Re: Undefined behaviour code used in sysdeps/unix/sysv/linux/x86_64/makecontext.c

[Godmar Back <godmar@gmail.com>]

If you're looking at just the C standard, uintptr_t wouldn't help
since it's only guaranteed to hold object pointers, not function
pointers

I would dig deeper in terms of "implementation-defined". The C
standards says that a conforming implementation should define the
behavior of pointer to int conversions (except where undefined, which
conversion of function pointers to integers are not). So somewhere,
gcc/clang etc. - if conforming - will have to define what a function
pointer converts to when converted to an int. This is, IMO, different
from undefined behavior where the compiler can do whatever they please
and doesn't owe anyone an explanation.

See J.3 pg 566 http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf

On Mon, May 21, 2018 at 9:18 AM, Remus Clearwater
<remus.clearwater@gmail.com> wrote:
> PS:
>
> The definition of uintptr_t in C99 is:
>
> “an unsigned integer type with the property that any valid pointer to void
> can be converted to this type, then converted back to pointer to void, and
> the result will compare equal to the original pointer”
>
> On Mon, May 21, 2018 at 9:08 PM, Remus Clearwater
> <remus.clearwater@gmail.com> wrote:
>>
>> Sorry for forgot to CC libc-help.
>>
>> ----
>>
>> Thanks a lot Godmar.
>>
>> But it didn't say `sizeof(function_pointer)` are must equal or less than
>> `sizeof(void*)`.
>>
>> I found this in POSIX.1-2008
>> http://pubs.opengroup.org/onlinepubs/9699919799.2008edition/functions/V2_chap02.html#tag_15_12_03:
>>
>> "All function pointer types shall have the same representation as the type
>> pointer to void. Conversion of a function pointer to void * shall not alter
>> the representation. A void * value resulting from such a conversion can be
>> converted back to the original function pointer type, using an explicit
>> cast, without loss of information.
>>
>> Note:
>> The ISO C standard does not require this, but it is required for POSIX
>> conformance."
>>
>> So under POSIX.1-2008 the kinda usage of `function_address = (uintptr_t)
>> funcfp;` is correct, but in POSIX.1-2017 this section 2.13.3 has been
>> removed. This means in POSIX.1-2017 that kinda conversition is still not
>> defined.
>>
>