Question

Here is my Makefile:

.PHONY: all homework1
CFLAGS= -g -O0 -Wall -Werror -Wno-unused-function
LDFLAGS= -lm

all : homework1

homework1 : program.tab.o program.lex.o
%.o : %.c
    gcc -o$@ -c $(CFLAGS) $<
%.lex.c : %.lex %.tab.h
    flex -o$@ $<
%.tab.c %.tab.h : %.y
    bison --verbose -o$@ -d $<

Whenever I try to compile, I get the warning make: Circular program.lex <- program.lex.o dependency dropped. I don’t see how program.lex is dependent on program.lex.o at all in the makefile. I see how the dependency tree is about 4 layers deep, but it doesn’t look circular.

How can I improve my makefile?

Answer 1

@aaa carp is correct that is an implicit rule, but it’s not one of the LEX rules, because they build N.c or N.r from N.l, and this is a rule that’s building N from N.o. That looks to me like the “Linking a single object file” rule, described in info (make) Catalogue of Rules. Renaming your .lex files to .l files will fix this problem.

Also: running flex, I don’t think you really need the .tab.h file to build the lexer source. Try this instead:

%.l.c: %.l
    flex -o$@ $<
program.l.o: program.tab.h

This will add the extra dependency to program.l.o.