Implicit assumptions (was: Re: Some fun with -O2)

[Mark Millard]

On 2021-Jan-15, at 10:35, Mark Millard <marklmi at yahoo.com> wrote:

> On 2021-Jan-15, at 10:11, Walter von Entferndt <walter.von.entferndt at posteo.net> wrote:
> 
>> At Freitag, 15. Januar 2021, 08:02:49 CET Mark Millard wrote:
>>> FYI: C itself has, in <limits.h> , CHAR_BIT for the number of bits in a
>>> Byte for how sizeof counts Bytes:  sizeof(char), sizeof(signed char),
>>> and sizeof(unsigned char) are each always 1.
>>> 
>> No, CHAR_BIT is the #bits in a *char*, which is the (standard) datatype for 
>> the binary representation of a character/letter/symbol in written human 
>> language, and for small integers.  The name also suggests that, as well as the 
>> comment in the header file.  That does not necessarily equal a "byte", which 
>> (by commonly accepted knowledge) is the smallest addressable entity in a 
>> computer's memory.  Of course, e.g. https://code-reference.com/c/datatypes/
>> char tells a *char* occupies one byte.  Sadly, AFAIK C itself does not define 
>> what a "byte" is, although that term is mentioned many times in reference 
>> manuals (implicit assumption).  So /theoretically/ CHAR_BIT and NBBY can 
>> differ.  In fact, many library funtions operating on characters/letters take 
>> an *int* instead of a *char* for performance reasons.  From https://code-reference.com/c/stdlib.h/sizeof: "the *sizeof* operator returns the number of 
>> bytes to be reserved for a variable or a data type".  Of course, for practical 
>> reasons, we can safely assume that a *char* will take one byte in storage 
>> space for the foreseeable future, since the consequences of changing that 
>> would be disastrous.
> 
> Have you read a (fairly modern) C standard or its officially
> published rationle? You might want to.
> 
> From the officially published C99 rationale (page labeled 11,
> Terms and definitions):
> 
> QUOTE
> ) All objects in C must be representable as a contiguous sequence of bytes,
>  each of which is at least 8 bits wide.
> 
> ) A char whether signed or unsigned, occupies exactly one byte.
> 
> (Thus, for instance, on a machine with 36-bit words, a byte can be defined
> to consist of 9, 12, 18, or 36 bits, these numbers being all the exact
> divisors of 36 which are not less than 8.) These strictures codify the
> widespread presumption that any object can be treated an an array of
> characters, the size of which is given by the sizeof operator with that
> object's type as its operand.
> END QUOTE


I should have reported that my rationale copy (.pdf)
is of:

QUOTE
Rationale for
International Standard--
Programming Languages --
C

Revisision 5.10
April-2003
END QUOTE


===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)