Re: Understanding 'volatile' in lstrlib.cpp (Lua 5.3)
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- Subject: Re: Understanding 'volatile' in lstrlib.cpp (Lua 5.3)
- From: Viacheslav Usov <via.usov@...>
- Date: Thu, 1 Oct 2020 10:13:52 +0200
On Thu, Oct 1, 2020 at 8:50 AM Andrew Gierth
<andrew@tao11.riddles.org.uk> wrote:
> The spec allows [...]
Note that the original claim was about "ANSI C", i.e., C89. Where
union-based type punning is implementation-defined.
On the other hand, I do not think that adding volatile would change
that, i.e., make it standard rather than implementation-defined.
Moreover, in C++ union-based type punning is undefined behavior, and
since the Lua source code intends to be consumable as C++, this is a
problem that volatile does not correctly address, either.
I believe memcpy() is the only way that would be truly portable with
all those things considered.
The code in str_unpack() might look like:
case Kfloat: {
char buff[sizeof (double)]; /* instead of volatile Ftypes u; */
lua_Number num;
copywithendian(buff, data + pos, size, h.islittle);
if (size == sizeof(u.f)) { float f; memcpy(&f, buff, sizeof
f); num = (lua_Number)f; }
else if (size == sizeof(u.d)) { double d; memcpy(&d, buff,
sizeof d); num = (lua_Number)d; }
else memcpy(&num, buff, sizeof num);
lua_pushnumber(L, num);
break;
}
Cheers,
V.