将变量从汇编程序传递给C

Question

#include <stdio.h>

int main(){
        __asm__ (
                "result:    \n\t"
                ".long 0    \n\t" 
                "rdtsc      \n\t"
                "movl %eax, %ecx\n\t"
                "rdtsc      \n\t"
                "subl %ecx, %eax\n\t"
                "movl %eax, result\n\t"
        );

        extern int result;
        printf("%d\n", result);
}

我想通过main变量将汇编程序中的一些数据传递给result。这可能吗？我的汇编程序代码导致Segmentation fault (core dumped)。我使用的是Ubuntu 15.10 x86_64，gcc 5.2.1。

Answer 1

更好的方法可能是：

int main (void)
{
    unsigned before, after;

    __asm__
    (
        "rdtsc\n\t"
        "movl %%eax, %0\n\t"
        "rdtsc\n\t"
        : "=rm" (before), "=a" (after)
        : /* no inputs */
        : "edx"
    );

    /* TODO: check for after < before in case you were unlucky
     * to hit a wraparound */
    printf("%u\n", after - before);
    return 0;
}