Calling real c-men...

[DanKarron at UCBVAX.BERKELEY.EDU]

Here is a problem at the border of the C-language. It is on the edge of 
c's value as a terse, compact language, and a slippery mess when it comes to
arrays and pointers. I would like someone to show me that this is really
a beautiful feature to be appreciated and loved!

Here is my problem. I have allocated a tree of structures, and I want to
pass a pointer to a piece of a structure to a subroutine. Now what I want to
pass is a Pointer to an 2 dim CONTIGUOUS array of floats (Like a Matrix typedef,
in that each float is in storage adjacent to each other, not an array of pointers to a list of floats)

In the receiving subroutine, I would like to be able to reference each
element in the array pointed to by the same indexes as in the main structure.

Here are the decls and deffs:

typedef struct LineTag {
	float l[2][3],V[2][3];
	int DrawMe,Inny,Outie;
	} Line;

typedef struct CompositeTag {
	struct LineTag *HotLinePointer;
	struct LineTag SkeweringLines[HOT_SPOTS][HOT_LINES];
	struct LineTag LineTween[HOT_SPOTS][HOT_LINES];
	Matrix CompositeMatrix;
	int NumberCompositeStacks,HotStack,
		HotSlice,HotCardinal,HotLine;
	char CompositeName[80];
	} Composite;

Now I want to call a subroutine that works on the vertices of the line as
an array of two sets of three plane normal coeff. Hence the decl
of the Line as a 2 by 3 array. If I want to operate on the substituent planes,
they I want to pass a pointer to the individual plane. If I want to work on
the line, I want to pass a pointer to a pair of planes.

I want to pass a LIST of lines, or a LIST of planes, where there can be an
arbitrary number of elements. I can do this by blinding the compiler with
void * and &p->LineInstance.V[0][0] but I would like the debugger and the
comiler to know the array bounds for what is pointed by.

So, How do I get this meta-code to fly?

I gets lots of compiler complaints, Here is what it says:

----------------------------------------------------------------------------

void NitherNoid(int mm,float llist[][2][3],float point[3]);

void KnitItTight()
{
register int i,j,l;
static int svd_line_count;
float *svd_line_list[2][3]; /* a list of pointers to lines */

svd_line_coune=0;
svd_line_list=(void *)0;

for(i=0;i<HOT_LINES;i++)
	{
	if(!ArenaDirectory->Composite->SkeweringLines[l][i].DrawMe)
		continue;
	svd_line_count++;
	if(svd_line_list) /* allocate list space as we need it */
		{
		svd_line_list=realloc(svd_line_list,svd_line_count*sizeof(float *));
		}
		else
		{
		svd_line_list=calloc(svd_line_count,sizeof(float *));
		}
	svd_line_list[svd_line_count-1]= &ArenaDirectory->Composite->SkeweringLines[l][i].V;
	/* attach a pointer to our line */
	}

if(svd_line_count > 1)
	{
	NitherNoid(svd_line_count,svd_line_list,ArenaDirectory->Composite->CardinalMark[l].v);
	ArenaDirectory->Composite->CardinalMark[l].DrawMe=TRUE;
	printf("%s:%f,%f,%f\n",ArenaDirectory->Composite->CardinalMark[l].MarkName,
			ArenaDirectory->Composite->CardinalMark[l].v[0],
			ArenaDirectory->Composite->CardinalMark[l].v[1],
			ArenaDirectory->Composite->CardinalMark[l].v[2]);
	}

free(svd_line_list);
}
/******************************************************************************/

The compiler won't let me attach a pointer to the array of line params.

karron:~/D.Image/D.Arenas/D.PixelPicker:159make
.c.o RULE PixelPicker.o : triggered by PixelPicker.c
        cc -DDEBUG -DDANS_HACK -DIRIS_NEWS -I/usr/people/karron/D.Include -g
2 -prototypes -float  -c PixelPicker.c
ccom: Error: PixelPicker.c, line 140: illegal lhs of assignment operator
      svd_line_list=(void *)0;
      -----------------------^
ccom: Error: PixelPicker.c, line 149: illegal lhs of assignment operator
                svd_line_list=realloc(svd_line_list,svd_line_count*sizeof(fl
oat *));
      ----------------------------------------------------------------------
-------^
ccom: Error: PixelPicker.c, line 153: illegal lhs of assignment operator
                svd_line_list=calloc(svd_line_count,sizeof(float *));
      --------------------------------------------------------------^
ccom: Warning: PixelPicker.c, line 155: & before array or function: ignored
        svd_line_list[svd_line_count-1]= &ArenaDirectory->Composite->Skeweri
ngLines[l][i].V;
      ----------------------------------------------------------------------
---------------^
ccom: Error: PixelPicker.c, line 155: illegal lhs of assignment operator
        svd_line_list[svd_line_count-1]= &ArenaDirectory->Composite->Skeweri
ngLines[l][i].V;
      ----------------------------------------------------------------------
---------------^
ccom: Error: PixelPicker.c, line 161: bounds of array argument and prototype
 disagree: 3 vs 2
        NitherNoid(svd_line_count,svd_line_list,ArenaDirectory->Composite->C
ardinalMark[l].v);
      ----------------------------------------------------------------------
----------------^

How do I make a list of pointers to a 2 dim array that works ?

Cheers!

dan.
| karron at nyu.edu (e-mail alias )         Dan Karron, Research Associate      |
| Phone: 212 263 5210 Fax: 212 263 7190  New York University Medical Center  |
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