Interfacing with c and platform dependent sizes

[Jonathan M Davis]

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.

- Jonathan M Davis