From: Charles Mills <cmills@...>
Date: 2005-07-26T05:51:01+09:00
Subject: Re: Ruby extensions on 64 bit linux

Berger, Daniel wrote:
> > -----Original Message-----
> > From: Joe Van Dyk [mailto:joevandyk@gmail.com]
> > Sent: Monday, July 25, 2005 12:24 PM
> > To: ruby-talk ML
> > Subject: Ruby extensions on 64 bit linux
> >
> >
> > http://www.ruby-talk.org/cgi-bin/scat.rb/ruby/ruby-talk/149446
> >  makes me wonder, when writing C extensions, are there
> > anything I should be watching out for when intending the code
> > to be run on a 64 bit OS?
> >
> > Thanks,
> > Joe
>
> * Don't assume a pointer and an int are the same size
> * Don't make assumptions about the relative size of variable types
> * Be wary of sign extension problems
> * Use pointer arithmetic rather than address arithmetic
> * Never cast malloc
> * Explicitly include header files (-Wall will warn you about implicit
> declarations)
> * Watch out for data truncation, e.g. passing a size_t to something that
> expects an int.
> * Pack (align) your data structures as tightly as possible
> * Watch out for data loss in constant expressions
> * Use %p in your (s)printf's as appropriate
> * Use #ifdef _LP64 as needed (or is that Solaris only?)
>
> Regards,
>
> Dan

That is a nice list.  It seems the first bullet on your list causes the
majority of problems.  From ruby.h:

#if SIZEOF_LONG != SIZEOF_VOIDP
# error ---->> ruby requires sizeof(void*) == sizeof(long) to be
compiled. <<----
#endif
typedef unsigned long VALUE;
typedef unsigned long ID;

----
On the vast majority of 64bit systems/compilers sizeof(long) >
sizeof(int), however on many 32bit systems sizeof(long) == sizeof(int).
When the type of a function return value is unknown most/all compilers
default to int.  If you have the following statement in your code

VALUE obj = my_func(arg);

(where my_func() returns a VALUE) and do not define or give a function
prototype for my_func() prior to the statement the result of my_func()
will be cast to an int (by default) and then cast to a VALUE, causing
truncation on most 64 bit systems.  Turning warnings on catches these
kind of errors.

Adding to the list:
* turn on warnings
* use ANSI C function prototypes

FYI: the current Ruby sources do not always this :(

-Charlie