Question

I am trying to link a static library while creating my program executable using the below makefile..

IDIR =../inc
CC=g++ -g
CFLAGS=-I$(IDIR)
WFLAGS=-Wall -W
OFLAGS=-O3
DLINUX=-D_LINUX

ODIR=obj
LDIR =../lib
LIBS=-lm

_OBJ = testclient.o 
OBJ = $(patsubst %,$(ODIR)/%,$(_OBJ))

$(ODIR)/testclient.o: testclient.c
    $(CC) -c $< $(CFLAGS) -o $@
$(ODIR)/file2.o: file2.c
    $(CC) -c $< $(CFLAGS) -o $@

testclient: $(OBJ)
    $(CC) -o $@ $^ $(LIBS) -lccn -pthread 

.PHONY: clean

clean:
    rm -f $(ODIR)/*.o *~ core $(INCDIR)/*~ 

I have tried everything available from changing the order of the ‘-lccn‘ parameter to checking whether the function exists in the library (nm libccn.a gives the required function ccn_create() in it). The error returned is :

obj/testclient.o: In function `main':
/root/testClient/src/testclient.c:91: undefined reference to `ccn_create()'

The library libccn.a is in /usr/local/lib. I have also tried changing the directory path and then using -L flag to look in that location. Doesn’t work either. 🙁 ..Any ideas as to how can i make it work ?

Answer 1

My guess is that libccn.a is a C library and the header that you use are not designed to be imported by a C++ compiler (there is no extern "C" { } block surrounding the function definition).

C++ supports function overloading by mangling name of function. When you put a function in a extern "C" { } block, C++ disable name mangling (and thus disable overloading). Here, in your error message, the function mentioned is ccn_create(). Notice the (), this means that the function type is known, and thus the named looked up was a mangled name.

When you do nm libccn.a you see the real name, and it is ccn_create. That is not a mangled name. So to fix this, you’ll need to surround the function definition in a export "C" { } block. The easiest way to do that is to surround the #include in such a block.

BTW, you can reproduce the error by doing this.

$ echo 'void ccn_create();' > ccn.h
$ echo '#include "ccn.h"
void ccn_create() { }' > ccn.c
$ echo '#include "ccn.h"
int main () {
  ccn_create();
  return 0;
}' > main.cc
$ gcc -o ccn.o -c ccn.c
$ g++ -o main main.cc ccn.o
Undefined symbols for architecture x86_64:
  "ccn_create()", referenced from:
      _main in cc8XnYRq.o
ld: symbol(s) not found for architecture x86_64
$ echo 'extern "C" {
#include "ccn.h"
}
int main () {
  ccn_create();
  return 0;
}' > main.cc
$ g++ -o main main.cc ccn.o