undefined reference static library makefile - makefile

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