[TUHS] yet another C discussion (YACD) and: Rust is not C++

[Dan Cross]

On Mon, Jan 30, 2023 at 8:49 PM Alejandro Colomar
<alx.manpages at gmail.com> wrote:
> Hello Ron,
>
> On 1/30/23 20:35, ron minnich wrote:
> > I don't know how many ways there are to say this, but Rust and C/C++ are
> > fundamentally different at the lowest level.
> >
> > If you are just looking at Rust syntax in a superficial way, you might be
> > excused for thinking it's "C with features / C++ with differences."
> >
> > But that's not how it is. It's like saying C is "just like assembly" because
> > labels have a ':' in them; or that Unix is "just like RSX" because they have
> > vaguely similar commands.
> >
> > Here's a real question that came up where I work: should the code shown below be
> > accepted (this is abstracted from a real example that is in use ... everywhere)?
> > We had one code analyzer that said, emphatically, NO; one person said YES,
> > another MAYBE. One piece of code, 3 answers :-)
> >
> > char f() {
> >     char *y;
> >     g(&y);
> >     return *y;
> > }
> >
> >
> > A specific question: should y be initialized to NULL?
>
> No.  At least not if you don't want to use the value NULL in your program.
> Using NULL as something to avoid Undefined Behavior is wrong, and it will
> contribute to hide programmer errors.

Sorry, I think this misses the point: how do you meaningfully tell
that `g` did something to `y` so that it's safe to indirect in the
`return`?

On the other hand, one could write,

char f() {
    char *y = NULL;
    g(&y);
    if (y == NULL)
        panic("g failed");
    return *y;
}

C, of course, can't tell in the original. And while you can now tell
that `g` did _something_ to `y`, you still really don't know that `y`
points to something valid.

> These days, compilers and static analyzers are smart enough to detect
> uninitialized variables, even across Translation Units, and throw an error,
> letting the programmer fix such bugs, when they occur.

In many cases, yes, but not in all. That would be equivalent to
solving the halting problem.

> The practice of initializing always to NULL and 0 provides no value, and
> silences all of those warnings, thus creating silent bugs, that will bite some
> cold winter night.
>
> I know some static analyzers (e.g., clang-tidy(1)) do warn when you don't
> initialize variables and especially pointers (well, you need to enable the
> warning that does that, but it can warn).  That warning is there due to some
> coding style or certifications that require it.  I recommend disabling those
> bogus warnings, and forgetting about the bogus coding style or certification
> that requires you to write bogus code.

Oh my.

> > The case to set y to NULL: otherwise it has an unknown value and it's unsafe.
>
> Is an undefined value less safe than an unexpected one?  I don't think so.  At
> least compilers can detect the former, but not the latter.
>
> > The case against setting y to NULL: it is pointless, as it slows the code down
> > slightly and g is going to change it anyway.
>
> Performance is a very minor thing.  But it's a nice side-effect that doing the
> right thing has performance advantages.  Readability is a good reason (and in
> fact, the compiler suffers that readability too, which is the cause of the
> silencing of the wanted warnings.
>
> > The case maybe: Why do you trust g() to always set it? Why don't you trust g()?
> > convince me.
>
> Well, it depends on the contract of g().  If the contract is that it may not
> initialize the variable, then sure, initialize it yourself, or even better,
> check for g()'s errors, and react when it fails and doesn't initialize it.
>
> If the contract is that it should always initialize it, then trust it blindly.
> The compiler will tell you when it doesn't happen (that is, when g() has a bug).

The number of situations where the compiler can't tell whether `g` has
a bug is unbounded.

> > You can't write this in Rust with this ambiguity. It won't compile. In fact, &
> > doesn't mean in Rust what it does in C.
>
> I don't know Rust.  Does it force NULL initialization?  If so, I guess it's a
> bad design choice.  Unless Rust is so different that it can detect such
> programmer errors even having defined default initialization, but I can't
> imagine how that is.

Rust enforces that all variables must be initialized prior to use.
Whether they're initialized with a zero value or something else is up
to the programmer; but not initializing is a compile-time error.

For example:

| fn main() {
|     let x;
|     if thing_is_true() {
|         x = 5;
|     } else {
|         x = 3;
|     }
|     println!("x={x}");
| }

In fact, this is good; this allows us to employ a technique called,
"Type-Driven Development", whereby we can create some type that
encodes an invariant about the object. An object of that type is
written in such a way that once it has been initialized, the mere
existence of the object is sufficient to prove that the invariant
holds, and need not be retested whenever the object is used. For
example:

| #[repr(transparent)]
| struct PageFrameAddr(u64);
| impl PageFrameAddr {
|     fn new_round_down(addr: u64) -> PageFrameAddr {
|         PageFrameAddr(addr & !0xFFF)
|     }
| }

Here, "PageFrameAddr" contains a 4KiB-aligned page address.  Since the
only way to create one of these is by the, `new_round_down` associated
method that masks off the low bits, we can be sure that if we get one
of these, the contained address is properly aligned.  In C, we'd
pretty much have to test at the site of use.

This is an extremely powerful technique; cf Alexis King's blog post,
"Parse Don't Validate"
(https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-validate/)
and Cliff Biffle's talk on the Hubris embedded RTOS
(https://talks.osfc.io/osfc2021/talk/JTWYEH/).

> > Sorry to be such a pedant, but I was concerned that we not fall into the "Rust
> > is C++ all over again" trap.
> >
> > As for replacing C, the velocity of Rust is just astonishing. I think folks have
> > been waiting for something to replace C for a long time, and Rust, with all its
> > headaches and issues, is likely to be that thing.
>
> Modern C is receiving a lot of improvements from C++ and other languages.  It's
> getting really good in fixing the small issues it had in the past (and GNU C
> provides even more good things).  GNU C2x is quite safe and readable, compared
> to say ISO C99 versions.

C23 looks like it will be a better language that C11, but I don't know
that even JeanHeyd would suggest it's "quite safe". :-/

        - Dan C.


> I don't think C will ever be replaced.  And I hope it doesn't.
>
> Possibly, something like with Plan9 and Unix/Linux will happen.  The good things
> from other languages will come back in one form or another to C.  The
> not-so-good ones will be discarded.
>
> >
> > Personally, I still prefer Go, but I can also see which way the wind is blowing,
> > especially when I see Rust use exploding in firmware and user mode, and now even
> > in the Linux kernel.
>
> Cheers,
>
> Alex