undefined reference to `yylex' && undefined reference to `yyin'

Question

I'm trying to use flex and bison to create a compiler. But i receive errors when i try to compile.
Here is the errors:

flex --header-file=lex.h --outfile=lex.cc lex.ll
g++ -w -Wall -Wextra -ansi -g -c -o lex.o lex.cc -lm -ll
bison -v --output-file=parse.cc --defines=parse.hh --warnings=all --feature=all parse.yy
g++ -w -Wall -Wextra -ansi -g   -c -o parse.o parse.cc
g++ -w -Wall -Wextra -ansi -g   -c -o main.o main.cc
g++ -w -Wall -Wextra -ansi -g -fkeep-inline-functions -o LFIO lex.o parse.o function.o main.o  -lfl -lm -ll
parse.o: In function `yyparse()':
/root/LFIO-combine/parse.cc:1199: undefined reference to `yylex'
main.o: In function `main':
/root/LFIO-combine/main.cc:21: undefined reference to `yyin'
/root/LFIO-combine/main.cc:21: undefined reference to `yyin'
collect2: error: ld returned 1 exit status
make: *** [LFIO] Error 1

Here is my makefile:

CXXFLAGS = -w -Wall -Wextra -ansi -g
OBJECTS = lex.o parse.o function.o main.o


LFIO : $(OBJECTS)
        g++ $(CXXFLAGS) -fkeep-inline-functions -o LFIO $(OBJECTS) -lfl -lm -ll

clean :
        rm -f *.o

lex.cc : lex.ll
        flex --header-file=lex.h --outfile=lex.cc lex.ll

parse.hh : parse.cc

parse.cc : parse.yy function.h algorithm.h
        bison -v --output-file=parse.cc --defines=parse.hh --warnings=all --feature=all parse.yy

%.o: %.cc %.h
        g++ $(CXXFLAGS) -c -o $@ $< -lm -ll

main.o : main.cc lex.h parse.hh function.h algorithm.h

function.o : function.h function.cc

parse.o : parse.cc parse.hh function.h algorithm.h

lex.o :lex.h algorithm.h function.h

Here is my lex.ll

%option c++
%option ecs
%option nodefault
%option stack
%option warn
%option yylineno

%{
#include "function.h"
#include "parse.hh"
#include <limits.h>
#include <iostream>
#include <sstream>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
int yyFlexLexer::yywrap() { return 1; }
%}

ATOM    [A-Z][a-zA-Z0-9_']*
VARIABLE        [a-z][a-zA-Z0-9_']*

%%
[ \t\n]         ;
{VARIABLE} {  return (TOK_VARIABLE);}
{ATOM} {  return (TOK_ATOM);}
":-" {}
","  {}
"."  {}
<<EOF>>  {yyterminate();}
%%

parse.yy is like this:

%defines
%define api.token.prefix {TOK_}

%{
#include "function.h"
#include "lex.h"
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

#ifdef MS_WINDOWS
#include <malloc.h>
#define alloca_alloca
#endif
void yyerror(const char *s)
{
   fprintf (stderr, "%s\n", s);
}
extern "C" int yylex(void);

%}

%union{
......
}

%type ......


%%
......

And then main.cc is like this:

#include "function.h"
#include "algorithm.h"
#include "parse.hh"
#include "lex.h"
extern "C" int yylex(void);
extern FILE *yyin;
extern FILE *yyout;
extern int fopen();
extern int yyparse(void *);

int main(int argc, char **argv){

if(argc > 1){
if(!(yyin = fopen(argv[1], "r"))) {
perror(argv[1]);
return (1);
}
}
//yylex();
yyparse();
......
return 0;
}

Does anybody know what cause these errors and how to solve them? I would be thankful if someone could make time to check these errors.

Please show the full output of make, so we can see if it tries to build and link with lex.o. — Some programmer dude
Just a hunch... You're compiling the lexer as C++, but still have extern "C" int yylex(void);. Try to remove the extern "C" part. — Some programmer dude
I'm less sure about the problem with yyin though... It was to long since I use Flex last. but it could be some similar issue. Try looking at the generated code, both the source and the header file to see what it says about yyin. — Some programmer dude

Some programmer dude Some programmer dude · Accepted Answer · 2016-08-20T13:25:51

_{Collecting my comments into an answer.}

There are two problems:

The first is that the code is compiled as C++, which means the definition of yylex will also be compiled as C++. That means it is not extern "C". To fix this problem remove the extern "C" from all declarations.

The second problem is two-fold: The first is that the symbol yyin doesn't seem to be defined anywhere, so you need to do it yourself. The second thing is that it should be a pointer to an std::istream object, and not a FILE *.

To solve the second problem, with yyin, I recommend you add the definition to your lexer declaration block:

%{
// Your includes...
int yyFlexLexer::yywrap() { return 1; }

// Add definition of yyin
std::istream* yyin;
%}

Then update the extern declaration in your main.cc file to match the definition.

undefined reference to `yylex' && undefined reference to `yyin'

1 Answers