ggiAddFlags(3) Set or get flags affecting operation on a visual

Other Alias

ggiSetFlags, ggiGetFlags, ggiRemoveFlags


#include <ggi/ggi.h>
int ggiSetFlags(ggi_visual_t vis, ggi_flags flags);
ggi_flags ggiGetFlags(ggi_visual_t vis);
#define ggiAddFlags(vis,flags) \
ggiSetFlags((vis), ggiGetFlags((vis)) | (flags))
#define ggiRemoveFlags(vis,flags) \
ggiSetFlags((vis), ggiGetFlags((vis)) & ~(flags))


ggiSetFlags sets the specified flags (bitwise OR'd together) on a visual.

ggiGetFlags obtains the flags currently in effect.

ggiAddFlags and ggiRemoveFlags are macros that set or unset the specified flags.

Flags are used to alter a visual's underlying behavior. All flags default to an unset value. Flags which are not supported by a given visual will remain unset even when an attempt is made to raise them. Thus, it is possible to tell by reading back the flags whether or not each of the flags is supported by the given visual.

Note: It is recommended to set the flags before setting a mode, i.e. right after ggiOpen(3).


ggiSetFlags, ggiAddFlags, and ggiRemoveFlags return 0 on success, a ggi-error(3) code on failure. This will only happen if the failure of a target to support the addition or removal of a flag will cause the target to behave in a way that the application is not expecting. As of this writing there are no such cases. On visuals where certain flags are unsupported but are inconsequential, these functions will return a successful return code, but will not actually set or clear the flag.

ggiGetFlags returns the current flags. This can be used by the curious to check whether a flag is being silently ignored as per above.


Some visuals allow different modes with regard to when the screen is updated and the actual drawing takes place.
  • In synchronous mode when the drawing command returns, it is already or will be executed very shortly. So the visible effect is that everything is drawn immediately. (It is not guaranteed in the strict sense that it is already drawn when the function call returns, but almost.) This is the default mode for all visuals.
  • The asynchronous mode does not guarantee that drawing commands are executed immediately, but is faster on many targets. If the visual does not support asynchronous mode, attempting to set it has no effect. Code written for asynchronous visuals will always perform correctly on synchronous visuals (but not vice-versa), so it is not necessary to adapt a program's behavior if this flag is not available.

    To make sure that all pending graphics operations are actually done and the screen is updated, you need to call ggiFlush(3). This call is not needed in synchronous mode.

Important: On some targets such as the X target there is no real synchronous mode, so LibGGI fakes one by periodically calling ggiFlush(3) in the background. This process can take about half the execution time of a program. So using synchronous mode can really slow things down.

However, the synchronous mode is the default, because it is what most programmers expect.

In either mode, all operations are guaranteed to be performed in the order in which they are called. Reordering is not done.

GGI guarantees that the effects of drawing operations on the final state of the buffer are consistant with the order in which they were invoked, but as to what order the operations visibly appear during a flush, that is entirely up to the target. You could draw a red square with the GPU (through the target), and then draw a green square inside it via software -- you will always end up with a red square inside a green square, but the user may see the red square appear first.

When it comes to directbuffer, though, that is the problem that the TIDYBUF flag is meant to fix. Or at least, the TIDYBUF flag fixes a problem with the way GGI fixes the serialization problem. The display is flushed entirely before the acquire completes, and then the db is flushed entirely before the db is released, so that is serialized. The TIDYBUF stuff lets you bypass this serialization for efficiency.

So the recommendation for all graphics applications is to set the asynchronous mode. It will be far more efficient on some platforms and will never be worse.

Setting up asynchronous mode:

ggiAddFlags(vis, GGIFLAG_ASYNC);      /* switches to asynchronous mode */
ggiFlush(vis);                        /* updates the screen */
ggiRemoveFlags(vis, GGIFLAG_ASYNC);   /* switches to synchronous mode */


Some visuals allow applications to manage their own dirty regions when using the directbuffer.
  • In the default dirty-buffering mode, visuals which use backbuffers to render to a display system will refresh the entire screen when the resource lock is held and then released for the write frame's directbuffer. This happens in both async and sync modes. In syncronous modes this full-screen refresh may be performed at regular intervals. This can be very inefficient, but it guarantees that naive applications will be rendered correctly even though they were not written with a backbuffered display in mind.

    These visuals may also perform dirty-region tracking, such that if the directbuffer is used, altered data may never reach the screen until the lock is released, because the visual does not know that a certain area of the backbuffer contains new (dirty) data. Even explicitly calling ggiFlushRegion(3) on the affected area may not cause the data to be sent to the screen.

  • In tidy-buffering mode, which is set by raising the flag GGIFLAG_TIDYBUF, visuals do not synchronize the screen at all when the write frame's directbuffer lock is held or upon its release. However, in this mode, ggiFlushRegion(3) will always cause the requested region of the screen to be updated.

    Note that this means that, as long as the lock is held, affected regions may also have to be flushed (and thus, should be flushed) after normal drawing primitives are called.

    Before releasing the lock, applications should be sure to flush all affected regions, because the visual may revert to its default dirty-region management behavior after the lock is released.

    Do note, also, that in multi-frame displays ggiFlushRegion(3) affects only the current write frame, so even though it is possible to use a directbuffer to alter a different frame, you must call ggiSetWriteFrame(3) to tell the visual that you will be altering the frame.

The GGIFLAG_TIDYBUF flag is not available on all visuals, but it is safe to attempt to set it whether or not it is available. Code written for the tidy-buffering mode will display correctly on visuals which do not have a tidy-buffering mode (but not vice-versa), so it is not necessary to adapt program behavior to its non-presence.

It is recommended that, if an application must use directbuffer, the application should attempt to place the visual in tidy-buffered mode. Do note, though, that many applications that use the directbuffer do not actually need to do so and probably should not, as it reduces portability.