//Global space
int A;
int B = 0;
int C = 0x01;
static int D;
static int E = 0;
static int F = 0x01;
void foo()
{
static int G;
static int H = 0;
static int I = 0x01;
}
My understanding is that all global & static variables that are either not initialized explicitly or initialized to 0 would go into BSS section. All the global & static variables that are initialized with any valu other than 0 would go into DATA section. Am I right?
In this case, following would be correct.
A -- BSS (No Explicit init)
B -- BSS (init to 0)
C -- DATA (!= 0)
D -- BSS
E -- BSS
F -- DATA
G -- BSS
H -- BSS
I -- DATA
Please correct me or acknowledge me. Thanks in advance.
CodePudding user response：
In C every object with static storage duration IS initialized. If you don't write an explicit initializer it is implicitly initialized to zero. Only objects with automatic storage (on the stack) are truly uninitialized.
This is from the compiler's point of view. If you choose to edit the linker script you can obviously get different results. A normal linker script will do the following:
Most compilers will put C, F and I in .data.
Most compilers will put B, D, E, G and H in .bss.
The only funny one is A; many compilers will put it in COMMON.
GCC with the option -fno-common will put A in .bss.
This option is turned on by default in recent versions of GCC on some platforms.
CodePudding user response：
1.Env.: *nix, gcc compiler, ELF (Executable and Linkable Format) binary
2.Code: According to your code, here's a way to display their runtime addresses:
#include <stdio.h>
//Global space
int A;
int B = 0;
int C = 0x01;
static int D;
static int E = 0;
static int F = 0x01;
void foo()
{
static int G;
static int H = 0;
static int I = 0x01;
printf("G : 8p\n",(void *) &G);
printf("H : 8p\n",(void *) &H);
printf("I : 8p\n",(void *) &I);
}
int main(int argc, char* argv[]) {
printf("A : 8p\n",(void *) &A);
printf("B : 8p\n",(void *) &B);
printf("C : 8p\n",(void *) &C);
printf("D : 8p\n",(void *) &D);
printf("E : 8p\n",(void *) &E);
printf("F : 8p\n",(void *) &F);
foo();
return 0;
}
3.Reminder:
ELF section to segment mapping: in ELF, BSS and Data sections will be mapped to the same memory segment :
Section to Segment mapping:
Segment Sections...
00
01 .interp
02 .interp .note.tag .hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame
03 .ctors .dtors .dynamic .got.plt .data .bss
04 .dynamic
05 .note.tag
06 .eh_frame_hdr
07
As you see above, the .data and .bss sections will be mapped to the segment #3
4.ELF Symtab
From the code above from 2., we compile it through gcc on *nix OS and we can see in the ELF binary (table below is sorted on the Value field):
Symbol table (.symtab) contains 56 entries:
Num: Value Size Type Bind Vis Ndx Name
---- ---------------- ----- ------- ------ -------- --- --------------------
24: 0000000000000000 0 FILE LOCAL DEFAULT ABS
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS crt1_c.c
4: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
13: 0000000000000000 0 FILE LOCAL DEFAULT ABS crtstuff.c
17: 0000000000000000 0 FILE LOCAL DEFAULT ABS mmap.c
