On Thu, May 02, 2024 at 01:45:27AM +0200, Alejandro Colomar wrote:
> [CC += libc-alpha (glibc), Richard]
> 
> Hi Rich Felker,
> 
> On Wed, May 01, 2024 at 07:04:38PM -0400, Rich Felker wrote:
> > On Wed, May 01, 2024 at 09:55:10PM +0200, Alejandro Colomar wrote:
> > > On Wed, May 01, 2024 at 01:21:39PM -0400, Rich Felker wrote:
> > > > It doesn't matter either way because musl's s[n]printf is AS-safe.
> > > 
> > > Hmm; interesting.  Thanks!
> > 
> > Yes, it's a pure function (aside from fenv, errno for %m, and possibly
> > LC_NUMERIC in the future) and has no reason to do anything AS-unsafe
> > unless you implement it with dynamic allocation, in which case you
> > have unforced failure cases which are very low QoI.
> > 
> > musl's printf core also has very low stack usage suitable for AS use,
> > at least in principle. LLVM and possibly modern GCC like to
> > inline-and-lift the slightly-large (IIRC something like 6-8k on
> > ld80/IEEE-quad archs, 2k on ld64 archs) floating point workspace to be
> > allocated unconditionally, but if you can suppress that, it should
> > only need a few hundred bytes of stack.
> > 
> > dprintf is also AS-safe (as intended by its creator; this was
> > discussed on the glibc list a few years back)

I've been digging into the archives, and found it:
<https://inbox.sourceware.org/libc-alpha/20130925180327.0351F2C097@topped-with-meat.com/>

But then it seems that, at least in 2013, it wasn't AS-safe:
<https://inbox.sourceware.org/libc-alpha/20130925212954.GQ20515@brightrain.aerifal.cx/>

It would be interesting to know the status as of today (if I have to
guess, I'd bet it's unsafe), and also if there could be any guarantees
that at least a subset of dprintf(3) was guaranteed to be AS-safe (e.g.,
ignoring '$', wide-char, ...).

> I realize that dprintf(3) is not documented in the ATTRIBUTES section of
> its manual page.  POSIX doesn't seem to document AS safety of it (or of
> most functions FWIW).  glibc's manual doesn't seem to document
> dprintf(3) at all.  I guess I should fix that.
> 
> The BSDs don't seem to document it as being AS-safe either.  NetBSD
> mentions the existence of snprintf_ss(3), but nothing about dprintf(3).
> FreeBSD is silent.  OpenBSD is silent too.
> 
> I've CCed glibc so that they confirm that this is MT-safe + AS-safe on
> glibc.  I guess if the original design was to have it AS-safe, we can
> report bugs to the BSDs so that they document their AS safety status,
> and that they make the function AS-safe if it isn't already.
> 
> Also, having dprintf(3) documented as AS-safe by design would be a great
> standard solution for my original interest, which was finding a libc
> portable AS-safe replacement for printf(3).  I could just
> 
> 	dprintf(STDOUT_FILENO, ...)
> 
> and avoid any hand-written wrappers around write(1).
> 
> > and even fprintf is
> > under the condition that you're not interrupting code accessing the
> > same FILE you pass to it.
> > 
> > Rich
> 
> Have a lovely night!
> Alex

-- 
<https://www.alejandro-colomar.es/>