Re: [PATCH (2) 34/34] unix.7: Use sizeof consistently

Michael Kerrisk (man-pages)
2020-09-05T14:39:10Z

Hello Alex,

On Sat, 5 Sep 2020 at 11:37, Alejandro Colomar <colomar.6.4.3@gmail.com> wrote:
>
> Hi Michael,
>
> On 9/5/20 10:27 AM, Michael Kerrisk (man-pages) wrote:
[...]
> So, I've still not processed patches 21, 22, and 29. And in review,
> > I see that I am wondering about whether I should maintain 1, 5, 17,
> > 18, and 19. These all involve the use of malloc() or similar.
> >
> > The existing pattern was something like:
> >
> > struct mytype *x; // Or some simple type such as 'int'
> > ...
> > x = malloc(n * sizeof(struct mytpe));
>
> Not to forget `malloc(sizeof(struct mytpe) * n);`

<Cough> yes <cough>...

> > and your patches change it to:
> >
> > struct mytype *x;
> > ...
> > x = malloc(n * sizeof(*x));>
> > I'm not sure that always helps readability.
> >
> > Part of the problem is the use of C90 in the code.
> >
> > Do you both agree with me that both of the following c99
> > forms are better than the original:
> >
> > struct mytype *x = malloc(n * sizeof(struct mytpe));
> > struct mytype *x = malloc(n * sizeof(*x));
> >
> > ?
>
> Yes, I would say both of these are an improvement.
> >
> > I *think* I mildly prefer the first form, but I'm open to
> > arguments that the latter form is preferable. Of course, the
> > fact that there might be more than one point where an 'alloc'
> > is done and assigned to 'x' may influence the argument. Thus
> >
> >
> > struct mytype *x = malloc(n * sizeof(struct mytpe));
> > ...
> > x = malloc(p * sizeof(struct mytype));
> >
> > vs
> >
> > struct mytype *x = malloc(n * sizeof(*x));
> > ...
> > x = malloc(p * sizeof(*x));
>
> In case there are 2 or more allocs, in general, I prefer the name of the
> variable.

Yes, by the time I'd written the two allocs examples, I'd started to
lean that way too, but didn't say so because I wanted to hear your
independent perspective .

> In case there is only 1 alloc in the same line as the declaration, I
> still prefer the name of the variable: for consistency, and because some
> day you may add another alloc, and then separate the original
> declaration+alloc in two lines, and forget to fix sizeof to use the name
> of the variable.

Yes.

> The cases where I see the type much better are cases where it is
> impossible for the type to change (and if it ever changed it would be an
> accident and cause a deserved bug) such as in those cases where you
> really need an (u)int64_t because of the API.

Yes.

> There's also cases where in real code I would prefer the name of the
> variable (to avoid future bugs because of type change), but in the man
> pages it is clearer if you write the type to be more explicit and
> consistent. Example: queue.3 (PATCH 24/34): It's clearer if you
> consistently use the type across all the code (and it may be therefore
> better to use it in the man-pages), because the name of the variable
> looks like it's different from one alloc to the next, but I can imagine
> some real code implementing a TAILQ and later deciding to use a CIRCLEQ,
> and if any of the types in the allocation are not updated accordingly,
> there will appear bugs, while if the name of the node is used for
> allocating the memory, the transition will be really simple.

Agreed.

I've applied patches 21, 22, and 29. And then in line with our
discussion above, I moved some pages to the style discussed above:

struct mytype *x = malloc(sizeof(*x));

See commit 48d05103071830b6708a3ecabeddcdef5f3daa44.

Thanks for your input Alex, it's been really helpful!

Cheers,

Michael

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/