Name

    KHR_fence_sync

Name Strings

    EGL_KHR_fence_sync
    GL_OES_EGL_sync
    VG_KHR_EGL_sync

Contributors

    Acorn Pooley
    Gary King
    Gregory Prisament
    Jon Leech

Contacts

    Acorn Pooley, NVIDIA Corporation (apooley 'at' nvidia.com)
    Gary King, NVIDIA Corporation (gking 'at' nvidia.com)
    Gregory Prisament, NVIDIA Corporation (gprisament 'at' nvidia.com)
    Jon Leech (jon 'at' alumni.caltech.edu)

Notice

    Copyright (c) 2006-2013 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Status

    Complete. Approved by the EGL Working Group on March 3, 2010.
    Approved by the Khronos Board of Promoters on April 30, 2010.

Version

    Version 24, January 31, 2014

Number

    EGL Extension #20
    OpenGL ES Extension #75
    OpenVG Extension #7

Dependencies

    Requires EGL 1.1

    This extension is written against the wording of the EGL 1.2
    Specification.

Overview

    This extension introduces the concept of "sync objects" into EGL.
    Sync objects are a synchronization primitive, representing events
    whose completion can be tested or waited upon.  This extension
    borrows heavily from the GL_ARB_sync extension and introduces a type
    of sync object known as a "fence sync object" comparable to the
    OpenGL fence sync object. The specification is designed to allow
    additional types of sync objects to be easily introduced in later
    extensions.

    Fence sync objects have corresponding fence commands, which are
    inserted into a client API command stream at the time the fence sync
    is created. A fence sync object is used to wait for completion of
    the corresponding fence command. This allows applications to request
    a partial Finish of an API command stream, wherein all commands
    issued in a particular client API context will be forced to complete
    before control is returned to the calling thread.

    This document describes three different extension strings
    collectively. The "EGL_KHR_fence_sync" string indicates that fence
    syncs and the corresponding interfaces (to create and place a fence,
    destroy, query, and wait on) are supported.

    The remaining extensions list valid client APIs for fence syncs. The
    "GL_OES_EGL_sync" string indicates that a fence sync object can be
    created in association with a fence command placed in the command
    stream of a bound OpenGL ES context. The "VG_KHR_EGL_sync" string
    indicates the same thing for a bound OpenVG context.

New Types

    /*
     * EGLSyncKHR is an opaque handle to an EGL sync object
     */
    typedef void* EGLSyncKHR;

    /*
     * EGLTimeKHR is a 64-bit unsigned integer representing intervals
     * in nanoseconds.
     */
    #include <khrplatform.h>
    typedef khronos_utime_nanoseconds_t EGLTimeKHR;


New Procedures and Functions

    EGLSyncKHR eglCreateSyncKHR(
                        EGLDisplay dpy,
                        EGLenum type,
                        const EGLint *attrib_list);

    EGLBoolean eglDestroySyncKHR(
                        EGLDisplay dpy,
                        EGLSyncKHR sync);

    EGLint eglClientWaitSyncKHR(
                        EGLDisplay dpy,
                        EGLSyncKHR sync,
                        EGLint flags,
                        EGLTimeKHR timeout);

    EGLBoolean eglGetSyncAttribKHR(
                        EGLDisplay dpy,
                        EGLSyncKHR sync,
                        EGLint attribute,
                        EGLint *value);


New Tokens

    Accepted by the <type> parameter of eglCreateSyncKHR, and returned
    in <value> when eglGetSyncAttribKHR is called with <attribute>
    EGL_SYNC_TYPE_KHR:

    EGL_SYNC_FENCE_KHR                      0x30F9

    Accepted by the <attribute> parameter of eglGetSyncAttribKHR:

    EGL_SYNC_TYPE_KHR                       0x30F7
    EGL_SYNC_STATUS_KHR                     0x30F1
    EGL_SYNC_CONDITION_KHR                  0x30F8

    Returned in <value> when eglGetSyncAttribKHR is called with
    <attribute> EGL_SYNC_STATUS_KHR:

    EGL_SIGNALED_KHR                        0x30F2
    EGL_UNSIGNALED_KHR                      0x30F3

    Returned in <value> when eglGetSyncAttribKHR is called with
    <attribute> EGL_SYNC_CONDITION_KHR:

    EGL_SYNC_PRIOR_COMMANDS_COMPLETE_KHR    0x30F0

    Accepted in the <flags> parameter of eglClientWaitSyncKHR:

