> That doesn't detect overflows and underflows

A trivial change does though: Just round-trip it.

    int s2d(double*restrict out,const char*restrict in){
     char tmp[99];int r,q=sscanf(in,"%lf%n",out,&r),s=sprintf(tmp,"%lf.",*out);
     return q&&!((r>s)||memcmp(in,tmp,r)||tmp[r]>'0');
    }

That's a feature: Where did the string come from if not the user?

> you cannot set locale, convert, then unset locale

So what? You're not handling multiple locales: You're using the user's locale. If you're making a server, using anything but LANG=C or a utf8 variant is asking for trouble.

I think the point is it's absolutely possible to portably detect errors, some libc's strtod() just don't which ain't the same thing.