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Re: ANSI requirement
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From:
Zefram
Date:
June 11, 2010 12:00
Subject:
Re: ANSI requirement
Message ID:
20100611185953.GA9553@lake.fysh.org
??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
* that all-bits-zero is a valid representation of a null pointer
-zefram
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