int
Tcl_LinkVar(interp, varName, addr, type)
Tcl_UnlinkVar(interp, varName)
Tcl_UpdateLinkedVar(interp, varName)
Tcl_LinkVar uses variable traces to keep the Tcl variable named by varName in sync with the C variable at the address given by addr. Whenever the Tcl variable is read the value of the C variable will be returned, and whenever the Tcl variable is written the C variable will be updated to have the same value. Tcl_LinkVar normally returns TCL_OK; if an error occurs while setting up the link (e.g. because varName is the name of array) then TCL_ERROR is returned and the interpreter's result contains an error message.
The type argument specifies the type of the C variable, and must have one of the following values, optionally OR'ed with TCL_LINK_READ_ONLY:
If the TCL_LINK_READ_ONLY flag is present in type then the variable will be read-only from Tcl, so that its value can only be changed by modifying the C variable. Attempts to write the variable from Tcl will be rejected with errors.
Tcl_UnlinkVar removes the link previously set up for the variable given by varName. If there does not exist a link for varName then the procedure has no effect.
Tcl_UpdateLinkedVar may be invoked after the C variable has changed to force the Tcl variable to be updated immediately. In many cases this procedure is not needed, since any attempt to read the Tcl variable will return the latest value of the C variable. However, if a trace has been set on the Tcl variable (such as a Tk widget that wishes to display the value of the variable), the trace will not trigger when the C variable has changed. Tcl_UpdateLinkedVar ensures that any traces on the Tcl variable are invoked.
Note that, as with any call to a Tcl interpreter, Tcl_UpdateLinkedVar must be called from the same thread that created the interpreter. The safest mechanism is to ensure that the C variable is only ever updated from the same thread that created the interpreter (possibly in response to an event posted with Tcl_ThreadQueueEvent), but when it is necessary to update the variable in a separate thread, it is advised that Tcl_AsyncMark be used to indicate to the thread hosting the interpreter that it is ready to run Tcl_UpdateLinkedVar.