Re: [pure-lang-users] C float

[Albert G. <Dr....@t-...>]

Eddie Rucker wrote:
> Then from Pure:
>  > using "lib:/t2.so";
>  > extern int foo(int);
>  > let a = 2147483646;
>  > foo (a div -1);
>  -2147483645

Nothing wrong there, a div -1 == -2147483646 in signed arithmetic, so 
foo (a div -1) == -2147483646+1 == -2147483645.

> Now, the following C program gives different results:
> [...]
>   unsigned int a = 2147483646;
> 
>   printf("%d\n", foo(a/-1));

Note that the 'a/-1' here does *unsigned* arithmetic (because you 
declared a as such), which means a/-1 == a/0xffffffffU == 0 != 
-2147483646 == (signed)a/-1. If you declare a as a signed int in your C 
program, you'll get exactly the same result as with Pure.

Under all circumstances, the C interface should behave as if the C int 
type is "cast" to the Pure int type and vice versa (using truncation and 
sign extension as necessary), if that's not the case then please report 
it and I will fix it. I think that this is the most reasonable way to 
implement the marshalling of C ints considering the limited repertoire 
of integer types available in Pure.

Of course, this means that a C unsigned int return value will become 
negative in Pure if it's big enough. But that can be easily undone with 
the following little Pure function which takes a Pure int x and returns 
a bigint equal to (unsigned)x in C:

uint x::int = if x>=0 then bigint x else x+0x100000000L;

This carries over to unsigned 8, 16 and 64 bit ints accordingly:

ubyte x::int = if x>=0 then x else x+0x100;
ushort x::int = if x>=0 then x else x+0x10000;
ulong x::bigint = if x>=0 then x else x+0x10000000000000000L;

These always use the smallest Pure int type capable of holding the 
result: int for ubyte and ushort, bigint for uint and ulong. (Note that 
in the case of 64 bit values the C interface returns a bigint, that's 
why ulong takes a bigint parameter.)

Maybe I should add those definitions to primitives.pure?

HTH,
Albert

-- 
Dr. Albert Gr"af
Dept. of Music-Informatics, University of Mainz, Germany
Email:  Dr....@t-..., ag...@mu...
WWW:    http://www.musikinformatik.uni-mainz.de/ag