How to link .so library properly in C++? - linux

Question

I have project structure like this,
a.pb.h --- includes --> protobuf.h
b.grpc.pb.h --- includes --> a.pb.h & grpcpp.h
Also there are a.pb.cc and b.grpc.cc files.
A C++ wrapper with extern C which is wrapper.cc and wrapper.h which includes b.grpc.pb.h and grpcpp.h.
The function inside extern C is char* helloWorld(const char*, const char*, const char*);
Creating .o of a.pb.h and b.grpc.pb.h:
g++ -fpic -std=c++11 `pkg-config --cflags protobuf grpc` -c -o a.pb.o a.pb.cc
g++ -fpic -std=c++11 `pkg-config --cflags protobuf grpc` -c -o b.grpc.pb.o b.grpc.pb.cc
Steps to create libcombined.so:
The grpc and protobuf so are already provided under /usr/local/lib.
First created .so of a.pb.o and b.grpc.pb.o to compile wrapper file as:
g++ -shared -o libcombined.so *.o
Compiled wrapper as:
g++ -fpic wrapper.cc -l:./libcombined.so -c -o wrapper.o -std=c++11
.so of a.pb.o, b.grpc.pb.o and wrapper.o as libcombined.so:
g++ -shared -o libcombinedwrapper.so *.o
Compiled main.c as:
gcc main.c -l:./libcombinedwrapper.so -o main -ldl
I am calling helloWorld from my main.c file which is:
#include <stdio.h>
#include <dlfcn.h>
int main(){
char* (*fn)(const char*,const char*,const char*);
void *handle = dlopen("path_to/libcombined.so",RTLD_NOW);
if(handle==NULL){
fprintf(stderr, "Error: %s\n", dlerror());
}
fn = (char* (*)(const char*,const char*,const char*))dlsym(handle, "helloWorld");
if (!fn) {
/* no such symbol */
fprintf(stderr, "Error: %s\n", dlerror());
dlclose(handle);
return 0;
}
char* msg = fn("asd","asdas","asdasd");
printf("%s",msg);
return 0;
}
Error after executing: ./main
Error: path_to/libcombinedwrapper.so: undefined symbol: _ZN6google8protobuf2io20ZeroCopyOutputStream15WriteAliasedRawEPKvi
Error: ./main: undefined symbol: helloWorld
Segmentation fault (core dumped)
The first above error is from symbol from protobuf.h file.
Can someone please suggest what I am doing wrong while linking or is there something I am doing wrong in main.c file?

Answer 1

g++ -shared -o libcombined.so *.o
You need to also link in all dependencies of the objects (libgrpc here).
You can add -Wl,--no-allow-shlib-undefined to verify that libcombined.so is linking everything it needs.
P.S. To avoid core dump, you should exit or return once dlopen fails.
P.P.S. It is generally a very bad idea(TM) to link *.o. Use proper Makefile to avoid unnecessary compilations and explicitly list objects that you are intending to put into libcombined.so.