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Re: What is the value after operator '!'?

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supercat@casperkitty.com writes:
> On Monday, October 26, 2015 at 5:16:07 PM UTC-5, Keith Thompson wrote:
>> supercat writes:
>> > In a system where (char*)x would yield a pointer to physical address x,
>> > and where physical address zero was valid, but where (char*)0 [i.e. NULL]
>> > was represented by a pointer with some physical address other than zero,
>> > would there be any way of writing a pointer constant equal to physical
>> > address zero?
>> 
>> No portable way -- but then pointer constants that refer to numeric
>> addresses are inherently non-portable anyway.  A compiler could provide
>> some special-case way of representing address zero if it were useful.
>
> Many CPU architectures are used on a variety of hardware configurations,
> and compilers are generally specialized per architecture, rather than 
> per hardware configuration.  Obviously a program which is designed for
> a system with various devices at different addresses will not be directly
> portable to other CPU platforms, but it would be nice if it could be
> reasonably portable among compilers for the same architecture.

If you're willing to settle for "reasonably portable", just assume that
a null pointer is all-bits-zero.  Add a runtime check to the beginning
of the program so the program dies if that's not the case.

>> > I guess "1-1" wasn't sufficiently subtle for a compiler to miss that it was
>> > really a constant; C# distinguishes between literal zeroes and integer
>> > constants whose value happens to equal zero, but I guess C doesn't?
>> 
>> Why guess?  N1570 6.3.2.3 defines null pointer constants.  6.6 defines
>> integer constant expressions.  Read The Fine Manual.
>
> Sorry, by "I guess" I meant "I guess my memory of the distinctions among the
> various C-ish languages isn't perfect".

The rules for null pointer constants have not changed across C89/90,
C99, and C11.  (C++ rules differ, but not in any way that's relevant to
the current discussion.)

>> > Would the value of (int)0.3 be considered an integer constant?  What about
>> > (int)(1.0E+17+1.0-1.0E+17), whose value would depend upon how floating-point
>> > precision was handled?
>> 
>> N1570 6.6.  Yes, it's possible to write an integer constant expression
>> whose value is implementation-defined.  (CHAR_BIT is another example of
>> such an expression.)
>
> CHAR_BIT is a constant whose meaning can be known before a program begins
> execution.  The value yielded by adding 1 to 1E+17 and then subtracting
> 1E17 from the result may vary depending upon the FPU configuration, and I
> believe some compilers have options to defer floating-point calculations
> until runtime for precisely that reason.

(int)(1.0E+17+1.0-1.0E+17) is an integer constant expression.  Its
evaluation cannot be deferred until runtime, at least not if it's used
in a context where a constant expression is required.

Its value can vary depending on various factors.  Has this ever been a
problem for you?  The only reason I can think of to use such an
expression is because you *want* its value to depend on the system's
floating-point characteristics.

[...]

>> >             I wonder how much code would break if the Standard were to
>> > require that the argument to the cast must have one of a small number of
>> > literal forms (e.g. 0, 0L, etc.) rather than merely being any constant
>> > expression whose value happens to be zero?
>> 
>> I wonder how useful such a change would really be.  If you want a
>> pointer value whose representation is all-bits-zero, define a pointer
>> object and call memset().  But I've never felt the need for such a
>> pointer value.
>
> You probably have never coded for a machine that put anything useful at
> address zero.

No, I haven't.

> One of the few places where the C Standard makes reference to the concept
> of a target architecture being something apart from the implementation is
> in the discussion of integer-to-pointer conversions, which are supposed to
> be done in a fashion unsurprising to someone familiar with the architecture.
> Having code work fine when configured to use the second parallel-I/O port
> (whose address is set to 0x0002 in a header file) but then break when changed
> to use the first (whose address is set to 0x0000) because the optimizer
> ripped it to shreds would not satisfy the "unsurprising" requirement.  If
> the Standard provided that casting or coercing anything other than a single
> digit zero directly to a pointer would invoke Implementation-Deined behavior,
> and recommended that 
>
>    (char*)(intptr_t)0
>
> should be interpreted according to the rules applicable to non-constant
> expressions yielding zero, that shouldn't break much code (compilers could
> include an option to interpret all forms of constant zero as the Standard
> used to require) but also make it easier to avoid "surprises" when using
> things that happen to use address zero.

Leaving the definition as it is won't break *any* code.

You propose changing the language definition to enable portable access
to address 0 on systems where a null pointer is not represented as
all-bits-zero.  Do you have a real-world use case for this?  If not, I
suggest not worrying about it.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

What is the value after operator '!'? fl <rxjwg98@gmail.com> - 2015-10-26 06:21 -0700
  Re: What is the value after operator '!'? Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-10-26 09:29 -0400
  Re: What is the value after operator '!'? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-26 13:42 +0000
    Re: What is the value after operator '!'? supercat@casperkitty.com - 2015-10-26 07:53 -0700
      Re: What is the value after operator '!'? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-26 17:37 +0000
        Re: What is the value after operator '!'? Keith Thompson <kst-u@mib.org> - 2015-10-26 11:05 -0700
          Re: What is the value after operator '!'? supercat@casperkitty.com - 2015-10-26 11:38 -0700
            Re: What is the value after operator '!'? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-26 20:27 +0000
              Re: What is the value after operator '!'? supercat@casperkitty.com - 2015-10-26 14:16 -0700
                Re: What is the value after operator '!'? Keith Thompson <kst-u@mib.org> - 2015-10-26 17:15 -0700
              Re: What is the value after operator '!'? Keith Thompson <kst-u@mib.org> - 2015-10-26 17:12 -0700
            Re: What is the value after operator '!'? Ian Collins <ian-news@hotmail.com> - 2015-10-27 10:27 +1300
            Re: What is the value after operator '!'? Keith Thompson <kst-u@mib.org> - 2015-10-26 15:15 -0700
              Re: What is the value after operator '!'? supercat@casperkitty.com - 2015-10-26 16:16 -0700
                Re: What is the value after operator '!'? Keith Thompson <kst-u@mib.org> - 2015-10-27 08:39 -0700
  Re: What is the value after operator '!'? Richard Heathfield <rjh@cpax.org.uk> - 2015-10-26 14:04 +0000
  Re: What is the value after operator '!'? Geoff <geoff@invalid.invalid> - 2015-10-26 09:50 -0700

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