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Re: ANSI requirement
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From:
karl williamson
Date:
June 11, 2010 12:37
Subject:
Re: ANSI requirement
Message ID:
4C12904D.5020700@khwilliamson.com
Zefram wrote:
> ??var Arnfj??r?? Bjarmason wrote:
>> I vaguely recall someone mentioning that we rely on some features of C
>> that aren't strictly guaranteed by ANSI C,
>
> Yes, we do.
>
>> such as the the offset into
>> a struct being the sum of the sizeof() of the struct members that came
>> before in the struct.
>
> But not this particular one, which is almost never true across a whole
> implementation. It would require that no data types have any alignment
> requirement. Implementations almost always use padding in structs.
> There is something similar, though: we do rely on the offset of a struct
> member being *a pure function of* the types of the preceding members.
> (That is, that where two structs begin with type-identical sequences
> of members, the layouts of those members will be compatible.) This is
> almost always true.
>
> We rely on a few other things that are almost always true but not
> guaranteed by the standards. Off the top of my head:
>
> * that all data pointers have bitwise-compatible representations
>
> * that function pointers are the same size as data pointers
>
> * that signed integers are represented in twos-complement
>
> * that signed integer arithmetic cleanly wraps on overflow
>
> * that the bits of multi-byte integers are divided up into bytes in
> adjacent groups, with all bits of each byte participating in the
> integer representation
>
> * that bytes consist of exactly eight bits
We do test that bytes are 8 bits in Configure.
>
> * that all-bits-zero is a valid representation of a null pointer
>
> -zefram
>
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