C语言引用链接脚本中定义的符号

1. 使用ld --verbose 查看当前编译器默认的链接脚本：

default.ld

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__bss_start_xxdk =.; --->>> 添加自定义符号 <<<---
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__bss_end_xxdk =.;
--->>> 添加自定义符号 <<<---
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2. 在上述默认链接脚本中添加了两个自定义的符号:

__bss_start_xxdk =;
--->>> 添加自定义符号,
<<<---
__bss_end_xxdk =.;
--->>> 添加自定义符号 <<<---

在符号表中创建项，__bss_start_xxdk, 其持有当前内存位置(地址)“.”，但是该内存位置中没有存放任何有效的值。也就是说，不能访问一个链接脚本中定义的符号的“值”，它没有有效值。只能使用链接脚本中定义符号的地址；

因此在源码中使用链接脚本定义的符号，应该使用该符号的地址，而不是试图访问该符号的地址中的值；

3. 源码中使用链接脚本中定义的符号:

#include <stdio.h>
extern long __bss_start_xxdk, __bss_end_xxdk;
int main()
{
/**< 定义符号a, 符号表中添加一项名称为a,并且在内存中预留以&a起始, 长度sizeof(int)空间 */
int a;
a = 10;
///< 向符号a对应的内存地址&a中写入值10;
int* p;
///< 定义符号p,符号表中添加一项名称为p, 并在内存中预留以&p起始，长度为sizeof(int*)的空间 
p = &a;
///< 向符号p对应的内存地址&p中写入符号a的起始地址&a
int b = *p; ///< 解引用符号p对应内存地址&p中的值(即&a), 相当于 int b = *(&a);
b = a;
///< 向符号b对应的内存地址&b中写入符号a的内存地址&a中的值
int** pp;
///< 定义符号pp，符号表中添加一项名称为pp, 并在内存中预留以&pp起始，长度为sizeof(int*)的空间 
pp = &p;
///< 向符号pp对应的内存地址&pp中写入符号p的内存起始地址&p
*pp = 0x10000000; ///< 向符号pp对应的内存地址&pp中的值，写入0x10000000
该值为上一步存放的符号p的内存起始地址&p, 相当于: &p = 0x10000000, 也就是我们改变了符号p的内存起始地址到0x10000000
/**< -------------------------------------------------------------*/
/**< access the value at symbol __bss_start_xxdk memory's address:
long v = __bss_start_xxdk; 访问符号__bss_start_xxdk的内存地址中的值 */
long
v =
__bss_start_xxdk;
printf("%ldn", v); ///< zero
/**< access symbol __bss_start_xxdk memory's address:
we need this symbol address 访问符号__bss_start_xxdk的内存地址 */
long* p = &__bss_start_xxdk; ///< 使用C语言标准语法访问符号的地址
printf("%pn", p);
p = &__bss_end_xxdk;
printf("%pn", p);
return 0;
}

• 编译: gcc elf.c -T default.ld 让链接器使用我们修改过的默认脚本
• 运行./a.out:

0
0x601038
0x601040

• 查看符号表: objdump -h a.out | grep xxdk

0000000000601040 g
.bss	0000000000000000
__bss_end_xxdk
0000000000601038 g
.bss	0000000000000000
__bss_start_xxdk

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