未初始化的 int 变量 will放在 .bss 部分。
但是如果我添加这样一个变量 - size 结果中的 bss 部分长度会变大到 8 个字节而不是 4 个 - 为什么会这样？
这是一个例子:
使用一个未初始化的整数:
#include <stdio.h>
int a;
//int b;
int main () {
printf("A size: %d\n", sizeof(a));
// printf("B size: %d\n", sizeof(b));
return 0;
}
大小的结果:
$ gcc int_szie.c -o int_szie $ size int_szie text data bss dec hex filename 1483 536 8 2027 7eb int_szie
取消注释 b var - 现在 bss 是 16 字节:
...
int a;
int b;
int main () {
printf("A size: %d\n", sizeof(a));
printf("B size: %d\n", sizeof(b));
return 0;
...
$ gcc int_szie.c -o int_szie $ size int_szie text data bss dec hex filename 1628 592 16 2236 8bc int_szie
sizeof() 结果:
$ ./int_szie A size: 4 B size: 4
我什至不敢问为什么 data 部分在整个 56 字节上变得更大:
data 536
对比:
data 592
gcc:
$ gcc --version gcc (GCC) 7.1.1 20170630
最佳答案
我运行了相同的代码，只是添加了 long long b .代码如下所示:
#include <stdio.h>
int a;
long long b;
int main () {
printf("A size: %d\n", sizeof(a));
// printf("B size: %d\n", sizeof(b));
return 0;
}
objdump输出如下:
mdeore@mdeore-Lenovo:~$ objdump -t ./a.out
./a.out: file format elf64-x86-64
SYMBOL TABLE:
0000000000400238 l d .interp 0000000000000000 .interp
0000000000400254 l d .note.ABI-tag 0000000000000000 .note.ABI-tag
0000000000400274 l d .note.gnu.build-id 0000000000000000 .note.gnu.build-id
0000000000400298 l d .gnu.hash 0000000000000000 .gnu.hash
00000000004002b8 l d .dynsym 0000000000000000 .dynsym
0000000000400318 l d .dynstr 0000000000000000 .dynstr
0000000000400358 l d .gnu.version 0000000000000000 .gnu.version
0000000000400360 l d .gnu.version_r 0000000000000000 .gnu.version_r
0000000000400380 l d .rela.dyn 0000000000000000 .rela.dyn
0000000000400398 l d .rela.plt 0000000000000000 .rela.plt
00000000004003c8 l d .init 0000000000000000 .init
00000000004003f0 l d .plt 0000000000000000 .plt
0000000000400420 l d .plt.got 0000000000000000 .plt.got
0000000000400430 l d .text 0000000000000000 .text
00000000004005c4 l d .fini 0000000000000000 .fini
00000000004005d0 l d .rodata 0000000000000000 .rodata
00000000004005e0 l d .eh_frame_hdr 0000000000000000 .eh_frame_hdr
0000000000400618 l d .eh_frame 0000000000000000 .eh_frame
0000000000600e10 l d .init_array 0000000000000000 .init_array
0000000000600e18 l d .fini_array 0000000000000000 .fini_array
0000000000600e20 l d .jcr 0000000000000000 .jcr
0000000000600e28 l d .dynamic 0000000000000000 .dynamic
0000000000600ff8 l d .got 0000000000000000 .got
0000000000601000 l d .got.plt 0000000000000000 .got.plt
0000000000601028 l d .data 0000000000000000 .data
0000000000601038 l d .bss 0000000000000000 .bss
0000000000000000 l d .comment 0000000000000000 .comment
0000000000000000 l df *ABS* 0000000000000000 crtstuff.c
0000000000600e20 l O .jcr 0000000000000000 __JCR_LIST__
0000000000400460 l F .text 0000000000000000 deregister_tm_clones
00000000004004a0 l F .text 0000000000000000 register_tm_clones
00000000004004e0 l F .text 0000000000000000 __do_global_dtors_aux
0000000000601038 l O .bss 0000000000000001 completed.7585
0000000000600e18 l O .fini_array 0000000000000000 __do_global_dtors_aux_fini_array_entry
0000000000400500 l F .text 0000000000000000 frame_dummy
0000000000600e10 l O .init_array 0000000000000000 __frame_dummy_init_array_entry
0000000000000000 l df *ABS* 0000000000000000 abc.c
0000000000000000 l df *ABS* 0000000000000000 crtstuff.c
0000000000400708 l O .eh_frame 0000000000000000 __FRAME_END__
0000000000600e20 l O .jcr 0000000000000000 __JCR_END__
0000000000000000 l df *ABS* 0000000000000000
0000000000600e18 l .init_array 0000000000000000 __init_array_end
0000000000600e28 l O .dynamic 0000000000000000 _DYNAMIC
0000000000600e10 l .init_array 0000000000000000 __init_array_start
00000000004005e0 l .eh_frame_hdr 0000000000000000 __GNU_EH_FRAME_HDR
0000000000601000 l O .got.plt 0000000000000000 _GLOBAL_OFFSET_TABLE_
00000000004005c0 g F .text 0000000000000002 __libc_csu_fini
0000000000000000 w *UND* 0000000000000000 _ITM_deregisterTMCloneTable
0000000000601028 w .data 0000000000000000 data_start
0000000000601040 g O .bss 0000000000000008 b <<<<<<<< 'b' is 8 bytes
0000000000601038 g .data 0000000000000000 _edata
00000000004005c4 g F .fini 0000000000000000 _fini
0000000000000000 F *UND* 0000000000000000 printf@@GLIBC_2.2.5
0000000000000000 F *UND* 0000000000000000 __libc_start_main@@GLIBC_2.2.5
0000000000601028 g .data 0000000000000000 __data_start
0000000000000000 w *UND* 0000000000000000 __gmon_start__
0000000000601030 g O .data 0000000000000000 .hidden __dso_handle
00000000004005d0 g O .rodata 0000000000000004 _IO_stdin_used
0000000000400550 g F .text 0000000000000065 __libc_csu_init
0000000000601050 g .bss 0000000000000000 _end
0000000000400430 g F .text 000000000000002a _start
0000000000601048 g O .bss 0000000000000004 a <<<<<<<<< 'a' is 4 bytes
0000000000601038 g .bss 0000000000000000 __bss_start
0000000000400526 g F .text 000000000000001f main
0000000000000000 w *UND* 0000000000000000 _Jv_RegisterClasses
0000000000601038 g O .data 0000000000000000 .hidden __TMC_END__
0000000000000000 w *UND* 0000000000000000 _ITM_registerTMCloneTable
00000000004003c8 g F .init 0000000000000000 _init
输出正确显示 int a是 4 个字节和 long long b是8个字节。
size <objfile>显示 .bss 的大小段而不是特定变量的大小。为了验证这个事实，删除两个 a和 b代码中的全局变量，现在检查 .bss 的大小段，您仍然会看到大小为 8。
有类似的thread Windows 操作系统上的大小不同。
关于C: 未初始化的 int 类型大小必须大于，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/46015902/