Data-flow analysis can be used to increase program understanding and to develop test cases based on data flow within the program. The data-flow testing technique is based on investigating the ways values are associated with variables and the ways that these associations affect the execution of the program. Data-flow analysis focuses on occurrences of variables, following paths from the definition (or initialization) of a variable to its uses. The variable values may be used for computing values for defining other variables or used as predicate variables to decide whether a predicate is true for traversing a specific execution path. A data-flow analysis for an entire program involving all variables and traversing all usage paths requires immense computational resources; however, this technique can be applied for select variables.
The simplest approach is to validate the usage of select sets of variables by executing a path that starts with definition and ends at uses of the definition. The path and the usage of the data can help in identifying suspicious code blocks and in developing test cases to validate the runtime behavior of the software. For example, for a chosen data definition-to-use path, with well-crafted test data, testing can uncover time-of-check-to-time-of-use (TOCTTOU) flaws. The ”Security Testing” section in [Howard 02] explains the data mutation technique, which deals with perturbing environment data. The same technique can be applied to internal data as well, with the help of data-flow analysis.