It is sometimes useful to take some action when a variable changes its value. The variable watchpoint facility provides the means to do so. Some possible uses for this feature include keeping display in sync with variable settings, and invoking the debugger to track down unexpected changes to variables (see Entering the debugger when a variable is modified).
The following functions may be used to manipulate and query the watch functions for a variable.
This function arranges for watch-function to be called whenever symbol is modified. Modifications through aliases (see Variable Aliases) will have the same effect.
watch-function will be called, just before changing the value of
symbol, with 4 arguments: symbol, newval,
operation, and where.
symbol is the variable being changed. newval is the value
it will be changed to. (The old value is available to
watch-function as the value of symbol, since it was not
yet changed to newval.) operation is a symbol
representing the kind of change, one of:
is a buffer if the buffer-local value of the variable is being
This function removes watch-function from symbol’s list of watchers.
This function returns the list of symbol’s active watcher functions.
There are a couple of ways in which a variable could be modified (or at least appear to be modified) without triggering a watchpoint.
Since watchpoints are attached to symbols, modification to the objects contained within variables (e.g., by a list modification function see Modifying Existing List Structure) is not caught by this mechanism.
Additionally, C code can modify the value of variables directly, bypassing the watchpoint mechanism.
A minor limitation of this feature, again because it targets symbols, is that only variables of dynamic scope may be watched. This poses little difficulty, since modifications to lexical variables can be discovered easily by inspecting the code within the scope of the variable (unlike dynamic variables, which can be modified by any code at all, see Scoping Rules for Variable Bindings).