Different GL implementations buffer commands in several different locations, including network buffers and the graphics accelerator itself. flush empties all of 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 GL program might be executed over a network, or on an accelerator that buffers commands, all programs should call flush whenever they count on having all of their previously issued commands completed. For example, call flush before waiting for user input that depends on the generated image.

Note that flush can return at any time. It does not wait until the execution of all previously issued GL commands is complete.

finish does not return until the effects of all previously called GL commands are complete. Such effects include all changes to GL state, all changes to connection state, and all changes to the frame buffer contents.

Note that finish requires a round trip to the server.