    EGL_SYNC_FLUSH_COMMANDS_BIT_KHR         0x0001

    Accepted in the <timeout> parameter of eglClientWaitSyncKHR:

    EGL_FOREVER_KHR                         0xFFFFFFFFFFFFFFFFull

    Returned by eglClientWaitSyncKHR:

    EGL_TIMEOUT_EXPIRED_KHR                 0x30F5
    EGL_CONDITION_SATISFIED_KHR             0x30F6

    Returned by eglCreateSyncKHR in the event of an error:

    EGL_NO_SYNC_KHR                         ((EGLSyncKHR)0)

Changes to Chapter 3 of the EGL 1.2 Specification (EGL Functions and Errors)

    Add a new subsection at the end of Section 3.8, page 43
    (Synchronization Primitives)

    "3.8.1  Sync Objects

    In addition to the aforementioned synchronization functions, which
    provide an efficient means of serializing client and native API
    operations within a thread, <sync objects> are provided to enable
    synchronization of client API operations between threads and/or
    between API contexts. Sync objects may be tested or waited upon by
    application threads.

    Sync objects have a status with two possible states: <signaled> and
    <unsignaled>. Initially, sync objects are unsignaled. EGL may be
    asked to wait for a sync object to become signaled, or a sync
    object's status may be queried.

    Depending on the type of a sync object, its status may be changed
    either by an external event, or by explicitly signaling and
    unsignaling the sync.

    Sync objects are associated with an EGLDisplay when they are
    created, and have <attributes> defining additional aspects of the
    sync object. All sync objects include attributes for their type and
    their status. Additional attributes are discussed below
    for different types of sync objects.

    <Fence sync objects> are created in association with a <fence
    command> in a client API. When the client API executes the fence
    command, an event is generated which signals the corresponding fence
    sync object. Fence sync objects may not be explicitly signaled, and
    may only change their status once, from the initial unsignaled
    status to signaled. Fence sync objects may be used to wait for
    partial completion of a client API command stream, as a more
    flexible form of glFinish / vgFinish.

    The command

        EGLSyncKHR eglCreateSyncKHR(
                            EGLDisplay dpy,
                            EGLenum type,
                            const EGLint *attrib_list);

    creates a sync object of the specified <type> associated with the
    specified display <dpy>, and returns a handle to the new object.
    <attrib_list> is an attribute-value list specifying other attributes
    of the sync object, terminated by an attribute entry EGL_NONE.
    Attributes not specified in the list will be assigned their default
    values.

    If <type> is EGL_SYNC_FENCE_KHR, a fence sync object is created. In
    this case <attrib_list> must be NULL or empty (containing only
    EGL_NONE). Attributes of the fence sync object are
    set as follows:

      Attribute Name         Initial Attribute Value(s)
      ---------------        --------------------------
      EGL_SYNC_TYPE_KHR      EGL_SYNC_FENCE_KHR
      EGL_SYNC_STATUS_KHR    EGL_UNSIGNALED_KHR
      EGL_SYNC_CONDITION_KHR EGL_SYNC_PRIOR_COMMANDS_COMPLETE_KHR

    When a fence sync object is created, eglCreateSyncKHR also inserts a
    fence command into the command stream of the bound client API's
    current context (i.e., the context returned by
    eglGetCurrentContext), and associates it with the newly created sync
    object.

    When the condition of the sync object is satisfied by the fence
    command, the sync is signaled by the associated client API context,
    causing any eglClientWaitSyncKHR commands (see below) blocking on
    <sync> to unblock. The only condition currently supported is
    EGL_SYNC_PRIOR_COMMANDS_COMPLETE_KHR, which is satisfied by
    completion of the fence command corresponding to the sync object,
    and all preceding commands in the associated client API context's
    command stream. The sync object will not be signaled until all
    effects from these commands on the client API's internal and
    framebuffer state are fully realized. No other state is affected by
    execution of the fence command.

    Each client API which supports fence commands indicates this support
    in the form of a client API extension. If the GL_OES_EGL_sync
    extension is supported by OpenGL ES (either version 1.x or 2.0), a
    fence sync object may be created when the currently bound API is
    OpenGL ES. If the VG_KHR_EGL_sync extension is supported by OpenVG,
    a fence sync object may be created when the currently bound API is
    OpenVG.

