Passing variable from assembler to 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);
}

I would like to pass some data from assembler to main via the result variable. Is this possible? My assembler code causes a Segmentation fault (core dumped). I am using Ubuntu 15.10 x86_64, gcc 5.2.1.

GCC has Extended ASM for this, allowing you to refer to an output variable in that __asm__ fragment. — user824425
– user824425, Commented Nov 19, 2015 at 15:32
To add to that: the code as it stands allocates space for result in the program's code segment, and .long 0 produces two add %al,(%rax) instructions. — Mark Plotnick
– Mark Plotnick, Commented Nov 19, 2015 at 15:49
If you want to read the clock, why not just use unsigned long long a = __builtin_ia32_rdtsc()? Then you don't need to write any asm. — David Wohlferd
– David Wohlferd, Commented Nov 19, 2015 at 22:27

Jester · Accepted Answer · 2015-11-19 16:23:03Z

1

A better approach could be:

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;
}

answered Nov 19, 2015 at 16:23

Jester

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