Re: [Xen-devel] [PATCH v9 1/5] xen: introduce ptrdiff_t, fix uintptr_t

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 2/12/19 4:03 PM, Jan Beulich wrote:
>>>> On 12.02.19 at 16:32, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 12/02/2019 15:26, Ian Jackson wrote:
>>>
>>>> But if we really want to have ptrdiff_t, then I think we should either
>>>> follow the uintptr_t model and use attribute((mode())), or we should
>>>> leverage the compiler's __PTRDIFF_TYPE__ (and then also
>>>> __UINTPTR_TYPE__ for uintptr_t, at least if available - not sure what
>>>> its availability depends on, but it's conditional in gcc's
>>>> c_stddef_cpp_builtins()).
>>> It is not unusual for porting something like Xen to a new architecture
>>> to involve writing a short header file with these kind of type
>>> definitions.  I don't know why we couldn't take that approach.
>>
>> stdint.h and inttypes.h are a freestanding header files, and are
>> intended for uses just like this.
>>
>> Lets stop second guessing our build environment, and use the solution to
>> the problem given to us by the C specification.
>>
>> And to be crystal clear.  This means including <stdint.h> and
>> <inttypes.h> in xen/types.h and deleting all of these typedefs
> 
> Well, this would certainly be a viable route if
> - these headers were truly freestanding (rather than coming in their
>   own incarnation with every gcc version, and perhaps also with
>   any other compiler)

Why would this be a problem?  Isn't that a feature -- that your
compiler/header file combination provides you with the proper runes to
get a working uintptr_t?  That seems a lot more robust and reliable than
trying to keep our own header that works on all possible compiler
combinations.

> - were guaranteed bug free on every distro we care about building on

This argument seems kind of backwards to me.

You are implicitly asserting that there are distros we care about
building on where the system stdint.h and inttypes.h have bugs.  It's
not up to others to show that this is false, but up to you to show that
it's true; by providing at least some examples (even if historical).

Or if you don't have examples ready to hand, saying something like:

"I seem to recall some distros having bugs in stdint.h in the past.  We
should make sure to test it with all distro versions we care about
before committing to it."  This should be somewhat easier now that we
have the docker build-testing infrastructure.

 -George

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