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Re: ANSI requirement

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June 11, 2010 12:00
Re: ANSI requirement
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??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


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