Re: [musl] fix various warnings/theoretical UB

[Rich Felker <dalias@libc.org>]

On Mon, Jul 03, 2023 at 11:23:00PM +0200, Jens Gustedt wrote:
> Hello,
> 
> Am 3. Juli 2023 21:59:57 MESZ schrieb Rich Felker <dalias@libc.org>:
> > On Mon, Jul 03, 2023 at 01:55:57PM -0400, Alex Xu (Hello71) wrote:
> > > See attached patches.
> > 
> > > From 978f2cded65ce73450277d3fde48f038b339d5f9 Mon Sep 17 00:00:00 2001
> > > From: "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>
> > > Date: Sun, 2 Jul 2023 20:28:23 -0400
> > > Subject: [PATCH 1/4] volatile static -> static volatile
> > > 
> > > C11 6.11.5p1:
> > > 
> > > > The placement of a storage-class specifier other than at the
> > > > beginning of the declaration specifiers in a declaration is an
> > > > obsolescent feature.
> > > 
> > > gcc also warns about this.
> > > ---
> > >  src/time/timer_create.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/src/time/timer_create.c b/src/time/timer_create.c
> > > index cd32c945..9216b3ab 100644
> > > --- a/src/time/timer_create.c
> > > +++ b/src/time/timer_create.c
> > > @@ -61,7 +61,7 @@ static void *start(void *arg)
> > >  
> > >  int timer_create(clockid_t clk, struct sigevent *restrict evp, timer_t *restrict res)
> > >  {
> > > -	volatile static int init = 0;
> > > +	static volatile int init = 0;
> > >  	pthread_t td;
> > >  	pthread_attr_t attr;
> > >  	int r;
> > > -- 
> > > 2.41.0
> > 
> > No objection to this change. It's contrary to usual style. I would say
> > let's convert to pthread_once, but this code is slated for removal
> > anyway once signals are no longer used for SIGEV_THREAD timers.
> > 
> > > From b98f243e7921ddff6978ee9b0ce9f08efaa17951 Mon Sep 17 00:00:00 2001
> > > From: "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>
> > > Date: Sun, 2 Jul 2023 20:29:41 -0400
> > > Subject: [PATCH 2/4] __year_to_secs: fix dangling pointer
> > > 
> > > C11 6.5.2.5p5:
> > > 
> > > > If the compound literal occurs outside the body of a function, the
> > > > object has static storage duration; otherwise, it has automatic
> > > > storage duration associated with the enclosing block.
> > > 
> > > gcc also warns about this.
> > > ---
> > >  src/time/__year_to_secs.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/src/time/__year_to_secs.c b/src/time/__year_to_secs.c
> > > index 2824ec6d..d215880a 100644
> > > --- a/src/time/__year_to_secs.c
> > > +++ b/src/time/__year_to_secs.c
> > > @@ -10,9 +10,9 @@ long long __year_to_secs(long long year, int *is_leap)
> > >  		return 31536000*(y-70) + 86400*leaps;
> > >  	}
> > >  
> > > -	int cycles, centuries, leaps, rem;
> > > +	int cycles, centuries, leaps, rem, tmp;
> > >  
> > > -	if (!is_leap) is_leap = &(int){0};
> > > +	if (!is_leap) is_leap = &tmp;
> > >  	cycles = (year-100) / 400;
> > >  	rem = (year-100) % 400;
> > >  	if (rem < 0) {
> > > -- 
> > > 2.41.0
> > 
> > Seems like a bogus warning. The enclosing block is the whole function,
> 
> No, the `if` statement forms a block of itself, and then the
> dependent statement forms yet another block.
> 
> We rectify the terminology a bit in C23 hopefully make it easier to
> read without changing semantics

Oh, yes, somehow I always forget this. I think we actually remedy it
somewhere else using ?: instead of if, which is a rather hilarious
footgun for anyone who goes gratuitously changing ?: to if for style
reasons...

Anyway, in that case this seems like a reasonable change, though
"dummy" would be a more appropriate var name than "tmp" I think.

Rich