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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   Copyright © 1991 X Consortium, Inc.

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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 sys
   tem counter */
   XSyncCounter        counter;     /* counter id of this system c
   ounter */
   XSyncValue          resolution;  /* resolution of this system c
   ounter */

   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/trans
   ition */
   XSyncValue       event_threshold;    /*send event if past thres
   hold */

   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 + XSyncCounterNot
   ify */
   unsigned long    serial;        /* number of last request proce
   ssed by server */
   Bool             send event;    /* true if this came from a Sen
   dEvent request */
   Display *        display;       /* Display the event was read f
   rom */
   XSyncCounter     counter;       /* counter involved in await */
   XSyncValue       wait_value;    /* value being waited for */
   XSyncValue       counter_value; /* counter value when this even
   t was sent */
   Time             time;          /* milliseconds */
   int              count;         /* how many more events to come
    */
   Bool             destroyed;     /* True if counter was destroye
   d */

   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 process
   ed by server */
   Bool            send_event;   /* true if this came from a SendE
   vent 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 req
   uest */
   unsigned char      error_code;   /* error_base + XSyncBadAlarm
   */
   unsigned char      request_code; /* Major op-code of failed req
   uest */
   unsigned char      minor_code;   /* Minor op-code of failed req
   uest */

   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 req
   uest */
   unsigned char      error_code;   /* error_base + XSyncBadCounte
   r */
   unsigned char      request_code; /* Major op-code of failed req
   uest */
   unsigned char      minor_code;   /* Minor op-code of failed req
   uest */
