28 ilave bayt bss

Question

struct abc 
{ 
    char cc[32]; 
} mystruct; 

int main() 
{ 
} 

Yukarıdaki programı çalıştırdığımda, .bss bölümünde 64 bayt vardır. 36 bayt olmasını bekliyordum. Diğer kütüphaneler tarafından alınan mystruct ve 4 bayt için 32 bayt.

char cc[32] değerini char cc[31] olarak değiştirirseniz, .bss içinde 36 bayt alırım.

-bash-3.00$ g++ bssSize.cc 

-bash-3.00$ readelf --sections ./a.out | grep bss 
    [23] .bss    NOBITS   08049580 000578 000040 00 WA 0 0 32 
-bash-3.00$ 

Neden .bss orada bu ekstra 28 byte hex 000.040 nedir?

Answer 1

readelf çıktısının son sütunu, hizalamadır ve gösterilen değer gerçekten "32" dir. Yani, BSS boyutu en yakın 32 bayta katına yuvarlanıyor.

Answer 2

objdump -sSx verim a.out, diğer şeyler arasında ... biz ** 5 2 ya da 2^5 bkz "Algn" altında

Sections: 
Idx Name   Size  VMA    LMA    File off Algn 
    0 .interp  0000001c 0000000000400238 0000000000400238 00000238 2**0 
        CONTENTS, ALLOC, LOAD, READONLY, DATA 
    1 .note.ABI-tag 00000020 0000000000400254 0000000000400254 00000254 2**2 
        CONTENTS, ALLOC, LOAD, READONLY, DATA 
    2 .note.gnu.build-id 00000024 0000000000400274 0000000000400274 00000274 2**2 
        CONTENTS, ALLOC, LOAD, READONLY, DATA 
    3 .gnu.hash  0000001c 0000000000400298 0000000000400298 00000298 2**3 
        CONTENTS, ALLOC, LOAD, READONLY, DATA 
    4 .dynsym  00000048 00000000004002b8 00000000004002b8 000002b8 2**3 
        CONTENTS, ALLOC, LOAD, READONLY, DATA 
    5 .dynstr  00000038 0000000000400300 0000000000400300 00000300 2**0 
        CONTENTS, ALLOC, LOAD, READONLY, DATA 
    6 .gnu.version 00000006 0000000000400338 0000000000400338 00000338 2**1 
        CONTENTS, ALLOC, LOAD, READONLY, DATA 
    7 .gnu.version_r 00000020 0000000000400340 0000000000400340 00000340 2**3 
        CONTENTS, ALLOC, LOAD, READONLY, DATA 
    8 .rela.dyn  00000018 0000000000400360 0000000000400360 00000360 2**3 
        CONTENTS, ALLOC, LOAD, READONLY, DATA 
    9 .rela.plt  00000018 0000000000400378 0000000000400378 00000378 2**3 
        CONTENTS, ALLOC, LOAD, READONLY, DATA 
10 .init   00000018 0000000000400390 0000000000400390 00000390 2**2 
        CONTENTS, ALLOC, LOAD, READONLY, CODE 
11 .plt   00000020 00000000004003b0 00000000004003b0 000003b0 2**4 
        CONTENTS, ALLOC, LOAD, READONLY, CODE 
12 .text   000001c8 00000000004003d0 00000000004003d0 000003d0 2**4 
        CONTENTS, ALLOC, LOAD, READONLY, CODE 
13 .fini   0000000e 0000000000400598 0000000000400598 00000598 2**2 
        CONTENTS, ALLOC, LOAD, READONLY, CODE 
14 .rodata  00000004 00000000004005a8 00000000004005a8 000005a8 2**2 
        CONTENTS, ALLOC, LOAD, READONLY, DATA 
15 .eh_frame_hdr 0000002c 00000000004005ac 00000000004005ac 000005ac 2**2 
        CONTENTS, ALLOC, LOAD, READONLY, DATA 
16 .eh_frame  000000a4 00000000004005d8 00000000004005d8 000005d8 2**3 
        CONTENTS, ALLOC, LOAD, READONLY, DATA 
17 .ctors  00000010 0000000000600e28 0000000000600e28 00000e28 2**3 
        CONTENTS, ALLOC, LOAD, DATA 
18 .dtors  00000010 0000000000600e38 0000000000600e38 00000e38 2**3 
        CONTENTS, ALLOC, LOAD, DATA 
19 .jcr   00000008 0000000000600e48 0000000000600e48 00000e48 2**3 
        CONTENTS, ALLOC, LOAD, DATA 
20 .dynamic  00000190 0000000000600e50 0000000000600e50 00000e50 2**3 
        CONTENTS, ALLOC, LOAD, DATA 
21 .got   00000008 0000000000600fe0 0000000000600fe0 00000fe0 2**3 
        CONTENTS, ALLOC, LOAD, DATA 
22 .got.plt  00000020 0000000000600fe8 0000000000600fe8 00000fe8 2**3 
        CONTENTS, ALLOC, LOAD, DATA 
23 .data   00000010 0000000000601008 0000000000601008 00001008 2**3 
        CONTENTS, ALLOC, LOAD, DATA 
24 .bss   00000040 0000000000601020 0000000000601020 00001018 2**5 
        ALLOC 
25 .comment  0000002a 0000000000000000 0000000000000000 00001018 2**0 
        CONTENTS, READONLY 

madde 24, nam 32. Bu işaret bss tahsis edilir 32 bit hizalı olduğu için 32 bit hizalıdır.

Answer 3

objdump -t a.out | grep bss size büyüklüğü 32 bayt içinde (hex 00000020)

08049660 l d .bss 00000000    .bss 
    08049660 l  O .bss 00000001    completed.5745 
    08049660 g  *ABS* 00000000    __bss_start 
    08049680 g  O .bss 00000020    mystruct 

beklenen verir.