// file a.c
// file-scope
int a = 0; // goes into BSS
after compilation of a.c
into object file a.o
, a
symbol goes into BSS section.
// file b.c
// file-scope
int b; // goes into COMMON section
after compilation of b.c
into object file b.o
, b
symbol goes into COMMON section.
After linking of a.o
and b.o
, both a
and b
symbols goes into BSS section. Common symbols only exist in object files, not in executable files. The idea of COMMON symbols in Unix is to allow multiple external definitions of a same variable (in different compilation units) under a single common symbol under certain conditions.