What is meant by code coverage? What are different types of code coverage?

- Code coverage can be defined as a measure to measure the extent to which the source code of a software system has been tested.
- Code coverage is categorized under white box testing techniques since the inspection of the code is carried out directly.
- Code coverage methodology was initially developed for systematic testing of software system. It was developed by two researchers:Miller and Maloney in year of 1963.
- Code coverage is regarded as one of the important considerations concerning the safety certification in the field of avionics equipment.
- Code coverage is not centralized on one idea. There are several different criteria or types to choose from. The criteria or type is chosen as per the requirement of the extent of testing. At a time one or more coverage criteria (types) can be used.

Different Coverage types have been discussed in details below:

Basic coverage type:
This category again has following criteria:
- Statement coverage: It determines whether or not each node in the program has been executed.
- Function coverage: It determines whether or not each function or sub routine in the program has been called.
- Decision coverage: It determines whether or not each edge in the program has been executed and it also ensures that requirements of every branch have been met.
- Predicate coverage: It is also known as condition coverage. It determines whether or each and every Boolean expression in the program has been evaluated to either true value or false value.
- Predicate/ decision coverage: It is combination of both predicate and decision coverage and determines whether or not both the types of coverage are satisfied. Fault injection technique becomes necessary here. It is done to ensure each and every part of the software system has got sufficient coverage.

Modified predicate/ decision coverage: It is usually abbreviated as MC/ DC. Some applications like avionics software applications are safety critical and they require satisfying the modified form of predicate/decisions coverage. The modification here is that each and every individual criterion for condition and decision should have a distinct affect on the outcome and it should be separately visible.

Multiple condition coverage: \
As the name suggests, this type of coverage has two or more conditions. The group of conditions is then tested. Multiple condition coverage determines whether or not all the combinations of the conditions in each decision are tested.

The above mentioned 3 code coverage types are frequently and mostly used coverage types. There are other types which are not so in use. They have also been discussed below:
- JJ path coverage:
It determines whether all the jump to jump kind of paths has been executed.
- Linear code sequence and jump coverage: It determines whether or not all LCSAJs have been executed.
- Entry coverage:
It determines whether or not every possible call has been executed.
- Exit coverage:
It determines whether or not every possible return of the functions has been executed.
- Path coverage:
It determines whether or not every possible path through a given unit has been executed.
- Loop coverage:
It determines whether or not all the loops have been executed.

Testing often achieves 100 percent of code coverage. This is verified through safety critical applications. Attaining full one kind of coverage is practically impossible. Most parts in a program are such that they cannot be accessed easily and therefore remain void of code coverage.There are certain things that are needed to be considered while implementing a code coverage type:

- Requirements of the code coverage for the certification of the finished product.
- Level of code coverage required.
- Testing of code coverage against the tests to verify the requirements.
- Direct generation of object code.