英文:
Difference between -ffreestanding and -nostdlib when compiling with gcc
问题
我使用一个64位的Linux机器,使用x84-elf64-gcc编译器。我刚开始进行低级编程,想要理解C代码是如何被翻译成二进制的。这主要用于操作系统开发,因为我知道处理器不理解ELF或任何其他格式,只理解二进制。
例如,以下的C文件:
//test.c
int func()
{
return 0x12345678;
}
当我使用gcc编译时:
gcc test.c
我得到以下错误:
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
所以我猜想可能与链接器有关。
我执行以下操作:
gcc test.c -c
我得到一个ELF目标文件,然后我使用objdump,得到了预期的结果:
0000000000000000 <func>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: b8 78 56 34 12 mov $0x12345678,%eax
9: 5d pop %rbp
a: c3 retq
但是当我使用"-m32"选项进行交叉编译32位版本并使用objdump时,得到:
hello.o: file format elf32-i386
Disassembly of section .text:
00000000 <func>:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: e8 fc ff ff ff call 4 <func+0x4>
8: 05 01 00 00 00 add $0x1,%eax
d: b8 78 56 34 12 mov $0x12345678,%eax
12: 5d pop %ebp
13: c3 ret
Disassembly of section .text.__x86.get_pc_thunk.ax:
00000000 <__x86.get_pc_thunk.ax>:
0: 8b 04 24 mov (%esp),%eax
3: c3 ret
我之前读到过这与位置无关代码(position independent code)有关:https://stackoverflow.com/questions/59070109/undefined-reference-to-global-offset-table-in-gcc-32-bit-code-for-a-trivial
为什么使用"-m32"选项编译时会发生这样的变化?
此外,有人建议我在编译时使用"-ffreestanding"选项,但似乎没有产生效果。我了解"-ffreestanding"告诉编译器没有标准库,那么"-nostdlib"是什么意思?
注意:我对这种“硬核”C编程相对新手,我认为主要问题是我不太了解链接器/编译器的工作原理。
英文:
I am using a 64 bit linux machine with a x84-elf64-gcc compiler. I have just started low level programming and would like to understand how C code is actually translated into binary. This is mainly for Operating Systems developement, as I know that a processor doesn't understand ELF, or any other format, and only understands binary.
For example, the following c file:
//test.c
int func()
{
return 0x12345678;
}
When I compile with gcc:
gcc test.c
I get the following error:
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
So I'm guessing that there is an issue with the linker.
I do:
gcc test.c -c
I get an ELF object file, and I do an objdump and get the expected:
0000000000000000 <func>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: b8 78 56 34 12 mov $0x12345678,%eax
9: 5d pop %rbp
a: c3 retq
But when I "cross compile" a 32 bit version using the -m32 option and objdump, I get:
hello.o: file format elf32-i386
Disassembly of section .text:
00000000 <func>:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: e8 fc ff ff ff call 4 <func+0x4>
8: 05 01 00 00 00 add $0x1,%eax
d: b8 78 56 34 12 mov $0x12345678,%eax
12: 5d pop %ebp
13: c3 ret
Disassembly of section .text.__x86.get_pc_thunk.ax:
00000000 <__x86.get_pc_thunk.ax>:
0: 8b 04 24 mov (%esp),%eax
3: c3 ret
I've read in a previous answer that this has to do with position independent code: https://stackoverflow.com/questions/59070109/undefined-reference-to-global-offset-table-in-gcc-32-bit-code-for-a-trivial
Why is there such a change when you compile with the -m32 option?
Moreover, I was advised to use the -ffreestanding option when I compile, but it doesn't seem to have an effect here. I've read that -ffreestanding tells the compiler that there is no standard library, so what is -nostdlib then?
Note: I'm relatively new to this hardcore c programming, and I think that the main issue here is that I don't really understand how linkers/compilers work.
答案1
得分: 4
以下是翻译好的内容:
-
-freestanding 指示编译器应该是一个独立运行的编译器,据我所知,其唯一作用是禁用一些内置函数,比如_memcpy_;
-
-nostdlib 表示默认情况下不应链接任何库和启动文件。
英文:
The options control two parts of the process:
-
-freestanding indicates to the compiler that it should be a freestanding one, AFAIK the only effect is disabling some built-in functions like memcpy;
-
-nostdlib indicates that no libraries and no start up files should be linked by default.
答案2
得分: 2
I don't know what -ffreestanding
does exactly, that part is a good question.
但很不幸,你的问题牵涉到了32位PIE代码:
为什么在使用-m32选项进行编译时会发生这种变化?
因为你没有加入任何-O
优化选项,而32位模式没有EIP相对寻址数据的方式(只有相对跳转/调用)。因此,显然调试模式总是会设置一个寄存器作为GOT指针,作为寻址静态数据的基地址,即使在不使用它的函数中也是如此。
除非你明确希望创建一个PIE可执行文件,否则请始终使用-fno-pie
来关闭该默认选项。
你可能还想使用-mcmodel=kernel
- 如果你正在编译一个64位高半部内核(静态地址可以用作32位符号扩展立即数,但不能用作32位零扩展)。但我不知道它是否对32位代码有任何影响。
英文:
I don't know what -ffreestanding
does exactly, that part is a good question.
But unfortunately your question has a big side-track into 32-bit PIE code:
> Why is there such a change when you compile with the -m32 option?
Because you left out any -O
optimization option, and 32-bit mode doesn't have an EIP-relative addressing mode for data (only relative jumps/calls). So apparently debug-mode always sets up a register as a GOT pointer as a base for addressing static data, even in functions that don't use it.
Always use -fno-pie
to turn off that default unless you specifically want to make a PIE executable.
You might also want -mcmodel=kernel
- that's a good idea if you're compiling a 64-bit high-half kernel (static addresses can be used a 32-bit sign-extended immediates, but not 32-bit zero extended). But IDK if it does anything for 32-bit code.
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库,让每个人都能够通过互相帮助和分享经验来进步。
评论