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I'm trying to use flex and bison to create a simple scripting language. Right now, I'm just trying to get a calculator working.

I can't get it to compile, though. When I run this makefile:

OBJECTS = hug.tab.o hug.yy.o
PROGRAM = hug.exe
CPP = g++
LEX = flex
YACC = bison
.PHONY: all clean
all: $(OBJECTS)
$(CPP) $^ -o $(PROGRAM)
clean:
$(RM) *.o *.output *.tab.* *.yy.* $(PROGRAM)
%.tab.o: %.tab.cpp
$(CPP) -c -o $@ $<
%.tab.cpp: %.ypp
$(YACC) -vd $<
%.yy.o: %.yy.c
$(CPP) -c -o $@ $<
%.yy.c: %.l
$(LEX) -o $@ $<
%.o: %.cpp
$(CPP) -c -o $@ $<
on my .l and .ypp files, I get this error:
undefined reference to `yylex()'
And if I make the command for all like this:
$(CPP) $^ -o $(PROGRAM) -lfl
it says it couldn't find -lfl. And if I make it like this:
$(CPP) $^ -o -lfl $(PROGRAM)
it goes back to the undefined reference error.
Sorry I'm kind of clueless about this.
EDIT: I have flex installed. I tried changing it from -lfl to C:/GnuWin32/lib/libfl.a (I'm trying to use Windows because Linux has odd problems on my computers and I don't have a Mac yet), but it still has the same error.
                You need to read the documentation for the C/C++ compiler that you're using. "-o" is an option that produces an output file; the very next argument must specify the name of that file. "-l" specifies a library; the documentation will tell you the naming convention and location where the compiler will look for this library. As a guess, you haven't installed the flex library.
– kdgregory
                Sep 26, 2009 at 15:46
The problem is that you're compiling hug.yy.c with g++ (treating it as C++) instead of gcc.  This is the file that defines yylex, so by compiling it as C++ you end up with a C++ yylex function while the other files are looking for a C yylex function.
Try sticking extern "C" int yylex(); into the first section of your hug.l file so that it will use C linkage for yylex instead of C++
Basically, do either of the following:
Adding %option noyywrap in the flex file and removing the -lfl part in the command would be OK.
#define yylex as extern "C" works too. Since, it prevents C++ mixing up with the name.
Install flex-old rather than flex. flex-old bydefault solves the problem like solution 2 so u dont have to bother about it.
                Can you summarize the solution in your answer? It would be helpful in case the link is no longer valid.
– Lauren Rutledge
                Jul 16, 2019 at 16:35
                I have it installed. I tried changing it from -lfl to C:/GnuWin32/lib/libfl.a (I'm trying to use Windows because Linux has odd problems on my computers and I don't have a Mac yet), but it still has the same error.
– Micah
                Sep 30, 2009 at 0:19
        
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