    Errors
    ------

      * If <dpy> is not the name of a valid, initialized EGLDisplay,
        EGL_NO_SYNC_KHR is returned and an EGL_BAD_DISPLAY error is
        generated.
      * If <attrib_list> is neither NULL nor empty (containing only
        EGL_NONE), EGL_NO_SYNC_KHR is returned and an EGL_BAD_ATTRIBUTE
        error is generated.
      * If <type> is not a supported type of sync object,
        EGL_NO_SYNC_KHR is returned and an EGL_BAD_ATTRIBUTE error is
        generated.
      * If <type> is EGL_SYNC_FENCE_KHR and no context is current for
        the bound API (i.e., eglGetCurrentContext returns
        EGL_NO_CONTEXT), EGL_NO_SYNC_KHR is returned and an
        EGL_BAD_MATCH error is generated.
      * If <type> is EGL_SYNC_FENCE_KHR and <dpy> does not match the
        EGLDisplay of the currently bound context for the currently
        bound client API (the EGLDisplay returned by
        eglGetCurrentDisplay()) then EGL_NO_SYNC_KHR is returned and an
        EGL_BAD_MATCH error is generated.
      * If <type> is EGL_SYNC_FENCE_KHR and the currently bound client
        API does not support the client API extension indicating it can
        place fence commands, then EGL_NO_SYNC_KHR is returned and an
        EGL_BAD_MATCH error is generated.

    The command

        EGLint eglClientWaitSyncKHR(
                            EGLDisplay dpy,
                            EGLSyncKHR sync,
                            EGLint flags,
                            EGLTimeKHR timeout);

    blocks the calling thread until the specified sync object <sync> is
    signaled, or until <timeout> nanoseconds have passed.

    More than one eglClientWaitSyncKHR may be outstanding on the same
    <sync> at any given time. When there are multiple threads blocked on
    the same <sync> and the sync object is signaled, all such threads
    are released, but the order in which they are released is not
    defined.

    If the value of <timeout> is zero, then eglClientWaitSyncKHR simply
    tests the current status of <sync>. If the value of <timeout> is the
    special value EGL_FOREVER_KHR, then eglClientWaitSyncKHR does not
    time out. For all other values, <timeout> is adjusted to the closest
    value allowed by the implementation-dependent timeout accuracy,
    which may be substantially longer than one nanosecond.

    eglClientWaitSyncKHR returns one of three status values describing
    the reason for returning. A return value of EGL_TIMEOUT_EXPIRED_KHR
    indicates that the specified timeout period expired before <sync>
    was signaled, or if <timeout> is zero, indicates that <sync> is
    not signaled. A return value of EGL_CONDITION_SATISFIED_KHR
    indicates that <sync> was signaled before the timeout expired, which
    includes the case when <sync> was already signaled when
    eglClientWaitSyncKHR was called. If an error occurs then an error is
    generated and EGL_FALSE is returned.

    If the sync object being blocked upon will not be signaled in finite
    time (for example, by an associated fence command issued previously,
    but not yet flushed to the graphics pipeline), then
    eglClientWaitSyncKHR may wait forever. To help prevent this behavior
    (footnote1), if the EGL_SYNC_FLUSH_COMMANDS_BIT_KHR bit is set in
    <flags>, and <sync> is unsignaled when eglClientWaitSyncKHR is
    called, then the equivalent of Flush() will be performed for the
    current API context (i.e., the context returned by
    eglGetCurrentContext()) before blocking on <sync>. If no context is
    current for the bound API, the EGL_SYNC_FLUSH_COMMANDS_BIT_KHR bit
    is ignored.

       [footnote 1: The simple Flush behavior defined by
        EGL_SYNC_FLUSH_COMMANDS_BIT_KHR will not help when waiting for a
        fence command issued in a different context's command stream.
        Applications which block on a fence sync object must take
        additional steps to ensure that the context from which the
        associated fence command was issued has flushed that command to
        the graphics pipeline.]

    Errors
    ------

      * If <sync> is not a valid sync object for <dpy>, EGL_FALSE is
        returned and an EGL_BAD_PARAMETER error is generated.
      * If <dpy> does not match the EGLDisplay passed to
        eglCreateSyncKHR when <sync> was created, the behaviour is
        undefined.


    The command

        EGLBoolean eglGetSyncAttribKHR(
                            EGLDisplay dpy,
                            EGLSyncKHR sync,
                            EGLint attribute,
                            EGLint *value);

    is used to query attributes of the sync object <sync>. Legal values
    for <attribute> depend on the type of sync object, as shown in table
    3.cc. Assuming no errors are generated, EGL_TRUE is returned and the
    value of the queried attribute is returned in <value>.

