Re: [PATCH] Various pages: SYNOPSIS: Use VLA syntax in function parameters

Martin Uecker
2022-09-03T12:54:20Z

...
> >
> > Whether or not you feel like the manpages are the best place to
> > start that, I'll leave up to you!
>
> I'll try to defend the reasons to start this in the man-pages.
>
> This feature is mostly for documentation purposes, not being meaningful
> for code at all (for some meaning of meaningful), since it won't change
> the function definition in any way, nor the calls to it. At least not
> by itself; static analysis may get some benefits, though.

GCC will warn if the bound is specified inconsistently between
declarations and also emit warnings if it can see that a buffer
which is passed is too small:

https://godbolt.org/z/PsjPG1nv7

BTW: If you declare pointers to arrays (not first elements) you
can get run-time bounds checking with UBSan:

https://godbolt.org/z/TvMo89WfP

>
> Also, new code can be designed from the beginning so that sizes go
> before their corresponding arrays, so that new code won't typically be
> affected by the lack of this feature in the language.
>
> This leaves us with legacy code, especially libc, which just works, and
> doesn't have any urgent needs to change their prototypes in this regard
> (they could, to improve static analysis, but not what we'd call urgent).

It would be useful step to find out-of-bounds problem in
applications using libc.

> And since most people don't go around reading libc headers searching for
> function declarations (especially since there are manual pages that show
> them nicely), it's not like the documentation of the code depends on how
> the function is _actually_ declared in code (that's why I also defended
> documenting restrict even if glibc wouldn't have cared to declare it),
> but it depends basically on what the manual pages say about the
> function. If the manual pages say a function gets 'restrict' params, it
> means it gets 'restrict' params, no matter what the code says, and if it
> doesn't, the function accepts overlapping pointers, at least for most of
> the public (modulo manual page bugs, that is).
>
> So this extension could very well be added by the manual pages, as a
> form of documentation, and then maybe picked up by compilers that have
> enough resources to implement it.
>
>
> Considering that this feature is mostly about documentation (and a bit
> of static analysis too), the documentation should be something appealing
> to the reader.
>
>
> Let's take an example:
>
>
> int getnameinfo(const struct sockaddr *restrict addr,
> socklen_t addrlen,
> char *restrict host, socklen_t hostlen,
> char *restrict serv, socklen_t servlen,
> int flags);
>
> and some transformations:
>
>
> int getnameinfo(const struct sockaddr *restrict addr,
> socklen_t addrlen,
> char host[restrict hostlen], socklen_t hostlen,
> char serv[restrict servlen], socklen_t servlen,
> int flags);
>
>
> int getnameinfo(socklen_t hostlen;
> socklen_t servlen;
> const struct sockaddr *restrict addr,
> socklen_t addrlen,
> char host[restrict hostlen], socklen_t hostlen,
> char serv[restrict servlen], socklen_t servlen,
> int flags);
>
> (I'm not sure if I used correct GNU syntax, since I never used that
> extension myself.)
>
> The first transformation above is non-ambiguous, as concise as possible,
> and its only issue is that it might complicate the implementation a bit
> too much. I don't think forward-using a parameter's size would be too
> much of a parsing problem for human readers.

I personally find the second form not terrible. Being
able to read code left-to-right, top-down is helpful in more
complicated examples.

> The second one is unnecessarily long and verbose, and semicolons are not
> very distinguishable from commas, for human readers, which may be very
> confusing.
>
> int foo(int a; int b[a], int a);
> int foo(int a, int b[a], int o);
>
> Those two are very different to the compiler, and yet very similar to
> the human eye. I don't like it. The fact that it allows for simpler
> compilers isn't enough to overcome the readability issues.

This is true, I would probably use it with a comma and/or
syntax highlighting.

> I think I'd prefer having the forward-using syntax as a non-standard
> extension --or a standard but optional language feature-- to avoid
> forcing small compilers to implement it, rather than having the GNU
> extension standardized in all compilers.

The problems with the second form are:

- it is not 100% backwards compatible (which maybe ok though) as
the semantics of the following code changes:

int n;
int foo(int a[n], int n); // refers to different n!

Code written for new compilers could then be misunderstood
by old compilers when a variable with 'n' is in scope.

- it would generally be fundamentally new to C to have
backwards references and parser might need to be changes
to allow this

- a compiler or tool then has to deal also with ugly
corner cases such as mutual references:

int foo(int (*a)[sizeof(*b)], int (*b)[sizeof(*a)]);

We could consider new syntax such as

int foo(char buf[.n], int n);

Personally, I would prefer the conceptual simplicity of forward
declarations and the fact that these exist already in GCC
over any alternative. I would also not mind new syntax, but
then one has to define the rules more precisely to avoid the
aforementioned problems.

Martin