Re: [eclipse-clp-users] C Interface Functions does not support int64

From: Kish Shen <kisshen_at_...5...>
Date: Sun, 25 Oct 2009 23:04:19 +0000
Hendrik Skubch wrote:
> So basically, what you are saying is, that it is not possible to pass a 
> double word integer (a long long in C usually) from C to Eclipse.

long long has only become standard in C with C99, and I think it is 
still not standard for C++. Secondly, as with all standard C types, you 
should not make assumptions about its size (other than it is at least as 
large as long); it is commonly a double word integer only on 32 bit 
platforms. On most C on 64 bit platforms, long long is word size, i.e. 
64 bit.

In short, there is no portable way of dealing with "double word" 
integers in C/C++, so ECLiPSe does not provide any support to do so.

ECLiPSe need to be compiled with GMP to support multi-word integers 
(refer to as bignums in ECLiPSe). Otherwise ECLiPSe only supports word 
sized integers. It may be valid to extend the C interface to support the 
conversion of C long long to ECLiPSe, converting to bignum if required 
and supported.


> As a workaround, one could pass multiple integers of word size, and call 
> a predicate, which shifts and adds them back together.
> 
> concat32(X1,X2,Res):-
> 	Res  is (X1<<32) + X2.
> 
> Not pretty, and one has to take extra care when decomposing signed 
> integers in C, but should do the trick.
> 

Your code assumes the word size is 32 bit, and you also need to be 
careful of byte ordering, so your decomposition must break the double 
word integer correctly.

A more standard way of passing larger integers values on 32 bit 
platforms is to use doubles -- on 32 bit machines, doubles can normally 
represent integers >32 bits precisely (I think this is normally 40 bits).

Cheers,

Kish

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Received on Sun Oct 25 2009 - 23:04:36 CET

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