Question

It appears GCC linker doesn’t care for one variable being defined in two files. I suspect this is the cause of trouble a 3rd party library is causing us.

Take this:

File a.cpp contains:

 int foo;
 //do things with it.

File b.cpp contains:

 int foo;
 //do other things with it.

File c.cpp contains:

 extern int foo;
 //do other things with it.

They are all compiled by gcc to .o files, then linked as shared object.

 gcc -fPIC -c a.cpp
 gcc -fPIC -c b.cpp
 gcc -fPIC -c c.cpp
 ld *.o -shared  -soname,mylib -o mylib

The linker doesn’t complain at all, but the resulting binary misbehaves. We suspect there are at least a few conflicts of this kind and would like to locate them. What kind of linker options would let us detect them?

(interestingly, if the variables are initialized (int foo=0) in both files, it produces an error).

Answer 1

It seems compiler option -fno-common forces all the variables to be initialized, so it triggers errors upon linking.