Interfacing with c and platform dependent sizes

[Jonathan M Davis]

On Friday, February 25, 2011 17:35:02 Jonathan M Davis wrote:
> On Friday, February 25, 2011 17:16:31 simendsjo wrote:
> > On 26.02.2011 02:06, bearophile wrote:
> > > simendsjo:
> > >> So.. A long in C is the same as the platform size? And long long
> > >> doesn't exist in 64 bit?
> > > 
> > > In D the size of int/uint is 32 bits and long/ulong is 64 bits.
> > > 
> > > In C the size of int, unsigned int, long, long long int, unsigned long
> > > long int, etc are not fixed, the change according to the CPU.
> > > sizeof(int)<= sizeof(long)<= sizeof(long long).
> > > 
> > > A help:
> > > http://www.digitalmars.com/d/2.0/phobos/std_stdint.html
> > > 
> > > Bye,
> > > bearophile
> > 
> > Ouch.. Any tips on porting C code that would work nicely with a
> > transition to 64 bit?
> > Should I change all long and int to int_atleast_32_t and long long to 64?
> 
> It depends entirely on what the code is doing. It could be completely safe
> to convert all ints, longs, and long longs to long to long. Or you may
> have to choose int or long depending on what system the code is supposed
> to be for.
> 
> In many cases, using a larger type wouldn't matter, since it can hold more
> than the smaller type, so whether the type in C/C++ was 32 bits or 64 bits
> is irrelevant. In other cases, the type needs to be an exact size, because
> the code is doing bit shifts or whatnot (in which case they _should_ have
> been using int32_t and int64_t on Linux and whatever the equivalent is on
> Windows, but unfortunately, many programmers don't). And if you're dealing
> with a struct, it's possible that that struct has to be an exact size
> (e.g. for some binary format), and using the wrong type in converting
> could make it the wrong size.
> 
> If you want to know what the appropriate D type is for the C/C++ code, you
> _need_ to know what it does.  Now, IIRC, int is almost guaranteed to be 32
> bits at this point, and long long is essentially guaranteed to be 64 bits,
> but long varies from system to system - both in terms of OS and
> architecture. IIRC, long is 32 bits on Solaris and Windows - both on x86
> and x86_64 - but it's 32 bits in x86 Linux and 64 bits in x86_64 Linux.
> So, if all the code uses is int and long long, then it's probably
> reasonably safe to use int for int and long for long long, but it's
> ultimately system dependent. Personally, I would argue that C/C++ code
> should use int when you don't care about the size of an integral type and
> use the intX_t types (where X is 8, 16, 32, or 64) when you _do_ care
> about the size, but there's no guarantee that programmers are going to be
> that disciplined about it.
> 
> In any case, if you really don't know the code and don't want to take the
> time to understand it, I'd use int for int, long for long long, and then
> if they have long, I'd do the research to figure out which OS and
> architecture the code is supposed to be for and use int if long long is 32
> bits and long if it's 64 bits. If you don't know what system it was built
> for, then I'd probably just use long and hoped that they weren't doing
> anything that made using an integral type which was too large a problem.

Actually, I just realized that I was thinking in terms of porting C++ code to D. 
You're going to have to be more strict if you're just converting header files. 
However, what you can do is just compile something like this in C/C++ on your 
system:

printf("int -> %d bytes", sizeof(int));
printf("long -> %d bytes", sizeof(long));
printf("long long -> %d bytes", sizeof(long long));

Then you'll know what they are on your system, and you can convert them over 
just fine. You just have to remember to do the same on any other system that you 
compile your code on. Bleh. At least D went the route of standarizing the size 
of its primitive types.

- Jonathan M Davis