X Version 11, Release 6.4
Version 3.0
Copyright 1991 by Olivetti Research Limited, Cambridge England and Digital Equipment Corporation, Maynard, Massachusetts
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Copyright © 1991 X Consortium, Inc.
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Table of Contents
Table of Contents
The core X protocol makes no guarantees about the relative order of execution of requests for different clients. This means that any synchronization between clients must be done at the client level in an operating system-dependent and network-dependent manner. Even if there was an accepted standard for such synchronization, the use of a network introduces unpredictable delays between the synchronization of the clients and the delivery of the resulting requests to the X server.
The core X protocol also makes no guarantees about the time at which requests are executed, which means that all clients with real-time constraints must implement their timing on the host computer. Any such timings are subject to error introduced by delays within the operating system and network and are inefficient because of the need for round-trip requests that keep the client and server synchronized.
The synchronization extension provides primitives that allow synchronization between clients to take place entirely within the X server. This removes any error introduced by the network and makes it possible to synchronize clients on different hosts running different operating systems. This is important for multimedia applications, where audio, video, and graphics data streams are being synchronized. The extension also provides internal timers within the X server to which client requests can be synchronized. This allows simple animation applications to be implemented without any round-trip requests and makes best use of buffering within the client, network, and server.
The mechanism used by this extension for synchronization within the X server is to block the processing of requests from a client until a specific synchronization condition occurs. When the condition occurs, the client is released and processing of requests continues. Multiple clients may block on the same condition to give inter-client synchronization. Alternatively, a single client may block on a condition such as an animation frame marker.
The extension adds Counter and
Alarm to the set of resources managed by
the server. A counter has a 64-bit integer value that may be increased or
decreased by client requests or by the server internally. A client can
block by sending an Await request that waits until
one of a set of synchronization conditions, called TRIGGERs, becomes TRUE.
The CreateCounter request allows a client to create
a Counter that can be changed by explicit
SetCounter and ChangeCounter
requests. These can be used to implement synchronization between
different clients.
There are some counters, called System Counters,
that are changed by the server internally rather than by client
requests. The effect of any change to a system counter is not visible
until the server has finished processing the current request. In other
words, system counters are apparently updated in the gaps between the
execution of requests rather than during the actual execution of a
request. The extension provides a system counter that advances with the
server time as defined by the core protocol, and it may also provide
counters that advance with the real-world time or that change each
time the CRT screen is refreshed. Other extensions may provide their own
extension-specific system counters.
The extension provides an Alarm mechanism that allows clients to receive an
event on a regular basis when a particular counter is changed.
Table of Contents
The C routines provide direct access to the protocol and add no additional semantics.
The include file for this extension is <X11/extensions/sync.h>. Most of the names in the language binding are derived from the protocol names by prepending XSync to the protocol name and changing the capitalization.
Most of the following functions generate SYNC protocol requests.
Status XSyncQueryExtension(Display *dpy, int *event_base_return, int *error_base_return);
If dpy supports the SYNC extension,
XSyncQueryExtension returns True,
sets *event_base_return to the event number for the first SYNC event, and
sets *error_base_return to the error number for the first SYNC error. If dpy
does not support the SYNC extension, it returns False.
Status XSyncInitialize(Display *dpy, int *major_version_return, int *minor_version_return);
XSyncInitialize sets *major_version_return and
*minor version return to the major/minor SYNC protocol version supported
by the server. If the XSync library is compatible with the version
returned by the server, this function returns True.
If dpy does not support the SYNC extension, or if there was an error
during communication with the server, or if the server and library protocol
versions are incompatible, this function returns False.
The only XSync function that may be called before this function is
XSyncQueryExtension. If a client violates this rule, the effects of all XSync
calls that it makes are undefined.
XSyncSystemCounter *XSyncListSystemCounters(Display *dpy, int *n_counters_return);
XSyncListSystemCounters returns a pointer to an array
of system counters supported by the display and sets *n_counters_return
to the number of counters in the array. The array should be freed with
XSyncFreeSystemCounterList. If dpy does not support
the SYNC extension, or if there was an error during communication with
the server, or if the server does not support any system counters,
this function returns NULL.
XSyncSystemCounter has the following fields:
char * name; /* null-terminated name of system counter */
XSyncCounter counter; /* counter id of this system counter */
XSyncValue resolution; /* resolution of this system counter */
void XSyncFreeSystemCounterList(XSyncSystemCounter *list);
XSyncFreeSystemCounterList frees the memory
associated with the system counter list returned by
XSyncListSystemCounters.
