atoi(3) is not broken by design. It actually has a good design. It was broken by history.

The standard could perfectly define the behavior on error, and it would be a decent API. They just decided to not define it.

If I were to define the behavior of atoi(3), I would make it equivalent to this:

int
atoi(const char *s)
{
    int  i, e;

    i = strtoi(s, NULL, 10, INT_MIN, INT_MAX, &e);
    errno = e;
    return i;
}

See also: https://man.netbsd.org/strtoi.3

alx‭ Yes they could define it but chose not to, and so the functions are broken. It's entirely a fault caused by the C standard and would have been easy to fix during C90 standardization, when they chose to pick up the function from the smelly swamp of sloppily designed, sloppily specified Unix crap. But this was never done. The C99 says the functions were kept around solely for backwards compatibility and for the reason that they might perform an itty bit better because of the lack of error handling. They have to justify the bad specification somehow...

Btw errno error handling is another huge designer facepalm originating from that same smelly swamp, so it's not something to encourage or specify in when designing new functions. Someone asked "surely it isn't possible to make something even worse than Windows awful GetLastError" and then *nix/POSIX proved them wrong...