Re: [Qemu-devel] Build broken

[Stefan Hajnoczi <stefanha@gmail.com>]

On Fri, Aug 5, 2011 at 8:29 AM, Kevin Wolf <kwolf@redhat.com> wrote:
> Am 05.08.2011 08:22, schrieb malc:
>>
>> /home/malc/x/rcs/git/qemuorg/coroutine-ucontext.c: In function 'coroutine_new':
>> /home/malc/x/rcs/git/qemuorg/coroutine-ucontext.c:160:16: error: 'arg.i[1]' may be used uninitialized in this function
>> /home/malc/x/rcs/git/qemuorg/coroutine-ucontext.c:136:18: note: 'arg.i[1]' was declared here
>>
>> diff --git a/coroutine-ucontext.c b/coroutine-ucontext.c
>> index 41c2379..42dc3e2 100644
>> --- a/coroutine-ucontext.c
>> +++ b/coroutine-ucontext.c
>> @@ -133,7 +133,7 @@ static Coroutine *coroutine_new(void)
>>      CoroutineUContext *co;
>>      ucontext_t old_uc, uc;
>>      jmp_buf old_env;
>> -    union cc_arg arg;
>> +    union cc_arg arg = {0};
>>
>>      /* The ucontext functions preserve signal masks which incurs a system call
>>       * overhead.  setjmp()/longjmp() does not preserve signal masks but only
>>
>> I guess gcc should yell not only here on ppc32 but on any machine where
>> pointer size is less than the size of two ints.
>
> Stefan, why does this code even exist again? I think at some point I had
> it changed to just use a static variable in order to avoid doing this
> kind of tricks with unions.

virtfs are using coroutines in multiple threads at the same time.
Introducing a global variable wouldn't be thread-safe.

The real problem is that makecontext(3) has a bad function signature.
There's no nice fix - whatever we do will be ugly.

Using a union is the way it should be done in C.  The code doesn't
look pretty but it doesn't introduce global state.

Stefan