The Feedback is an entity that classifies the outcome of an execution of the program under test as interesting or not. Typically, if an execution is interesting, the corresponding input used to feed the target program is added to a corpus.

Most of the time, the notion of Feedback is deeply linked to the Observer, but they are different concepts.

The Feedback, in most of the cases, processes the information reported by one or more observers to decide if the execution is interesting. The concept of "interestingness" is abstract, but typically it is related to a novelty search (i.e. interesting inputs are those that reach a previously unseen edge in the control flow graph).

As an example, given an Observer that reports all the sizes of memory allocations, a maximization Feedback can be used to maximize these sizes to sport pathological inputs in terms of memory consumption.

In terms of code, the library offers the Feedback and the FeedbackState traits. The first is used to implement functors that, given the state of the observers from the last execution, tells if the execution was interesting. The second is tied with Feedback and it is the state of the data that the feedback wants to persist in the fuzzers's state, for instance the cumulative map holding all the edges seen so far in the case of a feedback based on edge coverage.

Multiple Feedbacks can be combined into boolean formula, considering for instance an execution as interesting if it triggers new code paths or execute in less time compared to the average execution time using feedback_or.

On top, logic operators like feedback_or and feedback_and have a _fast option (feedback_or_fast where the second feedback will not be evaluated, if the first part already answers the interestingness question, to save precious performance.

Using feedback_and_fast in combination with ConstFeedback, certain feedbacks can be disabled dynamically.


While feedbacks are commonly used to decide if an Input should be kept for future mutations, they serve a double-purpose, as so-called Objective Feedbacks. In this case, the interestingness of a feedback indicates, if an Objective has been hit. Commonly, these would be a`crash or a timeout, but they can also be used to find specific parts of the program, for sanitization, or a differential fuzzing success.