Beginner Inline Assembly Segmentation fault

Question

I am writing Inline assembly for the first time and I don't know why I'm getting a Seg fault when I try to run it.

#include <stdio.h>
int very_fast_function(int i){
    asm volatile("movl %%eax,%%ebx;"
        "sall $6,%%ebx;"
        "addl $1,%%ebx;"
        "cmpl $1024,%%ebx;"
        "jle Return;"
        "addl $1,%%eax;"
        "jmp End;"
        "Return: movl $0,%%eax;"
        "End: ret;": "=eax" (i) : "eax" (i) : "eax", "ebx" );
    return i;
    /*if ( (i*64 +1) > 1024) return ++i;
    else return 0;*/
}

int main(int argc, char *argv[])
{
    int i;
    i=40;
    printf("The function value of  i is %d\n", very_fast_function(i));
    return 0;
}

Like I said this is my first time so if it's super obvious I apologize.

And have a look at the setg instruction. If you empty eax at the beginnning, after the cmpl you can do setg %al and be done with it. — Marco van de Voort
– Marco van de Voort, Commented Nov 18, 2013 at 9:17

starrify · Accepted Answer · 2013-11-18 05:38:55Z

1

You shall not use ret directly. Reason: there're initialization like push the stack or save the frame pointer when entering each function, also there're corresponding finalization. You just leave the stack not restored if use ret directly.

Just remove ret and there shall not be segmentation fault.

However I suppose the result is not as expected. The reason is your input/output constrains are not as expected. Please notice "=eax" (i) you write does not specify to use %%eax as the output of i, while it means to apply constraint e a and x on output variable i.

For your purpose you could simply use r to specify a register. See this edited code which I've just tested:

asm volatile("movl %1,%%ebx;"
    "sall $6,%%ebx;"
    "addl $1,%%ebx;"
    "cmpl $1024,%%ebx;"
    "jle Return;"
    "addl $1,%0;"
    "jmp End;"
    "Return: movl $0,%0;"
    "End: ;": "=r" (i) : "r" (i) : "ebx" );

Here To use %%eax explicitly, use "=a" instead of "=r".

For further information, please read this http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html

edited Nov 18, 2013 at 5:38

answered Nov 18, 2013 at 5:28

starrify

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2 Comments

user2926342 Over a year ago

Awesome, thank you very much for your help! Fixed everything!

starrify Over a year ago

@user2926342 It's always such a pleasure to help :D

user149341 · Accepted Answer · 2013-11-18 05:28:17Z

0

ret should not be used in inline assembly blocks - the function you're in needs some cleanup beyond what a simple ret will handle.

Remember, inline assembly is inserted directly into the function it's embedded in. It's not a function unto itself.

answered Nov 18, 2013 at 5:28

user149341

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