Question

I have been given an already working Makefile which is actually working fine.

Makefile Contents can be found here in this post …

Questions about Makefile – what is "$+" & where are .c files/dependencies called here ?

I am asking this question separately from my previous post mentioned above as it involves a different issue and adding it to that question would unnecessarily increase its length.

Now I have added one more functionality which is being used quite frequently at many places so I thought creating a separate file would be a nice idea so I created linklayer.c and added linklayer.o to $LIBOBJS.

I added this …

 LIBOBJS= linklayer.o csum.o compact.o protoname.o headers.o 
 parseargs.o cryptomod.o crc32.o

and this

 linklayer.o:    linklayer.c
    $(CC) -o $@ -c -I. $(CFLAGS) $+

I have declared function in sendip_module.h which is already declared and accessed in each of the module present in the project.

But now this multiple definition error is coming … Have I done something wrong or misunderstood something?

Note: “ipv6_opts” is defined in ipv6.h

$ make all
for subdir in mec ; do \
    cd $subdir ;\
    make  ;\
    cd ..  ;\
    done
make[1]: Entering directory `/home/udit/Desktop/sendip-2.5-mec-2/mec'
make[1]: Nothing to be done for `all'.
make[1]: Leaving directory `/home/udit/Desktop/sendip-2.5-mec-2/mec'
  gcc-4.4 -o ipv6.so -fPIC -fsigned-char -pipe -Wall -Wpointer-arith 
 -Wwrite-strings -Wstrict-prototypes -Wnested-externs -Winline -Werror
 -g -Wcast-align -DSENDIP_LIBS=\"/usr/local/lib/sendip\" -shared ipv6.c
  libsendipaux.a libsendipaux.a

 libsendipaux.a(linklayer.o):(.data.rel.local+0x0)
                              : multiple definition of `ipv6_opts'
 /tmp/ccxa4tMX.o:(.data.rel.local+0x0): first defined here
 collect2: ld returned 1 exit status
 make: *** [ipv6.so] Error 1

and why this libsendipaux.a libsendipaux.a two times ? Is there something wrong with the Makefile itself.

Do I first need to manually compile it and then add it to libsendipaux.a ?

I am new to this Makefile stuff, so please help me understand how this is all working out here ?

Thanks.

Edit :

Remake debugging output –

 remake -x

Reading makefiles...
Updating goal targets....
/home/udit/Desktop/sendip-2.5-mec-2/Makefile:33 File `all' does not exist.
/home/udit/Desktop/sendip-2.5-mec-2/Makefile:48 File `subdirs' does not exist.
/home/udit/Desktop/sendip-2.5-mec-2/Makefile:48 Must remake target `subdirs'.
for subdir in mec ; do \
    cd $subdir ;\
    make  ;\
    cd ..  ;\
    done
make[1]: Entering directory `/home/udit/Desktop/sendip-2.5-mec-2/mec'
make[1]: Nothing to be done for `all'.
make[1]: Leaving directory `/home/udit/Desktop/sendip-2.5-mec-2/mec'
/home/udit/Desktop/sendip-2.5-mec-2/Makefile:48 Successfully remade target file    
`subdirs'.
File `ipv6.so' does not exist.
Must remake target `ipv6.so'.
gcc-4.4 -o ipv6.so -fPIC -fsigned-char -pipe -Wall -Wpointer-arith 
-Wwrite-strings -Wstrict-prototypes -Wnested-externs -Winline -Werror 
-g -Wcast-align -DSENDIP_LIBS=\"/usr/local/lib/sendip\" -shared ipv6.c 
 libsendipaux.a libsendipaux.a
 libsendipaux.a(linklayer.o):(.data.rel.local+0x0)
                                  : multiple definition of `ipv6_opts'
 /tmp/ccb0oaXR.o:(.data.rel.local+0x0): first defined here
 collect2: ld returned 1 exit status
 remake: *** [ipv6.so] Error 1

 #0  ipv6.so at ??
 #1  all at /home/udit/Desktop/sendip-2.5-mec-2/Makefile:33

33rd line -> all: $(LIBS) subdirs sendip $(PROTOS) sendip.1 sendip.spec

I guess it could not help me out …. actual problem is with my understanding of scenario itself. Please help me bring out of the mess.

Answer 1

the problem you are facing, is that you are linking together several objects where at least two of them define de function ipv6_opts.
since there are two implementations of the function, your linker cannot decide which one to use and throws an error.

the problem most likely comes from the fact that you are linking libsendipaux.a twice into your final binary.

the reason why this happens is here:

 %.so: %.c $(LIBS)
        $(CC) -o $@ $(CFLAGS) $(LIBCFLAGS) $+ $(LIBS)

in this target, $+ will expand to all the dependencies of your target (that is: %.c $(LIBS), which will in turn be resolved to ipv4.c libsendipaux.a

the actual call to the compiler can then be read as $(CC) -o $@ $(CFLAGS) $(LIBCFLAGS) ipv4.c $(LIBS) $(LIBS), and $(LIBS) $(LIBS) will expand to libsendipaux.a libsendipaux.a, which will produce the erroneous double linkeage.

so the solution is to remove extraneous $(LIBS) from the .so target:

 %.so: %.c $(LIBS)
        $(CC) -o $@ $(CFLAGS) $(LIBCFLAGS) $+

btw, the errors you get in remake about non-existing files, are because all and subdirs are indeed no files, but phony targets (targets that do no generate files called after the target-name).

to prevent those warnings, add something like the following to your makefile:

.PHONY: all subdirs