🚀 go-pugleaf

schwarz@mips.complang.tuwien.ac.at (Konrad Schwarz) writes:
>Upon further reflection, I now am more inclined toward the following:

>	struct {
>		unsigned b [800][800]:1;
>	} the_array;

*Sigh*.  This is not legal C.

The standard says (Section 6.2.1, Semantics paragraph 5) that
	"A bit field shall have a type that is a qualified or
	unqualified version of one of int, unsigned int, or signed int."

The type here is "array [800] of array [800] of unsigned int", so this
is not legal.

There is *NO* way of directly getting a packed array of bits yourself
in standard C (or C++).  You will _have_ to program it yourself using
shifts and masks.

>Furthermore, in yesterdays posting making the size of the cell sizeof (unsigned)
>is arbitrary; in particular, it has nothing to do with the fact that the
>type of the bitfield is unsigned.  In the environment I am currently
>computing in, a cell size of sizeof (void *) would be more likely to make
>the cell size equal to the machine word length.

There are machines in which sizeof (void *) greatly exceeds the size of
any integral type.  If you want a packed array of bits, you will need
a plain array whose elements are of some integral type; it will be
simpler if they are unsigned.  The reasonable implementations are

	#define round_up(x,t) (((x) + (CHAR_BIT * sizeof (t) - 1)) \
			      / (CHAR_BIT * sizeof (t)) )

	unsigned char  uc[round_up(x, unsigned char)];

	unsigned short us[round_up(x, unsigned short)];

	unsigned int   ui[round_up(x, unsigned int)];

	unsigned long  ul[round_up(x, unsigned long)];

--- modulo mistakes in the round_up() macro ---
and the best of them is probably the third.

-- 
Result of state election = 50% Victorians didn't get MP of their choice.
Richard A. O'Keefe; http://www.cs.rmit.edu.au/~ok; RMIT Comp.Sci.