Code coverage

There are a number of coverage criteria, the main ones being:[5]

• Function coverage – has each function (or subroutine) in the program been called?
• Statement coverage – has each statement in the program been executed?
• Edge coverage – has every edge in the Control flow graph been executed?
• Branch coverage – has each branch (also called DD-path) of each control structure (such as in if and case statements) been executed? For example, given an if statement, have both the true and false branches been executed? This is a subset of edge coverage.
• Condition coverage (or predicate coverage) – has each Boolean sub-expression evaluated both to true and false?

For example, consider the following C function:

int foo (int x, int y)
{
    int z = 0;
    if ((x > 0) && (y > 0))
    {
        z = x;
    }
    return z;
}

Assume this function is a part of some bigger program and this program was run with some test suite.

• If during this execution function 'foo' was called at least once, then function coverage for this function is satisfied.
• Statement coverage for this function will be satisfied if it was called e.g. as foo(1,1), as in this case, every line in the function is executed including z = x;.
• Tests calling foo(1,1) and foo(0,1) will satisfy branch coverage because, in the first case, both if conditions are met and z = x; is executed, while in the second case, the first condition (x>0) is not satisfied, which prevents executing z = x;.
• Condition coverage can be satisfied with tests that call foo(1,0) and foo(0,1). These are necessary because in the first cases, (x>0) evaluates to true, while in the second, it evaluates false. At the same time, the first case makes (y>0) false, while the second makes it true.

Condition coverage does not necessarily imply branch coverage. For example, consider the following fragment of code:

if a and b then

Condition coverage can be satisfied by two tests:

• a=true, b=false
• a=false, b=true

However, this set of tests does not satisfy branch coverage since neither case will meet the if condition.

Fault injection may be necessary to ensure that all conditions and branches of exception handling code have adequate coverage during testing.

A combination of function coverage and branch coverage is sometimes also called decision coverage. This criterion requires that every point of entry and exit in the program has been invoked at least once, and every decision in the program has taken on all possible outcomes at least once. In this context the decision is a boolean expression composed of conditions and zero or more boolean operators. This definition is not the same as branch coverage,[6] however, some do use the term decision coverage as a synonym for branch coverage.[7]

Condition/decision coverage requires that both decision and condition coverage be satisfied. However, for safety-critical applications (e.g., for avionics software) it is often required that modified condition/decision coverage (MC/DC) be satisfied. This criterion extends condition/decision criteria with requirements that each condition should affect the decision outcome independently. For example, consider the following code:

if (a or b) and c then

The condition/decision criteria will be satisfied by the following set of tests:

• a=true, b=true, c=true
• a=false, b=false, c=false

However, the above tests set will not satisfy modified condition/decision coverage, since in the first test, the value of 'b' and in the second test the value of 'c' would not influence the output. So, the following test set is needed to satisfy MC/DC:

• a=false, b=true, c=false
• a=false, b=true, c=true
• a=false, b=false, c=true
• a=true, b=false, c=true

This criterion requires that all combinations of conditions inside each decision are tested. For example, the code fragment from the previous section will require eight tests:

• a=false, b=false, c=false
• a=false, b=false, c=true
• a=false, b=true, c=false
• a=false, b=true, c=true
• a=true, b=false, c=false
• a=true, b=false, c=true
• a=true, b=true, c=false
• a=true, b=true, c=true

Parameter value coverage (PVC) requires that in a method taking parameters, all the common values for such parameters be considered. The idea is that all common possible values for a parameter are tested.[8] For example, common values for a string are: 1) null, 2) empty, 3) whitespace (space, tabs, newline), 4) valid string, 5) invalid string, 6) single-byte string, 7) double-byte string. It may also be appropriate to use very long strings. Failure to test each possible parameter value may leave a bug. Testing only one of these could result in 100% code coverage as each line is covered, but as only one of seven options are tested, there is only 14.2% PVC.

There are further coverage criteria, which are used less often:

• Linear Code Sequence and Jump (LCSAJ) coverage a.k.a. JJ-Path coverage – has every LCSAJ/JJ-path been executed?[9]
• 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?
• Loop coverage – Has every possible loop been executed zero times, once, and more than once?
• State coverage – Has each state in a finite-state machine been reached and explored?
• Data-flow coverage – Has each variable definition and its usage been reached and explored?[10]

Safety-critical or dependable applications are often required to demonstrate 100% of some form of test coverage. For example, the ECSS-E-ST-40C standard demands 100% statement and decision coverage for two out of four different criticality levels; for the other ones, target coverage values are up to negotiation between supplier and customer.[11] However, setting specific target values - and, in particular, 100% - has been criticized by practitioners for various reasons (cf.[12]) Martin Fowler writes: "I would be suspicious of anything like 100% - it would smell of someone writing tests to make the coverage numbers happy, but not thinking about what they are doing".[13]

Some of the coverage criteria above are connected. For instance, path coverage implies decision, statement and entry/exit coverage. Decision coverage implies statement coverage, because every statement is part of a branch.

Full path coverage, of the type described above, is usually impractical or impossible. Any module with a succession of ${\displaystyle n}$ decisions in it can have up to ${\displaystyle 2^{n}}$ 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).[14] Basis path testing is for instance a method of achieving complete branch coverage without achieving complete path coverage.[15]

Methods 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.