From my book:
.bss:
Uninitialized global C variables
COMMON:
Uninitalized data objects that are not yet allocated
I have to say, I don't quite see a clear distinction. I don't even quite understand what an uninitizalied, non-allocated data object is...seems like nothing. I've used GNU's readelf tool to try to take a look at some simple C code, but can't find a single COMMON symbol. I've read things like FORTRAN's COMMON type is an example of a COMMON symbol - but I don't know FORTRAN
Can someone possibly distinguish the two for me? If at all possible, hopefully with a C example? Greatly appreciated.
edit: from this post, the variable c here:
int c;
int main() {} ...
should be COMMON. But using objdump -t shows for me that c is in .bss...
confused
Answer
// 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.