55: 0000000000000000 0 FUNC GLOBAL DEFAULT UND _init_tls@FBSD_1.0
36: 0000000000000000 0 FUNC GLOBAL DEFAULT UND atexit@FBSD_1.0
49: 0000000000000000 0 FUNC GLOBAL DEFAULT UND exit@FBSD_1.0
43: 0000000000000000 0 FUNC GLOBAL DEFAULT UND printf@FBSD_1.0
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
54: 0000000000400408 0 FUNC GLOBAL HIDDEN 9 _init
32: 0000000000400408 0 NOTYPE LOCAL DEFAULT 8 __rela_iplt_end
29: 0000000000400408 0 NOTYPE LOCAL DEFAULT 8 __rela_iplt_start
48: 0000000000400470 263 FUNC GLOBAL DEFAULT 11 _start
3: 0000000000400580 283 FUNC LOCAL DEFAULT 11 handle_static_init
2: 00000000004006a0 91 FUNC LOCAL DEFAULT 11 finalizer
7: 0000000000400700 0 FUNC LOCAL DEFAULT 11 deregister_tm_clones
8: 0000000000400730 0 FUNC LOCAL DEFAULT 11 register_tm_clones
9: 0000000000400770 0 FUNC LOCAL DEFAULT 11 __do_global_dtors_aux
12: 00000000004007f0 0 FUNC LOCAL DEFAULT 11 frame_dummy
44: 00000000004007f5 67 FUNC GLOBAL DEFAULT 11 foo
51: 0000000000400838 152 FUNC GLOBAL DEFAULT 11 main
16: 00000000004008d0 0 FUNC LOCAL DEFAULT 11 __do_global_ctors_aux
39: 0000000000400914 0 FUNC GLOBAL HIDDEN 12 _fini
34: 00000000004009a4 0 NOTYPE LOCAL DEFAULT 14 __GNU_EH_FRAME_HDR
15: 0000000000400a48 0 OBJECT LOCAL DEFAULT 15 __FRAME_END__
25: 0000000000600a4c 0 NOTYPE LOCAL DEFAULT 16 __fini_array_end
30: 0000000000600a4c 0 NOTYPE LOCAL DEFAULT 16 __fini_array_start
28: 0000000000600a4c 0 NOTYPE LOCAL DEFAULT 16 __init_array_end
33: 0000000000600a4c 0 NOTYPE LOCAL DEFAULT 16 __init_array_start
27: 0000000000600a4c 0 NOTYPE LOCAL DEFAULT 16 __preinit_array_end
26: 0000000000600a4c 0 NOTYPE LOCAL DEFAULT 16 __preinit_array_start
5: 0000000000600a50 0 OBJECT LOCAL DEFAULT 16 __CTOR_LIST__
14: 0000000000600a58 0 OBJECT LOCAL DEFAULT 16 __CTOR_END__
6: 0000000000600a60 0 OBJECT LOCAL DEFAULT 17 __DTOR_LIST__
41: 0000000000600a68 0 OBJECT GLOBAL HIDDEN 17 __DTOR_END__
31: 0000000000600a70 0 OBJECT LOCAL DEFAULT 18 _DYNAMIC
35: 0000000000600bd0 0 OBJECT LOCAL DEFAULT 19 _GLOBAL_OFFSET_TABLE_
53: 0000000000600c08 8 OBJECT GLOBAL DEFAULT 20 __progname
42: 0000000000600c10 0 OBJECT GLOBAL HIDDEN 20 __dso_handle
45: 0000000000600c18 4 OBJECT GLOBAL DEFAULT 20 C
20: 0000000000600c1c 4 OBJECT LOCAL DEFAULT 20 F
23: 0000000000600c20 4 OBJECT LOCAL DEFAULT 20 I.0
38: 0000000000600c24 0 NOTYPE GLOBAL DEFAULT 20 _edata
50: 0000000000600c24 0 NOTYPE GLOBAL DEFAULT 21 __bss_start
52: 0000000000600c28 0 OBJECT GLOBAL HIDDEN 20 __TMC_END__
40: 0000000000600c28 8 OBJECT GLOBAL DEFAULT 21 environ
10: 0000000000600c30 1 OBJECT LOCAL DEFAULT 21 completed.1
11: 0000000000600c38 8 OBJECT LOCAL DEFAULT 21 dtor_idx.0
46: 0000000000600c40 4 OBJECT GLOBAL DEFAULT 21 A
37: 0000000000600c44 4 OBJECT GLOBAL DEFAULT 21 B
18: 0000000000600c48 4 OBJECT LOCAL DEFAULT 21 D
19: 0000000000600c4c 4 OBJECT LOCAL DEFAULT 21 E
21: 0000000000600c50 4 OBJECT LOCAL DEFAULT 21 G.2
22: 0000000000600c54 4 OBJECT LOCAL DEFAULT 21 H.1
47: 0000000000600c58 0 NOTYPE GLOBAL DEFAULT 21 _end
As you see:
- data contains C, F and I
- bss contains A, B, D, E, G and H
5.Answer:
It seems your assumption is correct.