XLIB Programming Manual, Rel. 5, Third Edition

7.6 Getting Complete Visual Information

As mentioned earlier, some systems define more than one visual. The default visual might not be the most appropriate for your application. Moreover, the visual found using the technique described in 7.4.2 Code to Allocate Read-only Colors using XMatchVisualInfo() is fine for applications with routine color needs but is not necessarily the best. As you may recall, XMatchVisualInfo() returns a single visual arbitrarily selected from the list that matches the passed visual class and depth. The most thorough method is to get a complete list of visual information for every available visual, using XGetVisualInfo(), and then choose from these.

XGetVisualInfo() returns a list of visual structures that match the attributes specified by template and mask arguments. The template is an XVisualInfo structure with members set to the required values, and the mask indicates which members are matched with the list of available visuals. By passing an empty template structure, you can get a complete list of XVisualInfo structures.

7.6.1 The XVisualInfo Structure

The XVisualInfo structure returns information about the available visuals. It is used both to select a visual type from those available and as a source of information while using a particular visual.

The XVisualInfo structure is shown in Example 7-9.

Example 7-9. The XVisualInfo structure

typedef struct { Visual *visual; VisualID visualid; int screen_num; unsigned int depth; int class; unsigned long red_mask; ...