X Synchronization Extension Library

X Consortium Standard

Tim Glauert

Olivetti Research
MultiWorks

Dave Carver

Digital Equipment Corporation
MIT/Project Athena

Jim Gettys

Digital Equipment Corporation
Cambridge Research Laboratory

David P. Wiggins

X Consortium, Inc.

X Version 11, Release 7.7

Version 3.0

Copyright © 1991 Olivetti Research Limited, Cambridge England, Digital
Equipment Corporation, Maynard, Massachusetts

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Table of Contents

1. Synchronization Protocol

    Description

2. C Language Binding

    C Functions
    C Macros/Functions
    Events
    Errors

Chapter 1. Synchronization Protocol

Table of Contents

Description

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.

Description

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.

Chapter 2. C Language Binding

Table of Contents

C Functions
C Macros/Functions
Events
Errors

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.

C Functions

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/transition */
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.

C Macros/Functions

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.

Events

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 */

Errors

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 */

