在链接可执行文件之前找到 static 库未解析的依赖项

Question

So let's say we have static library mylib.a, which contains compiled cpp files.

file1.cpp:

int do_stuff();

int func_unres()
{
  int a = do_stuff();
  return a;
}

file2.cpp:

int do_other_stuff();

int func_res()
{
  int a = do_other_stuff();
  return a;
}

file3.cpp:

int do_other_stuff()
{
  return 42;
}

So, as we can see here, no file contains definition of do_stuff function. Library created this way:

g++ -c file1.cpp -o file1.o

g++ -c file2.cpp -o file2.o

g++ -c file3.cpp -o file3.o

ar r mylib.a file1.o file2.o file3.o

Now we try to make some binary with this library. Simple example main file:

#include <iostream>

int func_res();

int main()
{
  std::cout << func_res() << std::endl;
}

Compiling:

g++ main.cpp mylib.a -o my_bin

Everything works just fine. Now consider case of main file like this:

#include <iostream>

int func_unres();

int main()
{
  std::cout << func_unres() << std::endl;
}

In this case binary won't link, cause func_unres requires function do_stuff to be defined.

Is there a way to find out that static library requires symbol which no object file in the library contains before linking it with executable, which uses such symbol?

EDIT: The question is not how to simple list such symbols, but to get an output with linker like error. Like if i linked this library with executable using all of symbols it should contain.

Answer 1

It seems that as pointed in comments and in How to force gcc to link an unused static library , linker option --whole-archive is enough to force resolve all symbols and output linker error for all unresolved symbols in static library. So referring the question examples, compiling and linking this way first main file, which doesn't refer undefined symbol, will output linker error anyway:

g++ main.cpp -Wl,--whole-archive mylib.a -Wl,--no-whole-archive

Linking fails despite main doesn't use func_unres :

mylib.a(file1.o): In function func_unres(): file1.cpp:(.text+0x9): undefined reference to do_stuff()

Second option --no-whole-archive is used so the rest of required libraries' symbols will not be force resolved like this.