stylistic:tone, information:classification
Aware of input structure

Typically, fuzzers are used to generate inputs for programs that take structured inputs, such as a file, a sequence of keyboard or mouse events, or a sequence of messages. This structure distinguishes valid input that is accepted and processed by the program from invalid input that is quickly rejected by the program. What constitutes a valid input may be explicitly specified in an input model. Examples of input models are formal grammars, file formats, GUI -models, and network protocols. Even items not normally considered as input can be fuzzed, such as the contents of databases, shared memory, environment variables or the precise interleaving of threads. An effective fuzzer generates semi-valid inputs that are "valid enough" so that they are not directly rejected from the parser and "invalid enough" so that they might stress corner cases and exercise interesting program behaviours.