Re: Lua number type and 64-bit machines

[Sam Roberts <vieuxtech@...>]

On Fri, Mar 6, 2009 at 8:01 AM, John Barham <jbarham@gmail.com> wrote:
> I do recall being bitten by a bug in a 3rd party Python module running
> on 64-bit Linux that assumed that process ids could fit into 32-bit
> integers.

Strange, pid_t is 4 bytes wide on my AMD64 (linux 2.6.22-16 x86_64).
Also, I was pretty sure Linux can't handle more than 32K of pids, but
maybe its choosing pids that are scattered through a larger integer
range.

To the original poster:

Lack of larger numbers can be annoying. I've had the problem with a
codebase that used a lot of uint64s as identifiers. Here's my
conclusions.

There are people who like lua so much, they would use it for
everything. That's fine, but the way I look at is that it's not a
general purpose language like python or ruby, pre-existing in a
package ready to be used for many tasks.

For example, you can't even get a pid into your lua program with the
standard lua library. You have to build a C binding to fork(), or
getpid(), or something.

Lua exists as a set of flexible language mechanisms you can use to
create an efficient, portable, application-specific programming
language. I'm using it right now for implementing network protocol
tests, for example.

So, what needs to be done is to up the abstraction level from "64-bit
integers that might be pids", to something that is a pid.

After all, pid's aren't really numbers. You can't add two together, or
add 1 to a pid and get anything meaningful.

So, for your application of lua, if process manipulation is part of
what you need to do, you should implement a pid type that supports
whatever you need supported. Comparison operators, methods that look
up the name of the process, :kill(), etc. Whatever it is your
application needs to do to with pids.

A quick-and-easy way to do this if you don't want anything other than
a value you can pass to/from C code and identity comparisons between
values, is to use lightuserdata. It should be 64-bit wide on a 64-bit
system, and exists for this purpose.

Cheers,
Sam