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Re: How to add ssize_t a by size_t b?

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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.  The addition is then done with two size_t (unsigned) operands,
with the usual wraparound semantics if the result exceeds SIZE_MAX.

It's very likely that ssize_t is the signed type that corresponds to the
unsigned type size_t; for example, if size_t is unsigned long, then
ssize_t is probably long.  But I don't think POSIX actually guarantees
that.  (Which means, for example, that a "%zd" format specifier isn't
guaranteed to be correct for value of type ssize_t.)

This assumes that size_t and ssize_t have a rank greater than or equal
to that of int, which is not guaranteed but is almost certain.  If not
then both operands are promoted via the "integer promotions" before the
"usual arithmetic conversions" are applied.  (This is relevant if, for
example, you want to add operands of types short and unsigned short.)

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips
void Void(void) { Void(); } /* The recursive call of the void */

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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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