glXWaitGL.3G man page
glXWaitGL — complete GL execution prior to subsequent X calls
GL rendering calls made prior to glXWaitGL are guaranteed to be executed before X rendering calls made after glXWaitGL. Although this same result can be achieved using glFinish(), glXWaitGL does not require a round trip to the server, and it is therefore more efficient in cases where client and server are on separate machines.
glXWaitGL is ignored if there is no current GLX context.
glXWaitGL may or may not flush the X stream.
GLXBadCurrentWindow is generated if the drawable associated with the current context of the calling thread is a window, and that window is no longer valid.
glFinish(), glFlush(), glXWaitX(), XSync
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