XSyncCounter XSyncCreateCounter(Display *dpy, XSyncValue initial_value);
XSyncCreateCounter creates a counter on the dpy
with the given initial value and returns the counter ID. It returns
None if dpy does not support the SYNC extension.
Status XSyncSetCounter(Display *dpy, XSyncCounter counter, XSyncValue value);
XSyncSetCounter sets counter to value. It returns
False if dpy does not
support the SYNC extension; otherwise, it returns True.
Status XSyncChangeCounter(Display *dpy, XSyncCounter counter, XSyncValue value);
XSyncChangeCounter adds value to counter. It returns
False if dpy does not support the SYNC extension;
otherwise, it returns
True.
Status XSyncDestroyCounter(Display *dpy, XSyncCounter counter);
XSyncDestroyCounter destroys counter. It returns
False if dpy does not support the SYNC extension;
otherwise, it returns True.
Status XSyncQueryCounter(Display *dpy, XSyncCounter counter, XSyncValue *value_return);
XSyncQueryCounter sets *value_return to the current
value of counter. It returns False if there was an
error during communication with the server or if dpy does not support the
SYNC extension; otherwise, it returns True.
Status XSyncAwait(Display *dpy, XSyncWaitCondition *wait_list, int n_conditions);
XSyncAwait awaits on the conditions in wait_list.
The n_conditions is the number of wait conditions in wait_list. It
returns False if dpy does not support the SYNC
extension; otherwise, it returns True. The await is
processed asynchronously by the server; this function always returns
immediately after issuing the request.
XSyncWaitCondition has the following fields:
XSyncCounter trigger.counter; /*counter to trigger on */
XSyncValueType trigger.value_type; /*absolute/relative */
XSyncValue trigger.wait_value; /*value to compare counter to */
XSyncTestType trigger.test_type; /*pos/neg comparison/transtion */
XSyncValue event_threshold; /*send event if past threshold */
XSyncValueType can be either
XSyncAbsolute or XSyncRelative.
XSyncTestType can be one of
XSyncPositiveTransition,
XSyncNegativeTransition,
XSyncPositiveComparison, or
XSyncNegativeComparison.
XSyncAlarm XSyncCreateAlarm(Display *dpy, unsigned long values_mask, XSyncAlarmAttributes *values`);
XSyncCreateAlarm creates an alarm and returns the
alarm ID. It returns None if the display does not support the SYNC
extension. The values_mask and values specify the alarm attributes.
XSyncAlarmAttributes has the following fields. The
attribute_mask column specifies the symbol that the caller should OR
into values_mask to indicate that the value for the corresponding
attribute was actually supplied. Default values are used for all
attributes that do not have their attribute_mask OR’ed into values_mask.
See the protocol description for CreateAlarm for the
defaults.
type field name attribute_mask
XSyncCounter trigger.counter; XSyncCACounter
XSyncValueType trigger.value_type; XSyncCAValueType
XSyncValue trigger.wait_value; XSyncCAValue
XSyncTestType trigger.test_type; XSyncCATestType
XSyncValue delta; XSyncCADelta
Bool events; XSyncCAEvents
XSyncAlarmState state; client cannot set this
Status XSyncDestroyAlarm(Display *dpy, XSyncAlarm alarm);
XSyncDestroyAlarm destroys alarm. It returns
False if dpy does not support
the SYNC extension; otherwise, it returns True.
Status XSyncQueryAlarm(Display *dpy, XSyncAlarm alarm, XSyncAlarmAttributes *values_return);
XSyncQueryAlarm sets *values_return to the alarm’s
attributes. It returns False if there was an error
during communication with the server or if dpy does not support the
SYNC extension; otherwise, it returns True.
Status XSyncChangeAlarm(Display *dpy, XSyncAlarm alarm, unsigned long values_mask, XSyncAlarmAttributes *values);
XSyncChangeAlarm changes alarm’s attributes. The
attributes to change are specified as in
XSyncCreateAlarm. It returns
False if dpy does not support
the SYNC extension; otherwise, it returns True.
Status XSyncSetPriority(Display *dpy, XID client_resource_id, int priority);
XSyncSetPriority sets the priority of the client
owning client_resource_id to priority. If client_resource_id is None, it
sets the caller’s priority. It returns
False if dpy does not support the SYNC extension;
otherwise, it returns True.
Status XSyncGetPriority(Display *dpy, XID client_resource_id, int *return_priority);
XSyncGetPriority sets *return_priority to the
priority of the client owning client_resource_id. If client_resource_id
is None, it sets *return_priority to the caller’s priority. It returns
False if there was an error during communication
with the server or if dpy does not support the SYNC extension; otherwise, it
returns True.
