This protocol allows the Window Manager to determine if the Client is still processing X events. This can be used by the Window Manager to determine if a window which fails to close after being sent WM_DELETE_WINDOW has stopped responding or has stalled for some other reason, such as waiting for user confirmation. A Client SHOULD indicate that it is willing to participate in this protocol by listing _NET_WM_PING in the WM_PROTOCOLS property of the client window.

A Window Manager can use this protocol at any time by sending a client message as follows:

type = ClientMessage
window = the respective client window
message_type = WM_PROTOCOLS
format = 32
data.l[0] = _NET_WM_PING
data.l[1] = timestamp
data.l[2] = the respective client window
other data.l[] elements = 0

A participating Client receiving this message MUST send it back to the root window immediately, by setting window = root, and calling XSendEvent with the same event mask like all other root window messages in this specification use. The Client MUST NOT alter any field in the event other than the window. This includes all 5 longs in the data.l[5] array. The Window Manager can uniquely identify the ping by the timestamp and the data.l[2] field if necessary. Note that some older clients may not preserve data.l[2] through data.l[4].

The Window Manager MAY kill the Client (using _NET_WM_PID) if it fails to respond to this protocol within a reasonable time.

See also the implementation notes on killing hung processes.

This protocol uses the XSync extension (see the protocol specification (http://freedesktop.org/cgi-bin/viewcvs.cgi/xorg/xc/doc/hardcopy/Xext/sync.PS.gz) and the library documentation (http://freedesktop.org/cgi-bin/viewcvs.cgi/xorg/xc/doc/hardcopy/Xext/synclib.PS.gz)) to let client and window manager synchronize the repaint of the window manager frame and the client window. A client indicates that it is willing to participate in the protocol by listing _NET_WM_SYNC_REQUEST in the WM_PROTOCOLS property of the client window and storing the XID of an XSync counter in the property _NET_WM_SYNC_REQUEST_COUNTER. The initial value of this counter is not defined by this specification.

A window manager uses this protocol by preceding a ConfigureNotify event sent to a client by a client message as follows:

type = ClientMessage
window = the respective client window
message_type = WM_PROTOCOLS
format = 32
data.l[0] = _NET_WM_SYNC_REQUEST
data.l[1] = timestamp
data.l[2] = low 32 bits of the update request number
data.l[3] = high 32 bits of the update request number
other data.l[] elements = 0

After receiving one or more such message/ConfigureNotify pairs, and having handled all repainting associated with the ConfigureNotify events, the client MUST set the _NET_WM_SYNC_REQUEST_COUNTER to the 64 bit number indicated by the data.l[2] and data.l[3] fields of the last client message received.

By using either the Alarm or the Await mechanisms of the XSync extension, the window manager can know when the client has finished handling the ConfigureNotify events. The window manager SHOULD not resize the window faster than the client can keep up.

The update request number in the client message is determined by the window manager subject to the restriction that it MUST NOT be 0. The number is generally intended to be incremented by one for each message sent. Since the initial value of the XSync counter is not defined by this specification, the window manager MAY set the value of the XSync counter at any time, and MUST do so when it first manages a new window.