undefined reference to... when using intel inline asm

[Ian Lance Taylor]

Lennyk <lennyk430@gmail.com> writes:

> Code (Window-style):
> int bswap(int n)
> {
>    __asm
>    {
>       mov eax, n
>       bswap eax
>       mov n, eax
>    }
>    return n;
> }
>
> After looking at some guides/tutorials I've translated this code to Linux:
> int bswap(int n)
> {
>    asm(".intel_syntax noprefix");
>    asm("mov eax, n");
>    asm("bswap eax");
>    asm("mov n, eax");
>    asm(".att_syntax noprefix");
>    return n;
> }
>
> Compilation passes - but the linker shouts: "undefined reference to `n'"
>
> What am I doing wrong? Shouldn't it be straight forward to translate
> these simple commands to Linux?

gcc inline assembler does not work like that.  You can't simply refer to
local variables in the assembler code.  I recommend the friendly manual:

http://gcc.gnu.org/onlinedocs/gcc-4.4.0/gcc/Extended-Asm.html

In this case, though, you shouldn't use inline assembler at all, just
use __builtin_bswap32.

http://gcc.gnu.org/onlinedocs/gcc-4.4.0/gcc/Other-Builtins.html

Ian