[cfe-dev] On sizeof char, bytes, and bits in the C99 standard

[Török Edwin]

On 2009-01-02 17:50, Sebastian Redl wrote:
> Török Edwin wrote:
>   
>> Section 7.18.1.1 defines int8_t: "Thus, int8_t denotes a signed integer
>> type with a width of exactly 8 bits."
>>
>> This quote from C99Rationale V.5.10 " 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.)" shows that the intent was to allow for a definition of
>> byte that doesn't necessarily have 8 bits.
>>
>> However according this quote " These strictures codify the widespread
>> presumption that any object can be treated as an array of characters,
>> the size of which is given by the sizeof operator with that object’s
>> type as its
>> operand."  I should be able to treat any objects (thus including int8_t
>> type objects) as array of characters.
>> This implies that there exists an N such that: number_of_bits(char)*N =
>> number_of_bits(int8_t). Given what we know about char and int8_t this means:
>> there exists an N such that number_of_bits(byte)*N = 8, which implies
>> number_of_bits(byte) <= 8.
>>
>> Now according to C99Rationale V5.10: " All objects in C must be
>> representable as a contiguous sequence of bytes, each of which  is at
>> least 8 bits wide.",
>> number_of_bits(byte) >= 8.
>>
>> Thus number_of_bits(byte) = 8.
>>
>> Am I right, or am I wrong?
>>   
>>     
>
> You're wrong. 7.8.1.1p3 says that the exact forms are optional. An
> implementation where CHAR_BITS is > 8 cannot provide (u)int8_t, but is
> nevertheless conforming.

Ok, so I may assume that on any POSIX compliant platform CHAR_BITS is 8?
(POSIX requires (u)int8_t:
http://www.opengroup.org/onlinepubs/000095399/basedefs/stdint.h.html)

Best regards,
--Edwin