如何让strcmp在汇编中返回0

发布于 2024-11-07 07:18:41 字数 803 浏览 0 评论 0原文

我希望对 strcmp 函数的调用返回 0，这意味着

int strncmp(const char *s1, const char *s2, size_t n);

const char *s1 和 const char *s2 应包含相同的字符串。如果s2指向字符串“hello”并且n是4，我如何将一个也对应于<的十进制值传递给s1代码>你好？

 8049e87:       c7 44 24 08 04 00 00    movl   $0x4,0x8(%esp) // 4
 8049e8e:       00
 8049e8f:       c7 44 24 04 80 bd 04    movl   $0x804bd80,0x4(%esp) // the constant is "hello"
 8049e96:       08 
 8049e97:       89 04 24                mov    %eax,(%esp) // The contents of %eax are a decimal (%d)
 8049e9a:       e8 61 ec ff ff          call   8048b00 <strncmp@plt>
 8049e9f:       85 c0                   test   %eax,%eax // I want this to be 0!

我尝试以 ASCII 形式传递“h”的十进制值，这似乎是正确的方向，但不完全正确。

原文

I want the call to the strcmp function to return 0, which means

int strncmp(const char *s1, const char *s2, size_t n);

const char *s1 and const char *s2 should contain the same string. If s2 points to the string "hello" and n is 4, how can I pass to s1 a decimal value that will also correspond to hello?

 8049e87:       c7 44 24 08 04 00 00    movl   $0x4,0x8(%esp) // 4
 8049e8e:       00
 8049e8f:       c7 44 24 04 80 bd 04    movl   $0x804bd80,0x4(%esp) // the constant is "hello"
 8049e96:       08 
 8049e97:       89 04 24                mov    %eax,(%esp) // The contents of %eax are a decimal (%d)
 8049e9a:       e8 61 ec ff ff          call   8048b00 <strncmp@plt>
 8049e9f:       85 c0                   test   %eax,%eax // I want this to be 0!

I tried passing in the decimal value for "h" in ASCII, and it seemed to be the right direction, but not fully.

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绅士风度i 2024-11-14 07:18:41

根据定义，对于大小写和长度相同的两个字符串，strncmp 的返回值为零。

查看您的汇编代码，行：

test   %eax,%eax

不是 strncmp 函数的一部分。

使用调试器，在此指令处放置一个断点。检查EAX寄存器，它应该为零（取决于strncmp函数是否在EAX寄存器中返回其结果）。

test 汇编指令将根据参数的值设置条件代码。流行的条件代码位是零位，指示表达式为零。如果条件代码为零，则下一条指令可能是跳转。

如果您在数学语句或表达式中使用 strncmp 函数的结果，编译器可能会生成不同的代码。

试试这个片段：

  volatile int result = 0x55;
  volatile int a_value = 3;
  result = (strncmp("Hausaufgaben", "Hausaufgaben", 256) + 27) / 3;
  printf("Result is: %d\n", result);

您需要编译器保存 strncmp 中的值是否有原因？

您需要编译器将该值与常量数字零进行比较是否有原因？

By definition, the return value of strncmp is zero for two strings that are the same in case and length.

Looking at your assembly code, the line:

test   %eax,%eax

is not part of the strncmp function.

Using a debugger, put a breakpoint at this instruction. Examine the EAX register, it should be zero (depending if the strncmp function returns its result in the EAX register).

The test assembly instruction will set condition codes depending on the value of the parameters. A popular condition code bit is the zero bit indicating an expression is zero. The next instruction may be a jump if condition code is zero.

If you use the result of the strncmp function in a mathematical statement or expression, the compiler may generate different code.

Try this fragment:

  volatile int result = 0x55;
  volatile int a_value = 3;
  result = (strncmp("Hausaufgaben", "Hausaufgaben", 256) + 27) / 3;
  printf("Result is: %d\n", result);

Is there a reason you need the compiler to save the value from strncmp?

Is there a reason you need the compiler to compare the value to constant numeric zero?

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