1. 使用ld --verbose 查看当前编译器默认的链接脚本：
default.ld
/* GNU ld (GNU Binutils for Ubuntu) 2.26.1
Supported emulations:
elf_x86_64
elf32_x86_64
elf_i386
elf_iamcu
i386linux
elf_l1om
elf_k1om
i386pep
i386pe
using internal linker script: */
/* ================================================= */
/* Script for -z combreloc: combine and sort reloc sections */
/* Copyright (C) 2014-2015 Free Software Foundation, Inc.
Copying and distribution of this script, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved. */
OUTPUT_FORMAT("elf64-x86-64", "elf64-x86-64",
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OUTPUT_ARCH(i386:x86-64)
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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("%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);
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
