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From: Ben Bacarisse <ben@bsb.me.uk>
Newsgroups: comp.lang.c
Subject: Re: "undefined behavior"?
Date: Sat, 15 Jun 2024 23:13:01 +0100
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Richard Harnden <richard.nospam@gmail.invalid> writes:

> On 15/06/2024 19:57, David Brown wrote:
>> If you want BBX_RGBA to be a typedef for an unsigned 32-bit integer,
>> write:
>>      typedef uint32_t BBX_RGBA;
>> If you want bbx_rgba() to be a function that is typesafe, correct, and
>> efficient (for any decent compiler), write :
>>      static inline BBX_RGBA bbx_rgba(uint32_t r, uint32_t g,
>>              uint32_t b, uint32_t a)
>>      {
>>          return (r << 24) | (g << 16) | (b << 8) | a;
>>      }
>> 
>
> Shouldn't that be ... ?
>
> static inline BBX_RGBA bbx_rgba(uint8_t r, uint8_t g,
>         uint8_t b, uint8_t a)

Maybe, but the function then needs more care as uint8_t will promote to
int and then r << 24 can be undefined.  One needs

  ((BBX_RGBA)r << 24) | (g << 16) | (b << 8) | a

(assuming that int is never going to be 16 bits or the same issue comes
up with the g << 16 shift).  Given this assumption, I'd just check that
unsigned int is at least 32 bits are use that for BBX_RGBA.

-- 
Ben.