What does "COM" means in the Ndx column of the .symtab section?
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 sec开发者_JS百科tion too, and so get a "3" in the Ndx column.
Thank you
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 andint a;
in another object is OK: both refer to the samea
, initialised to 0int a;
in one object andint a = 42;
in another object is OK: both refer to the samea
, initialised to 42int a = 23;
in one object andint 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.)
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.
They're uninitialized global variables that are allocated by the linker. Sometimes referred to as communal variables.
Edit: Hrmm, Paul Baker beat me to it, with links no less. use his answer :)
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