如何在NASM x86_64 Linux Assembly中将字符串中的一个字符与另一个字符进行比较 [英] How to compare a char in a string with another char in NASM x86_64 Linux Assembly

问题描述

我实际上是想了解Intel x64语法中NASM汇编的基本概念，但是在尝试使strchr等效时遇到了一个问题...

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...

我一直在网上获取最大的信息，但是我不明白如何将一个字符串的当前字符(如str [i])与一个简单的字符进行比较.

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.

这里是测试主体:

#include <stdio.h>

extern char* my_strchr(char*, char);

int     main(void)
{
  char* str;

  str = my_strchr("foobar", 'b');
  printf("%s\n", 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为我提供了用c编写的strchr函数的输出，因此反汇编了:

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

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

但是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]

预先感谢您，希望有人会知道

Thank you in advance, Hope someone will know

推荐答案

rdi是一个指针，因此cmp rdi, 0检查空指针.您的意思是cmp byte [rdi + rcx], 0检查字符串的结尾.请注意，您需要检查当前字符，因此必须明显添加索引.

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毫无意义，因为您正在比较8字节的整个rsi.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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