The following procedures manipulate 64-bit values. They are defined both as macros and as functions. By default, the macro form is used. To use the function form, #undef the macro name to uncover the function.
void XSyncIntToValue(XSyncValue *pv, int i);
Converts i to an XSyncValue and stores it in *pv.
Performs sign extension (*pv will have the same sign as i.)
void XSyncIntsToValue(XSyncValue *pv, unsigned int low, int high);
Stores low in the low 32 bits of *pv and high in the high 32 bits of *pv.
Bool XSyncValueGreaterThan(XSyncValue a, XSyncValue b);
Returns True if a is greater than b, else returns
False.
Bool XSyncValueLessThan(XSyncValue a, XSyncValue b);
Returns True if a is less than b, else returns
False.
Bool XSyncValueGreaterOrEqual(XSyncValue a, XSyncValue b);
Returns True if a is greater than or equal to b,
else returns False.
Bool XSyncValueLessOrEqual(XSyncValue a, XSyncValue b);
Returns True if a is less than or equal to b,
else returns False.
Bool XSyncValueEqual(XSyncValue a, XSyncValue b);
Returns True if a is equal to b,
else returns False.
Bool XSyncValueIsNegative(XSyncValue v);
Returns True if v is negative,
else returns False.
Bool XSyncValueIsZero(XSyncValue v);
Returns True if v is zero,
else returns False.
Bool XSyncValueIsPositive(XSyncValue v);
Returns True if v is positive,
else returns False.
unsigned int XSyncValueLow32(XSyncValue v);
Returns the low 32 bits of v.
unsigned int XSyncValueHigh32(XSyncValue v);
Returns the high 32 bits of v.
void XSyncValueAdd(XSyncValue *presult, XSyncValue a, XSyncValue b, Bool *poverflow);
Adds a to b and stores the result in *presult. If the result could not
fit in 64 bits, *poverflow is set to True, else it is
set to False.
void XSyncValueSubtract(XSyncValue *presult, XSyncValue a, XSyncValue b, Bool *poverflow);
Subtracts b from a and stores the result in *presult. If the result could not
fit in 64 bits, *poverflow is set to True, else it is
set to False.
void XSyncMaxValue(XSyncValue *pv);
Sets *pv to the maximum value expressible in 64 bits.
void XSyncMinValue(XSyncValue *pv);
Sets *pv to the minimum value expressible in 64 bits.
Let event_base be the value event base return as defined in the function
XSyncQueryExtension.
An XSyncCounterNotifyEvent’s type field has the value
event_base + XSyncCounterNotify. The fields of this
structure are:
int type; /* event base + XSyncCounterNotify */
unsigned long serial; /* number of last request processed by server */
Bool send event; /* true if this came from a SendEvent request */
Display * display; /* Display the event was read from */
XSyncCounter counter; /* counter involved in await */
XSyncValue wait_value; /* value being waited for */
XSyncValue counter_value; /* counter value when this event was sent */
Time time; /* milliseconds */
int count; /* how many more events to come */
Bool destroyed; /* True if counter was destroyed */
An XSyncAlarmNotifyEvent’s type field has the value
event_base + XSyncAlarmNotify. The fields of
this structure are:
int type; /* event_base + XSyncAlarmNotify */
unsigned long serial; /* number of last request processed by server */
Bool send_event; /* true if this came from a SendEvent request */
Display * display; /*Display the event was read from */
XSyncAlarm alarm; /* alarm that triggered */
XSyncValue counter_value /* value that triggered the alarm */
XSyncValue alarm_value /* test value of trigger in alarm */
Time time; /* milliseconds */
XSyncAlarmState state; /* new state of alarm */
Let error_base be the value
error_base_return as defined in the function
XSyncQueryExtension.
An XSyncAlarmError’s error_code field has
XSyncBadAlarm. The fields of this structure are:
int type
Display * display; /* Display the event was read from */
XSyncCounter counter; /* resource id */
unsigned long serial; /* serial number of failed request */
unsigned char error_code; /* error_base + XSyncBadAlarm */
unsigned char request_code; /* Major op-code of failed request */
unsigned char minor_code; /* Minor op-code of failed request */
An XSyncCounterError’s error code field has the value
error_base + XSyncBadCounter. The fields of this
structure are:
int type
Display * display; /* Display the event was read from */
XSyncCounter counter; /* resource id */
unsigned long serial; /* serial number of failed request */
unsigned char error_code; /* error_base + XSyncBadCounter */
unsigned char request_code; /* Major op-code of failed request */
unsigned char minor_code; /* Minor op-code of failed request */