GCC：命令行参数组装code引用不同 [英] gcc: command line args referenced differently in assembly code

问题描述

我看惯了通过引用的命令行参数（约定（A））：

I am used to seeing command line arguments referenced by (convention (A)):

pushl %ebp  
movl %esp, %ebp  
movl (%ebp), %eax    # argc  
movl 4(%ebp), %ebx   # pointer to argv[0] string      
movl 8($ebp), %ecx   # pointer to argv[1] string

有时候，我见过的清单开始以8偏移量，这不是（主）的问题。我也注意到一个程序是这样的翻译和参考，我感到困惑，要获得的argv [1] （约定（B））：

movl 0xc(%ebp), %eax   # pointer to a pointer to argv[0] (argc is at offset 8)  
addl $0x4, %eax    # argv[1] is a pointer at offset 4 from the pointer to argv[0]   
movl (%eax), %eax    # load where it points to, which is the argv[1] string  

（偏移 16（％EBP）我看到一个指向一个环境变量）

(at offset 16(%ebp) I see a pointer to an environmental variable)

（1）是否有任何原因不同的约定？结果
（2）有一个编译器选项强制GCC使用什么我认为是标准的约定（A）以上？结果
（3）是否有一个理由GCC使用约定（B）？结果
（4）为什么额外的8偏移？

(1) Is there any reason for this different convention?
(2) Is there a compiler option to force gcc to use what I believe to be the standard convention (A) above?
(3) Is there a reason gcc uses convention (B)?
(4) Why the additional offset of 8?

系统信息：结果
- Ubuntu的12.04结果
- GCC 4.6.3结果
- 与FNO堆栈保护器编译

System info:
- Ubuntu 12.04
- gcc 4.6.3
- compiled with fno-stack-protector

推荐答案

如果你正在处理已链接到C运行一个程序，那么 ARGC 和的argv 参数在 EBP + 8 ARGC C $ C>和的argv 在 EBP + 12 。这是因为C运行时执行它自己的初始化并传递参数的main（）使用正常的C ABI。

If you're dealing with a program that has been linked to the C runtime, then the argc and argv parameters are passed (assuming x86) with argc at ebp+8 and argv at ebp+12. that's because the C runtime performs it's own initialization and passes the arguments to main() using the normal C ABI.

这是你说你看惯了（与 ARGC 在堆栈的顶部，然后按调用约定的argv [0] .. ARGV [ARGC] ）是由启动一个新的程序在Linux系统调用设置堆栈的状态。

The calling convention that you say you're used to seeing (with argc at the top of the stack, followed by argv[0]..argv[argc]) is the state of the stack as set up by the Linux system call that starts a new program.

请注意，您的装配导向code例如：

Note that your assembly oriented code example:

pushl %ebp  
movl %esp, %ebp  
movl (%ebp), %eax    # argc  
movl 4(%ebp), %ebx   # pointer to argv[0] string      
movl 8($ebp), %ecx   # pointer to argv[1] string

看起来由4对每个由于初始 pushl 指令的最后三行处于关机状态。

looks to be off by 4 for each of the last three lines because of the initial pushl instruction.

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