XLIB Programming Manual, Rel. 5, Third Edition

C.3 GC Caching

GCs are cached by the library to allow merging of independent change requests to the same GC into single protocol requests. This is typically called a write-back cache. Any extension routine whose behavior depends on the contents of a GC must flush the GC cache to make sure the server has up-to-date contents in its GC.

The FlushGC() macro checks the dirty bits in the library’s GC structure and calls _XFlushGCCache if any elements have changed. The FlushGC() macro is defined as follows:

FlushGC(display, gc)
      Display *display;
      GC gc;

display

Specifies the connection to the X server.

gc

Specifies the GC.

Note that if you extend the GC to add additional resource ID components, you should ensure that the library stub sends the change request immediately. This is because a client can free a resource immediately after using it, so if you only stored the value in the cache without forcing a protocol request, the resource might be destroyed before being set into the GC. You can use the _XFlushGCCache procedure to force the cache to be flushed. The _XFlushGCCache procedure is defined as follows:

_XFlushGC()Cache(display, gc)
      Display *display;
      GC gc;

display

Specifies the connection to the X server.

gc

Specifies the GC.