To measure how well the program is exercised by a test suite, one or more coverage criteria are used. There are a number of coverage criteria, the main ones being:
Function coverage - Has each function in the program been executed?
Statement coverage - Has each line of the source code been executed?
Condition coverage - Has each evaluation point (such as a true/false decision) been executed?
Path coverage - Has every possible route through a given part of the code been executed?
Entry/exit coverage - Has every possible call and return of the function been executed?
Safety-critical applications are often required to demonstrate that testing achieves 100% of some form of code coverage.
Some of the coverage criteria above are connected. For instance, path coverage implies condition, statement and entry/exit coverage. Statement coverage does not imply condition coverage, as the code (in the C programming language) below shows:
void foo(int bar)
{
printf("This is ");
if (bar <= 0)
{
printf("not ");
}
printf("a positive integer.\n");
return;
}
If the function foo were called with variable bar set to -1, statement coverage would be achieved. Condition coverage, however, would not.
Full path coverage, of the type described above, is usually impractical or impossible. Any module with a succession of n decisions in it can have up to 2n paths within it; loop constructs can result in an infinite number of paths. Many paths may also be infeasible, in that there is no input to the program under test that can cause that particular path to be executed. However, a general-purpose algorithm for identifying infeasible paths has been proven to be impossible (such an algorithm could be used to solve the halting problem). Techniques for practical path coverage testing instead attempt to identify classes of code paths that differ only in the number of loop executions, and to achieve "basis path" coverage the tester must cover all the path classes.
