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From: Ben Bacarisse <ben@bsb.me.uk>
Newsgroups: comp.lang.c
Subject: Re: "undefined behavior"?
Date: Fri, 14 Jun 2024 22:29:00 +0100
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Malcolm McLean <malcolm.arthur.mclean@gmail.com> writes:

> On 14/06/2024 12:44, Ben Bacarisse wrote:
>> Malcolm McLean <malcolm.arthur.mclean@gmail.com> writes:
>> 
>>> On 14/06/2024 00:55, Ben Bacarisse wrote:
>>>> Malcolm McLean <malcolm.arthur.mclean@gmail.com> writes:
>>>>
>>>>> On 13/06/2024 19:01, bart wrote:
>>>>
>>>>>> And here it just gets even uglier. You also get situations like this:
>>>>>>        uint64_t i=0;
>>>>>>        printf("%lld\n", i);
>>>>>> This compiles OK with gcc -Wall, on Windows64. But compile under Linux64
>>>>>> and it complains the format should be %ld. Change it to %ld, and it
>>>>>> complains under Windows.
>>>>>> It can't tell you that you should be using one of those ludicrous macros.
>>>>>> I've also just noticed that 'i' is unsigned but the format calls for
>>>>>> signed. That may or may not be deliberate, but the compiler didn't say
>>>>>> anything.
>>>>>>
>>>>> Exactly. We can't have this just to print out an integer.
>>>> This is how C works.  There's no point in moaning about it.  Use another
>>>> language or do what you have to in C.
>>>>
>>>>> In Baby X I provide a function called bbx_malloc(). It's is guaranteed
>>>>> never to return null. Currently it just calls exit() on allocation failure.
>>>>> But it also limits allocation to slightly under INT_MAX. Which should be
>>>>> plenty for a Baby program, and if you want more, you always have big boy's
>>>>> malloc.
>>>> And if you need to change the size?
>>>>
>>>>> But at a stroke, that gets rid of any need for size_t,
>>>> But sizeof, strlen (and friends like the mbs... and wcs... functions),
>>>> strspn (and friend), strftime, fread, fwrite. etc. etc. all return
>>>> size_t.
>>>>
>>> But these are not Baby X functions.
>> Neither is malloc but you wanted t replace that to get rid of the need
>> for size_t.
>> I confess that I am all at sea about what you are doing.  In essence, I
>> don't understand the rules of the game so I should probably just stop
>> commenting.
>> 
> Yes, I really need to get that website together so that people cotton on to
> what Baby X is, what it can and cannot do, and what is the point.

I know what Baby X is.  I don't know why "these are not Baby X
functions" applies to the ones I listed and not to malloc.

....
> However if you need to pass a colour value to a fuction, you normall pass a
> BBX_RGBA value, which is typedefed to unsigned long, and is opaque, and you
> query the channels using the macros in bbx_color.h
>
> #ifndef bbx_color_h
> #define bbx_color_h
>
> typedef unsigned long BBX_RGBA;


Curious.  The macros below seem to assume that int is 32 bits, so why
use long?

> #define bbx_rgba(r,g,b,a) ((BBX_RGBA) ( ((r) << 24) | ((g) << 16) | ((b) <<
> 8) | (a) ))

This is likely to involve undefined behaviour when r >= 128.  (I presume
you are ruling out int narrower than 32 bits or there are other problems
as well.)

> #define bbx_rgb(r, g, b) bbx_rgba(r,g,b, 255)
> #define bbx_red(col) ((col >> 24) & 0xFF)
> #define bbx_green(col) ((col >> 16) & 0xFF)
> #define bbx_blue(col) ((col >> 8) & 0xFF)
> #define bbx_alpha(col) (col & 0xFF)

It might not be an issue (as col is opaque and unlikely to be an
expression) but I'd still write (col) here to stop the reader having to
check or reason that out.

> #define BBX_RgbaToX(col) ( (col >> 8) & 0xFFFFFF )
>
> #endif
>
> The last macro is to make it easier to interface with Xlib, and has the
> prefix BBX_ (upper case) indicating that it is for internal use by the bbx
> library / system and not meant for user programs.

As a reader of the code, I made exactly the reverse assumption.  When I
see lower-case macros I assume they are for internal use.

-- 
Ben.