.bss vs COMMON: what goes where?

// 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.