Makefile：“对cos的未定义引用”

Question

I have just started learning about makefile files. I created a program that consists of two functions and wanted to use a makefile to put it all together. This is my file:

#Makefile

all: main

main: main.o find_root.o
    clang -o main main.o find_root.o

main.o: main.c
    clang -c -Wall --pedantic -std=c11 main.c -lm

find_root.o: find_root.c
    clang -c -Wall --pedantic -std=c11 find_root.c -lm

clean: rm -f main *.o*

However, when I run this, I get an error - "undefined reference to cos". I am using the cosine functions in my program, but I have already linked the library to the compilation of those two files. I thought about adding "-lm" to the first clang option as well. This led to no errors, but it made a warning instead - saying that "-lm linker is unused". What should I change in my file?

Answer 1

The "-lm" is a linker option but you have only included it in your compilation rule ( main.o: main.c ). You need to include it in your linker rule ( main: main.o find_root.o ).

As it stand the -lm option is ignored during compilation and missing during linking.

Answer 2

The linker flags aren't used when compiling, but when linking, so the command for the main rule should have -lm , rather than the command for the *.o files.

Better would be just to set the appropriate variables, and let Make use its built-in rules:

#Makefile

LDLIBS += -lm
CFLAGS += -Wall --pedantic -std=c11
C = clang

all: main

main: main.o find_root.o
    $(LINK.c) $^ $(LDLIBS) -o $@

clean:
    $(RM) main *.o *~