Does anybody know what glFlush() is used for?

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Different OpenGL implementations buffer commands in several different locations, including network buffers and the graphics accelerator itself. The glFlush function empties all these buffers, causing all issued commands to be executed as quickly as they are accepted by the actual rendering engine. Though this execution may not be completed in any particular time period, it does complete in finite time.

Because any OpenGL program might be executed over a network, or on an accelerator that buffers commands, be sure to call glFlush in all programs whenever they require that all of their previously issued commands have been completed. For example, call glFlush before waiting for user input that depends on the generated image.

The glFlush function can return at any time. It does not wait until the execution of all previously issued OpenGL functions is complete.