y.tab.c: undefined reference to yylex

Question

I am trying to run an example I found online of a calculator. But I have this error showing every time I run my gcc command. Here are the commands that I run:

flex -l calc3.l
yacc -vd calc3.y
gcc y.tab.c -lm -ll

-> at this point I got this error message:

/tmp/ccPOq58f.o : In function 'yyparse':
y.tab.c: undefined reference to 'yylex' 
collect2: error: ld returned 1 exit status

Here is my code:

calc3.l

%{
#include <stdlib.h>
#include "calc3.h"
#include "y.tab.h"
void yyerror(char *);
%}

%%

[a-z]       { 
                yylval.sIndex = *yytext - 'a';
                return VARIABLE;
            }

0           {
                yylval.iValue = atoi(yytext);
                return INTEGER;
            }

[1-9][0-9]* {
                yylval.iValue = atoi(yytext);
                return INTEGER;
            }

[-()<>=+*/;{}.] {
                return *yytext;
             }

">="            return GE;
"<="            return LE;
"=="            return EQ;
"!="            return NE;
"while"         return WHILE;
"if"            return IF;
"else"          return ELSE;
"print"         return PRINT;

[ \t\n]+        ;       /* ignore whitespace */

.               yyerror("Unknown character");
%%
int yywrap(void) {
    return 1;
}

here is calc3.h

typedef enum { typeCon, typeId, typeOpr } nodeEnum;

/* constants */
typedef struct {
    int value;                  /* value of constant */
} conNodeType;

/* identifiers */
typedef struct {
    int i;                      /* subscript to sym array */
} idNodeType;

/* operators */
typedef struct {
    int oper;                   /* operator */
    int nops;                   /* number of operands */
    struct nodeTypeTag **op;    /* operands */
} oprNodeType;

typedef struct nodeTypeTag {
    nodeEnum type;              /* type of node */

    union {
        conNodeType con;        /* constants */
        idNodeType id;          /* identifiers */
        oprNodeType opr;        /* operators */
    };
} nodeType;

extern int sym[26];

and here is calc3.y

%{
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include "calc3.h"

/* prototypes */
nodeType *opr(int oper, int nops, ...);
nodeType *id(int i);
nodeType *con(int value);
void freeNode(nodeType *p);
int ex(nodeType *p);
int yylex(void);

void yyerror(char *s);
int sym[26];                    /* symbol table */
%}

%union {
    int iValue;                 /* integer value */
    char sIndex;                /* symbol table index */
    nodeType *nPtr;             /* node pointer */
};

%token <iValue> INTEGER
%token <sIndex> VARIABLE
%token WHILE IF PRINT
%nonassoc IFX
%nonassoc ELSE

%left GE LE EQ NE '>' '<'
%left '+' '-'
%left '*' '/'
%nonassoc UMINUS

%type <nPtr> stmt expr stmt_list

%%

program:
        function                { exit(0); }
        ;

function:
          function stmt         { ex($2); freeNode($2); }
        | /* NULL */
        ;

stmt:
          ';'                            { $$ = opr(';', 2, NULL, NULL); }
        | expr ';'                       { $$ = $1; }
        | PRINT expr ';'                 { $$ = opr(PRINT, 1, $2); }
        | VARIABLE '=' expr ';'          { $$ = opr('=', 2, id($1), $3); }
        | WHILE '(' expr ')' stmt        { $$ = opr(WHILE, 2, $3, $5); }
        | IF '(' expr ')' stmt %prec IFX { $$ = opr(IF, 2, $3, $5); }
        | IF '(' expr ')' stmt ELSE stmt { $$ = opr(IF, 3, $3, $5, $7); }
        | '{' stmt_list '}'              { $$ = $2; }
        ;

stmt_list:
          stmt                  { $$ = $1; }
        | stmt_list stmt        { $$ = opr(';', 2, $1, $2); }
        ;

expr:
          INTEGER               { $$ = con($1); }
        | VARIABLE              { $$ = id($1); }
        | '-' expr %prec UMINUS { $$ = opr(UMINUS, 1, $2); }
        | expr '+' expr         { $$ = opr('+', 2, $1, $3); }
        | expr '-' expr         { $$ = opr('-', 2, $1, $3); }
        | expr '*' expr         { $$ = opr('*', 2, $1, $3); }
        | expr '/' expr         { $$ = opr('/', 2, $1, $3); }
        | expr '<' expr         { $$ = opr('<', 2, $1, $3); }
        | expr '>' expr         { $$ = opr('>', 2, $1, $3); }
        | expr GE expr          { $$ = opr(GE, 2, $1, $3); }
        | expr LE expr          { $$ = opr(LE, 2, $1, $3); }
        | expr NE expr          { $$ = opr(NE, 2, $1, $3); }
        | expr EQ expr          { $$ = opr(EQ, 2, $1, $3); }
        | '(' expr ')'          { $$ = $2; }
        ;

%%

nodeType *con(int value) {
    nodeType *p;

    /* allocate node */
    if ((p = malloc(sizeof(nodeType))) == NULL)
        yyerror("out of memory");

    /* copy information */
    p->type = typeCon;
    p->con.value = value;

    return p;
}

nodeType *id(int i) {
    nodeType *p;

    /* allocate node */
    if ((p = malloc(sizeof(nodeType))) == NULL)
        yyerror("out of memory");

    /* copy information */
    p->type = typeId;
    p->id.i = i;

    return p;
}

nodeType *opr(int oper, int nops, ...) {
    va_list ap;
    nodeType *p;
    int i;

    /* allocate node */
    if ((p = malloc(sizeof(nodeType))) == NULL)
        yyerror("out of memory");
    if ((p->opr.op = malloc(nops * sizeof(nodeType *))) == NULL)
        yyerror("out of memory");

    /* copy information */
    p->type = typeOpr;
    p->opr.oper = oper;
    p->opr.nops = nops;
    va_start(ap, nops);
    for (i = 0; i < nops; i++)
        p->opr.op[i] = va_arg(ap, nodeType*);
    va_end(ap);
    return p;
}

void freeNode(nodeType *p) {
    int i;

    if (!p) return;
    if (p->type == typeOpr) {
        for (i = 0; i < p->opr.nops; i++)
            freeNode(p->opr.op[i]);
        free (p->opr.op);
    }
    free (p);
}

void yyerror(char *s) {
    fprintf(stdout, "%s\n", s);
}

int main(void) {
    yyparse();
    return 0;
}

Answer 1

If you just use

flex -l calc3.l

then flex produces a scanner called lex.yy.c. Similarly, if you just use

yacc -vd calc3.y

the bison will produce files called y.tab.c and y.tab.h. And

gcc y.tab.c -lm -ll

will produce a file called a.out.

None of that is a good idea. It's far better to give the files meaningful names, based on the input filenames. All three of these tools understand a -o command-line flag which specifies the output name file.

So you could do this:

flex -l calc3.l
yacc -vd calc3.y
gcc lex.yy.c y.tab.c -lm -ll

But I'd recommend something like this:

flex -o calc3.lex.c -l calc3.l
bison -o calc3.tab.c -vd calc3.y
gcc -o calc3 calc3.lex.c calc3.tab.c -lm -ll

(When you do this, you'll need to change the #include "y.tab.h" to #include "calc3.tab.h".)

Even better if you put it in a Makefile, or at least a script file.