LuaJIT FFI: Semantics of undefined numeric conversions (e.g. huge double to int)
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- Subject: LuaJIT FFI: Semantics of undefined numeric conversions (e.g. huge double to int)
- From: Philipp Kutin <p@...>
- Date: 2011年9月19日 00:01:20 +0200
Hi,
I'm one of the developers/maintainers of a certain (open source) game
port and one of our upcoming ideas is to replace the existing scripting
language with something based on Lua; more specifically LuaJIT because
of its efficiency and the fact that we mostly target x86/x64 anyway.
From reading the documentation, it looks like LuaJIT's FFI is the
perfect tool for making engine and game functions/structures accessible
from the scripting language, where of course only "safe" functions
should be made visible directly (for example, those taking only scalar
numbers). But as far as I can see, the FFI docs don't specify what
happens for numeric conversions that are undefined per C standard, such
as downcasting a number greater than INT_MAX to an int.
A few experiments with the following C function
void printint(int x) { printf("%d\n",x); }
give these results from LuaJIT for me:
printint(2147483648+1) --> -2147483648
printint(4*2147483648) --> -2147483648
printint(-2147483648-1) --> -2147483648
printint(0/0) --> -2147483648
printint(1/0) --> -2147483648
So, in every "interesting" case, the out-of-range double is apparently
converted to INT_MIN, but can I rely on this or is this a coincidental
side-effect of the implementation?
The closest thing to an answer I could find is the "Conversion between C
types" section in the FFI documentation:
http://luajit.org/ext_ffi_semantics.html#convert_between ,
but again I'm not sure what to make of the undefined cases here. For
example, the double to int conversion is listed as
double -->^trunc int,
but what is "truncation" supposed to mean in this particular context? It
can't conceptually mean "round the double to an infinite-precision
integer and take its 32 lowest-order bits", since it would be undefined
for inf/nan and it's inconsistent with my experimental findings.
Greetings,
Philipp