将 C 转换为具有分段问题的 NASM 程序集

Question

我正在尝试获取此 c 代码：

main()

 {int x, y, count ;

  count = 0 ;
  x = 10 ; 
  y = 2 ;
  while (y < x)
    { x = x + 1 ;
      y = y + 2 ;
      count = count + 1 ;
     } 

    printf(“ It took %d iterations to complete loop. That seems like a     lot\n”,count) ;
}

到我目前拥有的 NASM 等价物：

segment .data

out1    db "It took ", 0
out2    db "%i ", 0
out3    db "iterations to complete the loop. That seems like a lot.", 10, 0

segment .bss

segment .text

global     main
extern printf

main:

    mov    eax, 0        ;count
    mov    ebx, 10        ;x
    mov    ecx, 2        ;y

    jmp     lp

    mov    eax, 0
    ret

lp:
    cmp    ecx, ebx    ;compare y to x
    jge    end        ;jump to end if y >= x
    add    eax, 1
    add    ebx, 1
    add    ecx, 2
    jmp    lp

end:

    push    out1
    call    printf

   push    eax
   push    out2
   call    printf

   push    out3
   call    printf

我不断收到分段错误，但我不明白为什么它会一直发生。我尝试在任何地方添加打印语句，但找不到故障所在。任何建议都会很棒！谢谢！

Answer 1

我认为您没有遵循 printf 的调用约定（它是 cdecl IIRC）。

1. 调用者必须在调用后清理堆栈
2. 并且 eax 被第一次调用 printf 及其返回值所破坏

从main返回时，第一个会引起麻烦。实际上，在printfs 之后没有ret 指令导致执行继续，直到出现问题:)

打印后添加以下内容：

   add     esp, 16  ; Fix stack, usually it's done after each call
                    ; with the appropiate values.
                    ; In this way (one fixup for many calls) stack
                    ; grows unnecessarily
   xor     eax, eax ; set return value to 0
   ret              ; Return from main

虽然，这并不能解决第二个问题。

Answer 2

更新完整示例：

segment .data
out1    db      "It took %d iterations to complete loop. That seems like "
        db      "a lot.", 0aH, 00H
segment .bss
segment .text
        global  main
        extern  printf
main:   xor     eax, eax
        mov     ebx, 10
        mov     ecx, 2
loop0:  inc     ebx
        add     ecx, 2
        inc     eax
        cmp     ecx, ebx
        jl      loop0
        push    eax
        push    offset out1
        call    printf
        add     esp, 8
        xor     eax, eax
        ret
        end

Answer 3

如果您还没有解决问题，则问题是由于 eax 在您的 printf 通话中受到打击。怎么修？在第一次调用printf 之前将其保存到缓冲区，并在第二次调用之前加载eax。例如。添加：

segment .bss

buffer resd  1          ; reserve 32 bit buffer
...
mov     [buffer], eax   ; save eax in the buffer

push    out1
call    printf

mov     eax, [buffer]   ; load eax from the buffer

push    eax
push    out2
call    printf

push    out3
call    printf

xor     ebx, ebx         ; exit nicely 
mov     eax, 1
int     0x80

使用/输出

$ ./bin/pt
It took 8 iterations to complete the loop. That seems like a lot.