SYNCLIB(3)

NAME
synclib - 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/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().

  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 */

AUTHORS
-nosplit X Consortium Standard

X Version 11, Release 6 Version 3.0 -split 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. Copyright © 1991 Olivetti Research Limited, Cambridge England Digital Equipment Corporation, Maynard, Massachusetts

  Legal Notice
Permission to use, copy, modify, and distribute this documentation for any purpose and without fee is hereby granted, provided that the above copyright notice appear in all copies. Olivetti, Digital, MIT, and the X Consortium make no representations about the suitability for any purpose of the information in this document. This documentation is provided as is without express or implied warranty.

  Legal Notice
Copyright © 1991 X Consortium, Inc.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE X CONSORTIUM BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Except as contained in this notice, the name of the X Consortium shall not be used in advertising or otherwise to promote the sale, use or other dealings in this Software without prior written authorization from the X Consortium.

X Window System is a trademark of The OpenGroup.