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