func.cpp

int func() {
  return 100;
}

main.cpp

#include <stdio.h>

extern int func();
int main() {
  int a = func();
  printf("a - %d\n", a);
}

Makefile

all: main.o libfunc.so main

main:
	gcc -o main main.o -Wl,-rpath=. -L. -lfunc

main.o: main.cpp
	gcc -c main.cpp
	nm main.o

libfunc.so: func.cpp
	gcc -shared -o libfunc.so func.cpp
	nm libfunc.so

clean:
	-rm main main.o libfunc.so