    Attribute              Description                Supported Sync Objects
    -----------------      -----------------------    ----------------------
    EGL_SYNC_TYPE_KHR      Type of the sync object    All
    EGL_SYNC_STATUS_KHR    Status of the sync object  All
    EGL_SYNC_CONDITION_KHR Signaling condition        EGL_SYNC_FENCE_KHR only

    Table 3.cc  Attributes Accepted by eglGetSyncAttribKHR Command

    Errors
    ------

      * If <sync> is not a valid sync object for <dpy>, EGL_FALSE is
        returned and an EGL_BAD_PARAMETER error is generated.
      * If <dpy> does not match the display passed to eglCreateSyncKHR
        when <sync> was created, the behaviour is undefined.
      * If <attribute> is not one of the attributes in table 3.cc,
        EGL_FALSE is returned and an EGL_BAD_ATTRIBUTE error is
        generated.
      * If <attribute> is not supported for the type of sync object
        passed in <sync>, EGL_FALSE is returned and an EGL_BAD_MATCH
        error is generated.

    If any error occurs, <*value> is not modified.

    The command

        EGLBoolean eglDestroySyncKHR(
                            EGLDisplay dpy,
                            EGLSyncKHR sync);

    is used to destroy an existing sync object.

    If any eglClientWaitSyncKHR commands are blocking on <sync> when
    eglDestroySyncKHR is called, <sync> is flagged for deletion and will
    be deleted when it is no longer associated with any fence command
    and is no longer blocking any eglClientWaitSyncKHR command.

    If no errors are generated, EGL_TRUE is returned, and <sync> will no
    longer be the handle of a valid sync object.

    Errors
    ------

      * If <sync> is not a valid sync object for <dpy>, EGL_FALSE is
        returned and an EGL_BAD_PARAMETER error is generated.
      * If <dpy> does not match the display passed to eglCreateSyncKHR
        when <sync> was created, the behaviour is undefined.

Issues

    Note about the Issues
    ---------------------
    The wording for this extension was originally written as a single
    extension defining two types of sync object; a "reusable sync
    object" and a "fence sync object". That extension was split to
    produce standalone extensions for each type of sync object, and
    references to the other type removed from the specification
    language. This issues list has been simplied to remove references to
    reusable sync objects but is otherwise very similar to the
    EGL_KHR_reusable_sync extension issues list.

    1. [REMOVED - found in the reusable_sync extension.]

    2. [REMOVED - found in the reusable_sync extension.]

    3. What does this extension provide that can not be accomplished
    with the existing, more efficient eglWaitClient and eglWaitNative
    API functions?

    RESPONSE: eglWaitClient and eglWaitNative may be implemented in
    extremely lightweight manners, in some cases not blocking the
    calling thread at all; however, they can not be used to synchronize
    between client API contexts and native APIs executing in separate
    threads (or simply between client API contexts executing in separate
    threads), such as between a thread with an active OpenGL context and
    a second thread performing video decode.

    4. What does this extension provide that could not be accomplished
    with native platform synchronization primitives and the existing
    client API Finish commands?

    RESPONSE: This extension provides a lighter-weight mechanism for
    synchronizing an application with client API command streams than
    the all-or-nothing Finish commands, enabling applications to block
    until a subset of issued client API commands have completed.

    5. [REMOVED - found in the reusable_sync extension.]

    6. Please provide a more detailed description of how
    eglClientWaitSyncKHR behaves.

    RESOLVED: eglClientWaitSyncKHR blocks until the status of the sync
    object transitions to the signaled state. Sync object status is
    either signaled or unsignaled. More detailed rules describing
    signalling follow (these may need to be imbedded into the actual
    spec language):

      * A fence sync object has two possible status values: signaled or
        unsignaled.
      * When created, the status of the sync object is unsignaled.
      * A fence command is inserted into a command stream. A fence sync
        object is not.
      * A fence command, once its condition has been met, will set its
        associated sync object to the signaled state. The only condition
        currently supported is EGL_SYNC_PRIOR_COMMANDS_COMPLETE_KHR.
      * A wait function, such as ClientWaitSyncKHR, waits on a fence
        sync object, not on a fence command.
      * A wait function called on a sync object in the unsignaled state
        will block. It unblocks (note, not "returns to the application")
        when the sync object transitions to the signaled state.
      * A wait function called on a sync object in the signaled state
        will return immediately.

    7. [REMOVED - found in the reusable_sync extension.]

    8. [REMOVED - found in the reusable_sync extension.]

    9. Should eglDestroySyncKHR release all WaitSyncs placed on a fence
    sync object?

    RESOLVED: No. (note that this behavior differs from reusable syncs.)

