非常自我解释的代码。为什么不起作用呢?
#include <stdio.h>
int main() {
__asm__("number dw 0"); // declare number?
printf("%d",number);
__asm__("mov %eax,number"
"inc %eax"
"mov number,%eax");
printf("%d",number);
return 0;
}
cc ex1.c -o ex1
ex1.c: In function ‘main’:
ex1.c:22:17: error: ‘number’ undeclared (first use in this function)
ex1.c:22:17: note: each undeclared identifier is reported only once for each function it appears in
make: *** [ex1] Error 1
感谢。
我有很多知识空白要填补...... gcc手册让我对内联汇编感到困惑,谷歌搜索教程的结果......
使用intel i7处理器
答案 0 :(得分:5)
使用此语法,您可以从内联程序集
访问C
中声明的变量
#include <stdio.h>
int main() {
int number = 0;
printf("%d\n",number);
asm(
"mov %[number],%%eax\n"
"inc %%eax\n"
"mov %%eax,%[number]\n"
: [number] "=m" (number) : "m" (number) : "eax", "cc" );
printf("%d\n",number);
return 0;
}
您可以通过在输入上指定number
约束,让编译器为您eax
加载"a"
#include <stdio.h>
int main() {
int number = 0;
printf("%d\n",number);
asm(
"inc %%eax\n"
"mov %%eax,%[number]\n"
: [number] "=m" (number) : "a" (number) : "cc" );
printf("%d\n",number);
return 0;
}
由于x86 inc
指令可以直接对内存进行操作,因此可以将其减少到此
#include <stdio.h>
int main() {
int number = 0;
printf("%d\n",number);
asm(
"incl %[number]\n" /* incl -> "long" (32-bits) */
: [number] "=m" (number) : "m" (number) : "cc" );
printf("%d\n",number);
return 0;
}
有关更多信息,请参阅gcc文档: