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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: How to add ssize_t a by size_t b?
Date: Wed, 06 Oct 2021 04:06:51 -0700
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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?

For those who may be interested, here is a function to compute
the sum when the result is within the range of ssize_t, and
give an error indication when it isn't.


ssize_t
add_ssize_and_size( ssize_t const a, size_t const b ){
    if(  a >= 0  &&  SSIZE_MAX - a >= b       )  return  a + b;
    if(  a <  0  &&  b <= SSIZE_MAX           )  return  a + (ssize_t){ b };
    if(  a <  0  &&  b-1-SSIZE_MAX <= -(a+1)  )  return  b-1 - -(a+1);

    return   errno = ERANGE,   SSIZE_MAX;    // or some other suitable value
}