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Re: How to add ssize_t a by size_t b?

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James Kuyper <jameskuyper@alumni.caltech.edu> writes:

> On 10/1/21 1:39 PM, Guillaume wrote:
>
>> Le 01/10/2021 at 17:32, wij a ecrit:
>>
>>> To simply the question of "a+b":
>>>
>>> ssize_t add(ssize_t a, size_t b) {
>>>   if(a+b would overflow) { set errno=ERANGE; }
>>>   a+=b;     // ?
>>>   return a;
>>> }
>>>
>>> Another example:
>>> ssize_t a=SSIZE_T_MIN;
>>> size_t b=SIZE_T_MAX;
>>> a+=b;      // Is this OK?  Or, How the addition is done correctly?
>>
>> ssize_t is not standard C. It's defined in POSIX.
>> While it's a signed integer, whereas size_t is unsigned, I don't know
>> how it relates to size_t on a given implementation in terms of width.
>
> ssize_t is a signed integer type with the same width as size_t.

I don't see anything on the pubs.opengroup.org website that
requires that.

> Therefore, they should have the same integer conversion rank.  Since
> the sign bit is included in the width, SSIZE_MAX is guaranteed to be
> smaller than SIZE_MAX.
>
> The usual arithmetic conversions apply (6.5.6p5).  The ssize_t value
> is first converted to size_t (6.3.1.8p1).  That is a well-defined
> conversion - negative ssize_t values have SIZE_MAX+1 added to them
> (6.3.1.3p2).  The sum is calculated using size_t math, which will
> produce what I assume is the desired result even if a is negative,
> so long as it is smaller than b.
>
> Upon assignment, the result is converted to ssize_t.  Values greater
> than SSIZE_MAX would not result in undefined behavior, as you
> suggested.  Instead, they would result in an implementation-defined
> value or the raising of an implementation-defined signal (6.3.1.3p3).
> This code will probably not work as desired on an implementation that
> chooses to raise a signal, and the implementation- defined value is
> not guaranteed to be the one that he wants.  Therefore, setting errno
> would not be sufficient, the final conversion must be actively
> prevented from occurring if it would otherwise overflow.
>
> I would write the function as:
>
>    size_t c = a + b;
>    if(c > SSIZE_MAX)
>        return overflow_value;
>    else
>        return c;
>
> where overflow_value is whatever value he wants add() to return
> when there's an overflow.  [...]

An exemplary proposal:  short, simple, and wrong.

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Thread

How to add ssize_t a by size_t b? wij <wyniijj@gmail.com> - 2021-10-01 08:32 -0700
  Re: How to add ssize_t a by size_t b? Guillaume <message@bottle.org> - 2021-10-01 19:39 +0200
    Re: How to add ssize_t a by size_t b? scott@slp53.sl.home (Scott Lurndal) - 2021-10-01 18:31 +0000
      Re: How to add ssize_t a by size_t b? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-01 14:29 -0700
        Re: How to add ssize_t a by size_t b? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-01 20:17 -0400
          Re: How to add ssize_t a by size_t b? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-01 18:42 -0700
            Re: How to add ssize_t a by size_t b? Manfred <noname@add.invalid> - 2021-10-02 20:17 +0200
              Re: How to add ssize_t a by size_t b? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-02 15:03 -0400
    Re: How to add ssize_t a by size_t b? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-01 14:53 -0400
      Re: How to add ssize_t a by size_t b? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-10-02 15:13 -0700
  Re: How to add ssize_t a by size_t b? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-01 14:13 -0700
    Re: How to add ssize_t a by size_t b? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-10-02 14:52 -0700
  Re: How to add ssize_t a by size_t b? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-10-02 14:47 -0700
  Re: How to add ssize_t a by size_t b? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-10-06 04:06 -0700

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