# X Synchronization Extension Library

## Synchronization Protocol

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.

## C Language Binding

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 */