
        "else",                           token(ELSE),
        "if",                             token(IF),
        "int",                            token(INT),
        "return",                         token(RETURN),
        "while",                          token(WHILE)
    };
/* samplec.l snipped */

When I lex the code everything seems fine

$ lex -v samplec.l 
scanner options: -XvI8 -Cem
94/2000 NFA states
38/1000 DFA states (125 words)
16 rules
Compressed tables always back-up
Beginning-of-line patterns used
1/40 start conditions
60 epsilon states, 27 double epsilon states
10/100 character classes needed 144/500 words of storage, 0 reused
105 state/nextstate pairs created
62/43 unique/duplicate transitions
40/1000 base-def entries created
69/2000 (peak 60) nxt-chk entries created
2/2500 (peak 30) template nxt-chk entries created
0 empty table entries
3 protos created
2 templates created, 3 uses
15/256 equivalence classes created
1/256 meta-equivalence classes created
2 (1 saved) hash collisions, 27 DFAs equal
0 sets of reallocations needed
489 total table entries needed

But, when I try to compile it, it fails:

$ gcc -c lex.yy.c -o lex.yy.o
samplec.l:75: error: initializer element is not constant
samplec.l:75: error: (near initialization for ‘rwtable[0].rw_yylex’)
# etc for each token(T)

What’s up? Is token() a macro that should be evaluated (but isn’t for some reason)? Or, should token() have been evaluated by lex?

Sorry, these sort of naive introductions are bad specifically in this situation – where bugs occur that you weren’t taught to understand 🙁 I guess there’s no skimping in learning tools.

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1 Answer

  1. Editorial Team

    token(x) was in fact a macro defined in the file I was working on viz. samplec.l

    #define token(x) x

    but of course I had a typo which omitted the x

    #define token(x)

    Furthermore, I was missing 3 functions (undefined symbols): s_lookup, yyerror and yymark. The following chapter defines a small stub for yyerror and so I simply defined s_lookup and yymark as empty functions.

    I don’t know if I should have to stub out s_lookup or yymark but the resulting parser worked.

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