[Xcb] A simple question on XCBGCFunction [SOLVED]

[Ian Osgood]

There are a couple of ways to avoid this type of error.

First, you could fill in the values array during declaration:

CARD32 mask = XCBGCFunction/*0*/      | XCBGCForeground/*2*/   |
                  XCBGCBackground/*3*/ | XCBGCLineWidth/*4*/      |
                 XCBGCLineStyle/*5*/       | XCBGCGraphicsExposures/ 
*16*/;
CARD32 values[] = {
         XCBGXxor,                      // XCBGCFunction
         screen->white_pixel,     // XCBGCForeground
         0x1ecf48,                        // XCBGCBackground (light  
green)
         1,                                     // XCBGCLineWidth
         XCBLineStyleOnOffDash,  // XCBGCLineStyle
         0                                         //  
XCBGCGraphicsExposures
};


Second, there are some convenience structures and functions
in the XCBaux library that replace the value lists with structures:

#include <X11/XCB/xcb_aux.h>

XCBParamsGC params;
params.function = XCBGXxor;
params.foreground = screen->white_pixel;
params.background = 0x1ecf48;
params.line_width = 1;
params.line_style = XCBLineStyleOnOffDash;
params.graphics_exposures = 0;

XCBAuxCreateGC(c, sel_rect, win, mask, &params);

This method also avoids errors due to incorrect ordering
of the value list.

Ian

PS.  What the heck is up with the mailing list server on freedesktop??
This and a few other messages were posted almost a month ago!

On May 18, 2006, at 6:48 AM, Osmo Maatta wrote:

> Problem solved.
>
> The "values[]" variable was too small to store all values.
>   CARD32   values[2];
>
>   CARD32   values[10];
>
> Thanks.
>
> -------------------------------------------------------------
> This code is OK.
>
>
> sel_rect = XCBGCONTEXTNew (c);
>
> mask = XCBGCFunction/*0*/      | XCBGCForeground/*2*/   |
>              XCBGCBackground/*3*/ | XCBGCLineWidth/*4*/      |
>             XCBGCLineStyle/*5*/       | XCBGCGraphicsExposures/*16*/;
>
> // In same order as the mask.
> values[0] = XCBGXxor;                      // XCBGCFunction
> values[1] = screen->white_pixel;     // XCBGCForeground
> values[2] = 0x1ecf48;                        // XCBGCBackground  
> (light green)
> values[3] = 1;                                     // XCBGCLineWidth
> values[4] = XCBLineStyleOnOffDash;  // XCBGCLineStyle
> values[5] = 0;                                         //  
> XCBGCGraphicsExposures
>
> XCBCreateGC (c, sel_rect, win, mask, values);
> ....
>
> // Draw using the XOR (XCBGXxor) pen.
>
> case XCBExpose:
>   {
>     XCBPolyLine (c, CoordModeOrigin, win, sel_rect, 4, polyline);
>     XCBPolyRectangle (c, win, sel_rect, 2, rectangles);
>
>     XCBSync (c, 0);
>
>     break;
>   }
>
> case XCBButtonPress:
>  {
>     // Each second click will make the lines visible (XOR'ing pixels).
>     XCBPolyLine (c, CoordModeOrigin, win, sel_rect, 4, polyline);
>     XCBPolyRectangle (c, win, sel_rect, 2, rectangles);
>  }
>
> ...
>
>
>
>
> Osmo Maatta wrote:
>> Hello,
>>
>> I try to create a GC (graphic context) to draw some rubber lines  
>> using the XOR pen.
>>
>> I do this:
>>
>>  XCBGCONTEXT    sel_rect;
>>  ....
>>
>>  sel_rect = XCBGCONTEXTNew (c);
>>  mask =   XCBGCFunction | XCBGCForeground | XCBGCLineStyle |  
>> XCBGCGraphicsExposures;
>>  values[0] = XCBGXxor;  values[1] = screen->black_pixel;
>>  values[2] = XCBLineStyleOnOffDash;
>>  values[3] = 0;
>>  XCBCreateGC (c, sel_rect,  win, mask, values);
>>
>>
>> and later in
>>
>> case XCBExpose:
>> {
>>     printf("Debug expose\n");
>>
>>     XCBPolyRectangle (c, win, sel_rect, 2, rectangles);
>>           XCBPolyLine (c, CoordModeOrigin, win, sel_rect, 4,  
>> polyline);
>>
>>     break;
>> }
>> ---------------------------------
>>   But it does not draw anything. Only the debug line appears on  
>> the console.
>> -------------------
>>
>> I have even tried with  XCBGXcopy  value as XCBGCFunction.
>>
>> values[0] = XCBGXcopy;
>> Xlib manual says that GXcopy is the default GCFunction. I suppose  
>> it's the same in XCB.
>> But it does not work.
>>
>> It works (draws lines in a window) if I remove XCBGCFunction from  
>> the mask and remove the value[0]= line.
>> How to use the values for XCBGCFunction mask ?
>> -------------------------------------------------------------------
>>
>> I follow this tutorial
>> http://www.iecn.u-nancy.fr/~torri/files/xcb/doc/
>>
>> Many TIA.
>>  Osmo
>>
>> _______________________________________________
>> Xcb mailing list
>> Xcb at lists.freedesktop.org
>> http://lists.freedesktop.org/mailman/listinfo/xcb
>>
>
>
> _______________________________________________
> Xcb mailing list
> Xcb at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/xcb

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