With -fcommon (default in GCC up to version 10), uninitialized globals var2 are placed in the COMMON section, allowing multiple definitions across files.

With -fno-common (default in GCC 10 and later), uninitialized globals var2 are no longer placed in the COMMON section. Instead, it is given a specific section index, treated as strong symbols, that may resulting in linker errors if the same variable name appears uninitialized in multiple files.