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Re: How to add ssize_t a by size_t b?

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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

> wij <wyniijj@gmail.com> writes:
>
>> 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?
>
> In standard C, you don't, because ssize_t doesn't exist (unless you
> define it yourself).
>
> However, POSIX does define a type ssize_t, which is a signed integer
> type that can represent values up to SSIZE_MAX.
>
> The rules are given in N1570 6.3.1.8, "Usual arithmetic conversions".
>
> It's likely, but not guaranteed, that size_t and ssize_t have the same
> rank.  If so, then the ssize_t value is converted to size_t, and
> assuming SSIZE_MAX <= SIZE_MAX the conversion does not change the
> value.  [...]

If size_t and ssize_t have the same integer conversion rank then
SSIZE_MAX <= SIZE_MAX is guaranteed.  However, converting a value
of type ssize_t to type size_t may indeed change the value,
because the value might be negative.

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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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