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_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("%ld\n", 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("%p\n", p);
p =&__bss_end_xxdk;
printf("%p\n", p);
return0;
}
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:
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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