堆栈分配，为什么多余的空间？ [英] Stack allocation, why the extra space?

问题描述

为了更好地掌握调用约定以及堆栈是如何处理的，我玩了一段时间，但我无法弄清楚为什么在设置堆栈时主分配三个额外的双字（<主+ 0> ）。它没有对齐到8字节或16字节，所以这不是我所知道的原因。正如我所看到的，主要需要12个字节的两个参数func和返回值。

我缺少什么？

该程序是在x86架构上用gcc -ggdb编译的C代码。

编辑：我从gcc中删除了-O0标志，对输出没有影响。

$ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ b转储函数main的汇编代码
0x080483d1 <+ 0>：sub esp，0x18
0x080483d4 <+ 3>：mov DWORD PTR [esp + 0x4]，0x7
0x080483dc< + 11> ;: mov DWORD PTR [esp ]，0x3
0x080483e3< + 18> ;:呼叫0x80483b4< func>
0x080483e8 <+ 23>：mov DWORD PTR [esp + 0x14]，eax
0x080483ec <+ 27>：add esp，0x18
0x080483ef< + 30>：ret
汇编器转储结束。

编辑：当然我应该发布C代码：

  int func（int a，int b）{
 int c = 9; 
返回a + b + c; 
} 
 
 void main（）{
 int x; 
 x = func（3，7）; 
} 
  

该平台是Arch Linux i686。

解决方案

它是一致的。我假定出于某种原因， esp 会从一开始就对齐，显然不是。

gcc 将堆栈帧调整为每个默认值16字节，这就是发生了什么。

I was playing around a bit to get a better grip on calling conventions and how the stack is handled, but I can't figure out why main allocates three extra double words when setting up the stack (at <main+0>). It's neither aligned to 8 bytes nor 16 bytes, so that's not why as far as I know. As I see it, main requires 12 bytes for the two parameters to func and the return value.

What am I missing?

The program is C code compiled with "gcc -ggdb" on a x86 architecture.

Edit: I removed the -O0 flag from gcc, and it made no difference to the output.

(gdb) disas main
Dump of assembler code for function main:
    0x080483d1 <+0>:    sub    esp,0x18
    0x080483d4 <+3>:    mov    DWORD PTR [esp+0x4],0x7
    0x080483dc <+11>:   mov    DWORD PTR [esp],0x3
    0x080483e3 <+18>:   call   0x80483b4 <func>
    0x080483e8 <+23>:   mov    DWORD PTR [esp+0x14],eax
    0x080483ec <+27>:   add    esp,0x18
    0x080483ef <+30>:   ret    
End of assembler dump.

Edit: Of course I should have posted the C code:

int func(int a, int b) {
    int c = 9;
    return a + b + c;
}

void main() {
    int x;
    x = func(3, 7);
}

The platform is Arch Linux i686.

解决方案

It's alignment. I assumed for some reason that esp would be aligned from the start, which it clearly isn't.

gcc aligns stack frames to 16 bytes per default, which is what happened.

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