如何将字符串中的字符与 NASM x86_64 Linux 程序集中的另一个字符进行比较

问题描述

我实际上是在尝试理解 Intel x64 语法中 NASM 程序集的基本概念，但在尝试使 strchr 等效时遇到问题...

I'm actually trying to understand the basic concepts of NASM Assembly in Intel x64 syntax but facing an issue while trying to make a strchr equivalent...

我一直在网上航行以获取最大信息，但我无法理解如何将字符串的当前字符(如 str[i])与简单字符进行比较.

I've been sailing the web to get the maximum information but I can't understand how to compare the current char of a string (like str[i]) with a simple char.

这里是测试主要内容:

#include <stdio.h>

extern char* my_strchr(char*, char);

int     main(void)
{
  char* str;

  str = my_strchr("foobar", 'b');
  printf("%s
", str);
  return 0;
}

这是我的汇编代码:

我假设 rdi 是我的字符串，rsi 是我的一字节数据.

I assume that rdi is my string and rsi my one-byte data.

my_strchr:
    push    rcx                  ;Save our counter
    xor     rcx, rcx             ;Set it to 0
loop:
    cmp     rdi, byte 0          ;Check the end of string
    jz      end
    cmp     rsi, [byte rdi+rcx]  ;Here is the point ...
    jz      end
    inc     rcx                  ;increment our counter
    jmp     loop
end:
    mov     rax, [rdi+rcx]       ;Are the brackets needed ? Is it equivalent to '&' in C ?
    pop     rcx
    ret

Gdb 给了我这个用 c 编写的 strchr 函数的输出，所以反汇编了:

Gdb gave me this output for the strchr function written in c and so disassembled :

....
cmp    al,BYTE PTR [rbp-0x1c]
....

但我的实际上是这样做的:

But mine is actually doing this :

0x400550 <my_strchr>            push   rcx
0x400551 <my_strchr+1>          xor    rcx,rcx
0x400554 <loop>                 cmp    rdi,0x0
0x400558 <loop+4>               je     0x400566 <end>
0x40055a <loop+6>               cmp    rsi,QWORD PTR [rdi+rcx*1+0x0]

先谢谢你，希望有人知道

Thank you in advance, Hope someone will know

推荐答案

rdi 是一个指针，所以 cmp rdi, 0 检查空指针.你的意思是 cmp byte [rdi + rcx], 0 来检查字符串的结尾.注意您需要检查当前字符，因此必须添加索引.

rdi is a pointer, so cmp rdi, 0 checks for a null pointer. What you meant was cmp byte [rdi + rcx], 0 to check the end of string. Note you need to check the current character so have to add the index obviously.

至于 cmp rsi, [byte rdi+rcx] byte 没有任何意义，因为您正在比较整个 rsi这是 8 个字节.那应该是 cmp sil, [rdi + rcx].

As for cmp rsi, [byte rdi+rcx] the byte there makes no sense, since you are comparing the whole of rsi which is 8 bytes. That should be cmp sil, [rdi + rcx].

最后，strchr 应该返回一个指针，因此您应该将 mov rax, [rdi+rcx] 更改为 lea rax, [rdi + rcx].

Finally, strchr is supposed to return a pointer so you should change mov rax, [rdi+rcx] to lea rax, [rdi + rcx].

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