Coverage and Control Flow Graphs

Coverage and Control Flow Graphs

The application of coverage analysis is typically associated with the use of control and data flow models to represent program structural elements and data. The logic elements most commonly considered for coverage are based on the flow of control in a unit of code. For example,

(i) program statements;

(ii) decisions/branches (these influence the program flow of control);

(iii)           conditions (expressions that evaluate to true/false, and do not contain any other true/false-valued expressions);

(iv)combinations of decisions and conditions;

(v)             paths (node sequences in flow graphs).

These logical elements are rooted in the concept of a program prime. A program prime is an atomic programming unit. All structured programs can be built from three basic primes-sequential (e.g., assignment statements), decision (e.g., if/then/else statements), and iterative (e.g., while, for loops). Graphical representations for these three primes are shown in Figure 5.1.

Using the concept of a prime and the ability to use combinations of primes to develop structured code, a (control) flow diagram for the soft- ware unit under test can be developed. The flow graph can be used by the tester to evaluate the code with respect to its testability, as well as to develop white box test cases. This will be shown in subsequent sections of this chapter. A flow graph representation for the code example in Figure 5.2 is found in Figure 5.3. Note that in the flow graph the nodes represent sequential statements, as well as decision and looping predicates. For simplicity, sequential statements are often omitted or combined as a block that indicates that if the first statement in the block is executed, so are all the following statements in the block. Edges in the graph represent transfer of control. The direction of the transfer depends on the outcome of the condition in the predicate (true or false).

There are commercial tools that will generate control flow graphs from code and in some cases from pseudo code. The tester can use tool support for developing control flow graphs especially for complex pieces of code. A control flow representation for the software under test facilitates the design of white box-based test cases as it clearly shows the logic elements needed to design the test cases using the coverage criterion of choice.Zhu has formally described a set of program-based coverage criteria in the context of test adequacy criteria and control/data flow models.

This chapter will presents control-flow, or logic-based, coverage concepts in a less formal but practical manner to aid the tester in developing test data sets, setting quantifiable testing goals, measuring results, and evaluating the adequacy of the test outcome. Examples based on the logic elements listed previously will be presented. Subsequent sections will describe data flow and fault-based coverage criteria.