Linux assembler error "impossible constraint in ‘asm’"

linux gcc assembly inline-assembly
slashmais picture slashmais · Sep 25, 2009 · Viewed 30.9k times · Source

I'm starting with assembler under Linux. I have saved the following code as testasm.c
and compiled it with: gcc testasm.c -otestasm
The compiler replies: "impossible constraint in ‘asm’".

#include <stdio.h>
int main(void)
{
    int foo=10,bar=15;

    __asm__ __volatile__ ("addl %%ebx,%%eax"
        : "=eax"(foo) 
        : "eax"(foo), "ebx"(bar) 
        : "eax" 
    );

    printf("foo = %d", foo);

    return 0;
}

How can I resolve this problem? (I've copied the example from here.)

Debian Lenny, kernel 2.6.26-2-amd64
gcc version 4.3.2 (Debian 4.3.2-1.1)

Resolution:
See the accepted answer - it seems the 'modified' clause is not supported any more.

Answer

Stephen Canon picture Stephen Canon · Sep 25, 2009 
__asm__ __volatile__ ("addl %%ebx,%%eax" : "=a"(foo) : "a"(foo), "b"(bar));

seems to work. I believe that the syntax for register constraints changed at some point, but it's not terribly well documented. I find it easier to write raw assembly and avoid the hassle.

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