C程序中.bss段的模糊行为 [英] Ambiguous behaviour of .bss segment in C program
问题描述
#include< stdio.h>
int main()
{
return 0;
}
并执行以下命令来了解.bss段的大小更改。
gcc test.c -o test
size test
输出结果如下: -
文本数据bss十进制十六进制文件名
1115 552 8 1675 68b test
我没有声明全局或静态范围。所以请解释为什么bss段的大小是8字节。
我做了以下更改: - $ / $>
#包括< stdio.h中>
int x; //声明的全局变量
int main()
{
return 0;
}
但令我吃惊的是,输出与之前一样: -
文本数据bss十进制十六进制文件名
1115 552 8 1675 68b测试
请解释。
然后我初始化全局: -
$ p $ #include< stdio.h>
int x = 67; //初始化全局变量
int main()
{
return 0;
}
数据段的大小如预期般增加,但我没有预料到大小的bss段减少到4(相反,当没有声明时为8)。请解释。
文本数据bss十进制十六进制文件名
1115 556 4 1675 68b测试
我也尝试了objdump和nm命令,但它们也显示了变量x占用.bss(在第二种情况下)。然而,在size命令中没有显示bss大小的变化。
我按照以下步骤执行:
http://codingfox.com/10-7-memory-segments-code-data-bss/
其中输出如期完美。
当您编译一个简单的 main
程序你也链接启动代码。
此代码负责初始化bss。
该代码是使用8字节的代码,您在.bss部分看到的代码。
您可以使用 - nostartfiles gcc选项:
-nostartfiles
链接时不要使用标准系统启动文件。除非使用-nostdlib或-nodefaultlibs,否则通常会使用标准系统库。
要进行测试,请使用以下代码
#include< stdio.h>
int _start()
{
return 0;
}
并用
编译
gcc -nostartfiles test.c
你会看到。 bss设为0
文本数据bss十进制十六进制文件名
206 224 0 430 1ae测试
code>
I wrote the simple C program (test.c) below:-
#include<stdio.h>
int main()
{
return 0;
}
and executed the follwing to understand size changes in .bss segment.
gcc test.c -o test
size test
The output came out as:-
text data bss dec hex filename
1115 552 8 1675 68b test
I didn't declare anything globally or of static scope. So please explain why the bss segment size is of 8 bytes.
I made the following change:-
#include<stdio.h>
int x; //declared global variable
int main()
{
return 0;
}
But to my surprise, the output was same as previous:-
text data bss dec hex filename
1115 552 8 1675 68b test
Please explain. I then initialized the global:-
#include<stdio.h>
int x=67; //initialized global variable
int main()
{
return 0;
}
The data segment size increased as expected, but I didn't expect the size of bss segment to reduce to 4 (on the contrary to 8 when nothing was declared). Please explain.
text data bss dec hex filename
1115 556 4 1675 68b test
I also tried the comands objdump, and nm, but they too showed variable x occupying .bss (in 2nd case). However, no change in bss size is shown upon size command.
I followed the procedure according to: http://codingfox.com/10-7-memory-segments-code-data-bss/ where the outputs are coming perfectly as expected.
When you compile a simple main
program you are also linking startup code.
This code is responsible, among other things, to init bss.
That code is the code that "uses" 8 bytes you are seeing in .bss section.
You can strip that code using -nostartfiles gcc option:
-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib or -nodefaultlibs is used
To make a test use the following code
#include<stdio.h>
int _start()
{
return 0;
}
and compile it with
gcc -nostartfiles test.c
Youll see .bss set to 0
text data bss dec hex filename
206 224 0 430 1ae test
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