different ways of compilation cause the same dynamic library generate differents results - c

Question

I use different ways of compilation to compile my project for testing symbol of a same cover when loaded dynamic library, however, there are two different results in different compilations, I cannot find differences between this two compilations, please help?
when I use dlopen() and dlsym()， I see official description of dlsym() as below:
The dlsym() function shall search for the named symbol in all objects
loaded automatically as a result of loading the object referenced by
handle (see dlopen()). Load ordering is used in dlsym() operations
upon the global symbol object. The symbol resolution algorithm used
shall be dependency order as described in dlopen().
Note: load ordering is an mechanism to handle conflict when loaded symbols with same name.
So I want to run a test for load ordering, however, I find different results when use different compilations(as I think two of them are equivalent)
here is my project:
file tree:
├── CMakeLists.txt
├── fn.c
└── main.c
CMakeLists.txt
project(demo)
set(CMAKE_CXX_FLAGS " -fpic ${CMAKE_CXX_FLAGS}")
add_library(fn SHARED fn.c)
add_executable(demo main.c)
target_link_libraries(demo -ldl)
fn.c
#include<stdio.h>
int temp = 1;
void fn_print(){
printf("fn temp address is %p, value is %d\n", &temp, temp);
}
main.c
#include<stdio.h>
#include<dlfcn.h>
typedef void (*fn_p)();
int temp = 0;
int main(){
printf("main temp address is %p, value is %d\n", &temp, temp);
void *handle = dlopen("./libfn.so", RTLD_NOW);
fn_p dy_print = dlsym(handle, "fn_print");
dy_print();
return 0;
}
First, I use cmake to compile this project and test, we can see variable temp share the same address 0x60104c and value 0 in obj file and dynamic library.
root#acnszavl00033:~/temp/dylib_test# cmake .
-- The C compiler identification is GNU 5.4.0
-- The CXX compiler identification is GNU 5.4.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /root/temp/dylib_test
root#acnszavl00033:~/temp/dylib_test# make
Scanning dependencies of target demo
[ 25%] Building C object CMakeFiles/demo.dir/main.c.o
[ 50%] Linking C executable demo
[ 50%] Built target demo
Scanning dependencies of target fn
[ 75%] Building C object CMakeFiles/fn.dir/fn.c.o
[100%] Linking C shared library libfn.so
[100%] Built target fn
root#acnszavl00033:~/temp/dylib_test# ./demo
main temp address is 0x60104c, value is 0
fn temp address is 0x60104c, value is 0
according to description, varaible temp should have different addresses, then I use gcc to only recompile demo(don't make changes to dynamic library libfn.so) , then test, varaible temp have different addresses, why?
root#acnszavl00033:~/temp/dylib_test# gcc -o demo main.c -ldl
root#acnszavl00033:~/temp/dylib_test# ./demo
main temp address is 0x60104c, value is 0
fn temp address is 0x7f795cec0028, value is 1
I expect output of compilatoin using cmake should like this:
main temp address is 0x60104c, value is 0
fn temp address is 0x7f795cec0028, value is 1

Answer 1

The below CMakeLists.txt seems to solve the issue:
cmake_minimum_required(VERSION 3.4)
project(demo)
add_library(fn SHARED fn.c)
add_executable(demo main.c)
target_link_libraries(demo PRIVATE dl)
When you don't add cmake_minimum_required or just provide anything below VERSION 3.4, you have the problem. Otherwise, when you build and run, you get the following:
~/so/build> cmake ..
-- The C compiler identification is GNU 9.1.0
-- The CXX compiler identification is GNU 9.1.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /home/aytekin/so/build
~/so/build> cmake --build .
Scanning dependencies of target fn
[ 25%] Building C object CMakeFiles/fn.dir/fn.c.o
[ 50%] Linking C shared library libfn.so
[ 50%] Built target fn
Scanning dependencies of target demo
[ 75%] Building C object CMakeFiles/demo.dir/main.c.o
[100%] Linking C executable demo
[100%] Built target demo
~/so/build> ./demo
main temp address is 0x56135a414044, value is 0
fn temp address is 0x7f170b3cb028, value is 1
I am not sure if this has to do with the policy change introduced in v3.4, but the most relevant change seems to be CMP0065.