通过内联汇编操作 c 变量 [英] manipulating c variable via inline assembly

问题描述

可能的重复:
如何访问 c 变量进行内联汇编操作

鉴于此代码:

#include <stdio.h>

int main(int argc, char **argv)
{
  int x = 1;
  printf("Hello x = %d
", x);


  }

我想在内联汇编中访问和操作变量 x.理想情况下，我想使用内联汇编更改其值.GNU 汇编器，并使用 AT&T 语法.假设我想在 printf 语句之后将 x 的值更改为 11，我该怎么做?

I'd like to access and manipulate the variable x in inline assembly. Ideally, I want to change its value using inline assembly. GNU assembler, and using the AT&T syntax. Suppose I want to change the value of x to 11, right after the printf statement, how would I go by doing this?

推荐答案

asm() 函数遵循以下顺序:

asm ( "assembly code"
           : output operands                  /* optional */
           : input operands                   /* optional */
           : list of clobbered registers      /* optional */
);

并通过您的 c 代码将 11 与 x 结合起来:

and to put 11 to x with assembly via your c code:

int main()
{
    int x = 1;

    asm ("movl %1, %%eax;"
         "movl %%eax, %0;"
         :"=r"(x) /* x is output operand and it's related to %0 */
         :"r"(11)  /* 11 is input operand and it's related to %1 */
         :"%eax"); /* %eax is clobbered register */

   printf("Hello x = %d
", x);
}

您可以通过避免损坏的寄存器来简化上面的 asm 代码

You can simplify the above asm code by avoiding the clobbered register

asm ("movl %1, %0;"
    :"=r"(x) /* related to %0*/
    :"r"(11) /* related to %1*/
    :);

您可以通过避免输入操作数并使用来自 asm 的局部常量值而不是来自 c 的局部常量值来进一步简化:

You can simplify more by avoiding the input operand and by using local constant value from asm instead from c:

asm ("movl $11, %0;" /* $11 is the value 11 to assign to %0 (related to x)*/
    :"=r"(x) /* %0 is related x */
    :
    :);

另一个例子:比较2个数字和汇编

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