问题描述
我所知道的是,全局和静态变量保存在。数据
段和未初始化的数据都在的.bss
段。我不明白的是,为什么我们未初始化变量专用段?如果一个未初始化的变量在运行时赋值,不变量仍然存在于的.bss
段只?
在下面的程序中, A
是。数据
段和 B
是的.bss
段;那是对的吗?请纠正我,如果我的理解是错误的。
的#include<&stdio.h中GT;
#包括LT&;&stdlib.h中GT;诠释一个[10] = {1,2,3,4,5,6,7,8,9};
INT B〔20〕; / *未初始化,所以在.bss段和20 *的sizeof(int)的不占空间* /诠释的main()
{
;
}
此外,考虑下面的程序,
的#include<&stdio.h中GT;
#包括LT&;&stdlib.h中GT;
INT变种[10]; / *未初始化所以在.bss段* /
诠释的main()
{
VAR [0] = 20 / * **初始化,在这种变种会是什么?** * /
}
究其原因是为了减少程序大小。试想一下,你的C程序的嵌入式系统,其中code和所有的常量都保存在真正的ROM(闪存)上运行。在这种系统中,初始的复制下来必须执行将所有静态存储持续时间的对象,之前的主()被调用。它通常会是这样的伪:
为(i = 0; I< all_explicitly_initialized_objects;我++)
{
。数据[I] = init_value [I]
}memset的(.bss段,
0,
all_implicitly_initialized_objects);
其中的.data和.bss被存储在RAM中,但init_value存储在ROM中。如果它一直一个段,则该光盘必须与大量的零来填充,显著增加ROM的大小
基于RAM的可执行文件同样的工作,但他们当然没有真正的ROM。
另外,memset的很可能一些非常有效的联汇编,这意味着该启动拷贝下来可以执行得更快。
What I know is that global and static variables are stored in the .data
segment, and uninitialized data are in the .bss
segment. What I don't understand is why do we have dedicated segment for uninitialized variables? If an uninitialised variable has a value assigned at run time, does the variable exist still in the .bss
segment only?
In the following program, a
is in the .data
segment, and b
is in the .bss
segment; is that correct? Kindly correct me if my understanding is wrong.
#include <stdio.h>
#include <stdlib.h>
int a[10] = { 1, 2, 3, 4, 5, 6, 7, 8, 9};
int b[20]; /* Uninitialized, so in the .bss and will not occupy space for 20 * sizeof (int) */
int main ()
{
;
}
Also, consider following program,
#include <stdio.h>
#include <stdlib.h>
int var[10]; /* Uninitialized so in .bss */
int main ()
{
var[0] = 20 /* **Initialized, where this 'var' will be ?** */
}
The reason is to reduce program size. Imagine that your C program runs on an embedded system, where the code and all constants are saved in true ROM (flash memory). In such systems, an initial "copy-down" must be executed to set all static storage duration objects, before main() is called. It will typically go like this pseudo:
for(i=0; i<all_explicitly_initialized_objects; i++)
{
.data[i] = init_value[i];
}
memset(.bss,
0,
all_implicitly_initialized_objects);
Where .data and .bss are stored in RAM, but init_value is stored in ROM. If it had been one segment, then the ROM had to be filled up with a lot of zeroes, increasing ROM size significantly.
RAM-based executables work similarly, though of course they have no true ROM.
Also, memset is likely some very efficient inline assembler, meaning that the startup copy-down can be executed faster.
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