[英]If a global variable is initialized to 0, will it go to BSS?
All the initialized global/static variables will go to initialized data section . 所有初始化的全局/静态变量都将转到初始化数据部分 。 All the uninitialized global/static variables will go to uninitialed data section (BSS).
所有未初始化的全局/静态变量将转到uninitialed数据部分 (BSS)。 The variables in BSS will get a value 0 during program load time.
在程序加载时间内,BSS中的变量将获得值0。
If a global variable is explicitly initialized to zero ( int myglobal = 0
), where that variable will be stored? 如果全局变量显式初始化为零(
int myglobal = 0
),那么该变量将被存储在哪里?
Compiler is free to put such variable into bss
as well as into data
. 编译器可以自由地将这样的变量放入
bss
以及data
。 For example, GCC has a special option controlling such behavior: 例如,GCC有一个控制此类行为的特殊选项 :
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS.
如果目标支持BSS部分,则GCC默认将初始化为零的变量放入BSS。 This can save space in the resulting code.
这可以节省生成的代码中的空间。 This option turns off this behavior because some programs explicitly rely on variables going to the data section.
此选项会关闭此行为,因为某些程序明确依赖于转到数据部分的变量。 Eg, so that the resulting executable can find the beginning of that section and/or make assumptions based on that.
例如,以便生成的可执行文件可以找到该部分的开头和/或基于此做出假设。
The default is
-fzero-initialized-in-bss
.默认值为
-fzero-initialized-in-bss
。
Tried with the following example ( test.c
file): 尝试使用以下示例(
test.c
文件):
int put_me_somewhere = 0;
int main(int argc, char* argv[]) { return 0; }
Compiling with no options (implicitly -fzero-initialized-in-bss
): 没有选项编译(隐式
-fzero-initialized-in-bss
):
$ touch test.c && make test && objdump -x test | grep put_me_somewhere
cc test.c -o test
0000000000601028 g O .bss 0000000000000004 put_me_somewhere
Compiling with -fno-zero-initialized-in-bss
option: 使用
-fno-zero-initialized-in-bss
选项进行编译:
$ touch test.c && make test CFLAGS=-fno-zero-initialized-in-bss && objdump -x test | grep put_me_somewhere
cc -fno-zero-initialized-in-bss test.c -o test
0000000000601018 g O .data 0000000000000004 put_me_somewhere
It's easy enough to test for a specific compiler: 测试特定的编译器很容易:
$ cat bss.c
int global_no_value;
int global_initialized = 0;
int main(int argc, char* argv[]) {
return 0;
}
$ make bss
cc bss.c -o bss
$ readelf -s bss | grep global_
32: 0000000000400420 0 FUNC LOCAL DEFAULT 13 __do_global_dtors_aux
40: 0000000000400570 0 FUNC LOCAL DEFAULT 13 __do_global_ctors_aux
55: 0000000000601028 4 OBJECT GLOBAL DEFAULT 25 global_initialized
60: 000000000060102c 4 OBJECT GLOBAL DEFAULT 25 global_no_value
We're looking for the location of 0000000000601028
and 000000000060102c
: 我们正在寻找
0000000000601028
和000000000060102c
的位置:
$ readelf -S bss
There are 30 section headers, starting at offset 0x1170:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
...
[24] .data PROGBITS 0000000000601008 00001008
0000000000000010 0000000000000000 WA 0 0 8
[25] .bss NOBITS 0000000000601018 00001018
0000000000000018 0000000000000000 WA 0 0 8
It looks like both values are stored in the .bss
section on my system: gcc version 4.5.2 (Ubuntu/Linaro 4.5.2-8ubuntu4)
. 看起来这两个值都存储在我的系统的
.bss
部分: gcc version 4.5.2 (Ubuntu/Linaro 4.5.2-8ubuntu4)
。
The behavior is dependent upon the C implementation. 行为取决于C实现。 It may end up in either .data or .bss, and to increase changes that it does not end up in .data taking redundant space up, it's better not to explicitly initialize it to 0, since it will be set to 0 anyway if the object is of static duration.
它可能最终以.data或.bss结尾,并且为了增加它不会最终的变化.data占用冗余空间,最好不要将它显式初始化为0,因为无论如何它都将被设置为0对象具有静态持续时间。
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