What does "COM" means in the Ndx column of the .symtab section?

Question

add2.c:

int counter=0;
int a=0;
int b;
int c;
int add(int a, int b) {
    return a+b;
}

compilation: gcc -c add2.c -o add2.o

reading the symbol table: readelf --symbols add2.o

Symbol table '.symtab' contains 12 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 00000000     0 FILE    LOCAL  DEFAULT  ABS add2.c
     2: 00000000     0 SECTION LOCAL  DEFAULT    1 
     3: 00000000     0 SECTION LOCAL  DEFAULT    2 
     4: 00000000     0 SECTION LOCAL  DEFAULT    3 
     5: 00000000     0 SECTION LOCAL  DEFAULT    5 
     6: 00000000     0 SECTION LOCAL  DEFAULT    4 
     7: 00000000     4 OBJECT  GLOBAL DEFAULT    3 counter
     8: 00000004     4 OBJECT  GLOBAL DEFAULT    3 a
     9: 00000004     4 OBJECT  GLOBAL DEFAULT  COM b
    10: 00000004     4 OBJECT  GLOBAL DEFAULT  COM c
    11: 00000000    14 FUNC    GLOBAL DEFAULT    1 add

What does "COM" means in the Ndx column ? I understand that "counter" and "a" are defined in the section #3 (ie, .bss) and that "add" is defined in the section #1 (ie, .text), but i was expecting "b" and "c" to be defined in the .bss section too, and so get a "3" in the Ndx column.

Thank you

Answer 1

gcc treats uninitialised globals which are not explicitly declared extern as "common" symbols (hence "COM").

Multiple definitions of the same common symbol (across multiple object files) are merged together by the linker when creating the final executable, so that they all refer to the same storage. One of the object files may initialise it to a particular value (in which case it will end up in the data section); if no object files initialise it, is will end up in the BSS; if more than one object initialises it, you'll get a linker error.

In summary, if you have, say, two definitions of int a:

• int a; in one object and int a; in another object is OK: both refer to the same a, initialised to 0
• int a; in one object and int a = 42; in another object is OK: both refer to the same a, initialised to 42
• int a = 23; in one object and int a= 42; in another object will give a link error.

Do note that the use of multiple definitions of the same symbol across two objects is not technically allowed by standard C; but it is supported by many compilers, including gcc, as an extension. (It's listed under "Common extensions" - no pun intended - in the C99 spec.)

Answer 2

From this PDF, table 7-11:

SHN_COMMON
Symbols defined relative to this section are common symbols, such as FORTRAN COMMON or unallocated C external variables. These symbols are sometimes referred to as tentative.

Also, see this page.