The data flow testing method selects test paths of a program according to the locations of definitions and uses of variables in the program. A number of data flow testing strategies have been studied and compared.

To illustrate the data flow testing approach, assume that each statement in a program is assigned a unique statement number and that each function does not modify its parameters or global variables. For a statement with S as its statement number,

**DEF(S) = {X | statement S contains a definition of X}**

USE(S) = {X | statement S contains a use of X}

USE(S) = {X | statement S contains a use of X}

If statement S is an if or loop statement, its DEF set is empty and its USE set is based on the condition of statement S. The definition of variable X at statement S is said to be live at statement S' if there exists a path from statement S to statement S' that contains no other definition of X.

A definition-use (DU) chain of variable X is of the form [X, S, S'], where S and S' are statement numbers, X is in DEF(S) and USE(S'), and the definition of X in statement S is live at statement S'.

One simple data flow testing strategy is to require that every DU chain be covered at least once. We refer to this strategy as the DU testing strategy. It has been shown that DU testing does not guarantee the coverage of all branches of a program. However, a branch is not guaranteed to be covered by DU testing only in rare situations such as if-then-else constructs in which the then part has no definition of any variable and the else part does not exist. In this situation, the else branch of the if statement is not necessarily covered by DU testing.

Data flow testing strategies are useful for selecting test paths of a program containing nested if and loop statements. To illustrate this, consider the application of DU testing to select test paths for the PDL that follows:

**proc x**

B1;

do while C1

if C2

then

if C4

then B4;

else B5;

endif;

else

if C3

then B2;

else B3;

endif;

endif;

enddo;

B6;

end proc;

B1;

do while C1

if C2

then

if C4

then B4;

else B5;

endif;

else

if C3

then B2;

else B3;

endif;

endif;

enddo;

B6;

end proc;

To apply the DU testing strategy to select test paths of the control flow diagram, we need to know the definitions and uses of variables in each condition or block in the PDL. Assume that variable X is defined in the last statement of blocks B1, B2, B3, B4, and B5 and is used in the first statement of blocks B2, B3, B4, B5, and B6. The DU testing strategy requires an execution of the shortest path from each of Bi, 0 < i ≤ 5, to each of Bj, 1 < j ≤ 6. (Such testing also covers any use of variable X in conditions C1, C2, C3, and C4.) Although there are 25 DU chains of variable X, we need only five paths to cover these DU chains. The reason is that five paths are needed to cover the DU chain of X from Bi, 0 < i ≤ 5, to B6 and other DU chains can be covered by making these five paths contain iterations of the loop.

If we apply the branch testing strategy to select test paths of the PDL just noted, we do not need any additional information. To select paths of the diagram for BRO testing, we need to know the structure of each condition or block. (After the selection of a path of a program, we need to determine whether the path is feasible for the program; that is, whether at least one input exists that exercises the path.)

Since the statements in a program are related to each other according to the definitions and uses of variables, the data flow testing approach is effective for error detection. However, the problems of measuring test coverage and selecting test paths for data flow testing are more difficult than the corresponding problems for condition testing.