Re: [RFC,PATCH] C99 format specifiers for fixed-length integer types

[Javier Martín]

El mié, 22-07-2009 a las 21:08 -0400, Pavel Roskin escribió:
> I doubt about "runtime weirdness".  gcc is good at catching such
> problems at the compile time.
Yes, in naked expressions. However, once we start using casts the C
compiler tends to be quite silent about the whole nitty-gritty process.
For example, the following code generates no warnings (with -ansi
-pedantic -Wall -Wconversion):

    unsigned short a = (unsigned short) 123456789;
    uint16_t b = (uint16_t) UINT32_C(123456789);

That is, casts shut the compiler up (without the casts gcc just shows a
warning about the truncation being performed). The runtime value of both
a and b is 0xcd15 in my x86_64 Linux box.

> It would be a good idea to use standard modifiers if they were not so
> hard on the eye.
Well, I suggested GRUB_x names for the macros, where x is the C99
standard name, but we could use them directly as PRIx64 and the like.
About verbosity, though... Have you programmed in Java? Some appearances
of GRUB_PRId32 are nothing compared to System.out.println or
java.util.ArrayList<Integer>, yet I write the last two religiously.

> That's a biased example where the size of the arguments is obvious.  And
> even then, it's hard to read.  And if you put GRUB_PRI32o instead of
> GRUB_PRI16o there, the compiler won't tell you anything.
Hey, I'm the one proposing the patch. Surely you wouldn't expect me to
be unbiased ;)
Nevertheless, the issue you pointed happens because variadic arguments
are automatically promoted with the following rules [1][2] from the
ancient days of C:
 - Integral types narrower than "int" -> int
 - Floating point types narrower than "double" -> double
So, you could argue, we could do without at the very least PRI?8 (and,
here in grub, PRI?16 too). However, given how obscure this "feature" is
(see [1]), I'd go for keeping all of them: orthogonality means less
surprises for the future coders.

> 
> The patch includes the easy part, namely adding the macros.  But I
> doesn't think it would be so pretty with the ugly part, that is,
> "fixing" every almost every *printf call.  It will be very hard to
> review.
Well, of course. This patch only adds the infrastructure, or else it
would be too invasive. Once it is in, a small number of people can start
checking most source files and replace the old specifiers when
appropriate.

> 
> > -#if GRUB_CPU_SIZEOF_VOID_P == 8
> > +#if GRUB_CPU_SIZEOF_LONG == 8
> 
> This belongs to a separate patch.
Oops, sorry. It seems I had a little mixup between the trunk files and
Bean's lib branch. I might post this change as a separate patch indeed.


[1] I would point at section 6.5.2.2.6 (function calls, automatic
argument promotions) of the C99 standard, but I really do not understand
that paragraph quite well.
[2] Here it says so with more certainty, but they seem to be class notes
http://www.eumus.edu.uy/eme/c/c-notes_summit/intermediate/sx11.html#sx11c


-- 
-- Lazy, Oblivious, Recurrent Disaster -- Habbit

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