glFlush(3G)glFlush(3G)NAMEglFlush - force execution of GL commands in finite time
Different GL implementations buffer commands in several different loca‐
tions, including network buffers and the graphics accelerator itself.
glFlush() 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 glFlush()
whenever they count on having all of their previously issued commands
completed. For example, call glFlush() before waiting for user input
that depends on the generated image.
NOTESglFlush() can return at any time. It does not wait until the execution
of all previously issued GL commands is complete.
GL_INVALID_OPERATION is generated if glFlush() is executed between the
execution of glBegin() and the corresponding execution of glEnd().