    Fence sync objects are intended to be signalled by the graphics
    driver within a short period of time (typically less than 1 second
    after creation) and so should not cause waiting threads to hang
    forever. To reduce implementation complexity, fence sync objects are
    defined to not release waiting threads; waiting threads are released
    normally when their condition is satisfied or their timeout expires.
    The handle to a fence sync object immediately becomes invalid
    following a call to eglDestroySyncKHR.

Revision History

#24 (Jon Leech, January 31, 2014)
    - Clarify return value of ClientWaitSyncKHR when called with <timeout>
      of zero for an unsignaled <sync> (Bug 11576).
#23 (Jon Leech, April 23, 2013)
    - Simplify issues list to remove issues specific to reusable sync
      objects and general sync object design issues.
#22 (Jon Leech, June 15, 2010)
    - Correct minor typos in GL/VG extension names.
#21 (Jon Leech, May 5, 2010)
    - Correct minor typos, assign extension numbers for EGL, OpenGL ES,
      and OpenVG, and publish in the registry,
#20 (Robert Palmer, July 14, 2009)
    - Branch wording from draft KHR_sync specification. Remove ability
      to create "reusable sync objects and all tokens/wording specific
      to them.
#19 (Robert Palmer, July 22, 2009)
    - Replace specific eglCreateSyncKHR error cases for bad <type>
    argument with extensible catch-all case.
#18 (Robert Palmer, July 8, 2009)
    - Issues 8 and 9 declared resolved in EGL meeting 2009-07-08
#17 (Robert Palmer, July 8, 2009)
    - Update eglDestroySyncKHR to special-case deletion of fence sync
      objects.  This is explained in issue 9.
    - Corrected EGL_REUSABLE_SYNC_KHR -> EGL_SYNC_REUSABLE_KHR
    - Define value for EGL_SYNC_REUSABLE_KHR
    - Fix typo and whitespace
#16 (Jon Leech, July 7, 2009)
    - Update description of new tokens to match changes to the
      eglCreateSyncKHR entry point in revision 15.
#15 (Jon Leech, June 16, 2009)
    - Define separate one-time fence sync and reusable sync extensions
      and corresponding extension strings. Remove AUTO_RESET and
      eglFenceKHR. Rename eglCreateFenceSyncKHR to eglCreateSyncKHR and
      change initial status of reusable syncs to unsignaled. Clarify
      which functions apply to which types of sync objects. Update
      issues list.
#14 (Jon Leech, April 29, 2009)
    - Clarify that all waiters are woken up on signalling a sync.
      Remove tabs to cleanup some formatting issues.
#13 (Acorn Pooley, April 2, 2009)
    - Renamed
        GL_OES_egl_sync -> GL_OES_EGL_sync
        VG_KHR_egl_sync -> VG_KHR_EGL_sync
#12 (Jon Leech, April 1, 2009)
    - Changed sync flags type from EGLuint to EGLint and add issue 7.
#11 (Acorn Pooley, February 4, 2009)
    - add error case to eglGetSyncAttribKHR.
    - fix year on rev 8-10 (2008->2009)
#10 (Acorn Pooley, February 4, 2009)
    - clarify some error message descriptions
#9  (Greg Prisament, January 15, 2009)
    - Destroy now wakes up all waits (eglClientWaitSyncKHR)
    - Add EGLDisplay <dpy> as first parameter to all commands
    - Split into 3 extension strings, EGL_KHR_sync, GL_OES_egl_sync,
      VG_KHR_egl_sync, all described in this document.
    - Add attribute AUTO_RESET_KHR
    - Time type uses the type from khrplatform.h
    - Remove EGL_ALREADY_SIGNALLED
#8  (Jon Leech, November 11, 2009)
   - Assign enum values
#7   (Acorn Pooley, October 30, 2008)
   - Fix typos
   - remove obsolete wording about Native sync objects (see issue 5)
   - formatting: remove tabs, 80 columns
#6   (Acorn Pooley, October 27, 2008)
   - Corrected 'enum' to 'EGLenum' in prototypes.
#5   (Jon Leech, September 9, 2008)
   - Removed native sync support (eglCreateNativeSyncKHR and
     EGL_SYNC_NATIVE_SYNC_KHR), and re-flowed spec to fit in 80 columns.
#4   (Jon Leech, November 20, 2007)
   - Corrected 'enum' to 'EGLenum' in prototypes.
#3   (Jon Leech, April 5, 2007)
   - Added draft Status and TBD Number
#2   (November 27, 2006)
   - Changed OES token to KHR