Although Common Test was created primarly for the purpose of black box testing, nothing prevents it from working perfectly as a white box testing tool as well. This is especially true when the application to test is written in Erlang. Then the test ports are easily realized by means of Erlang function calls.

When white box testing an Erlang application, it is useful to be able to measure the code coverage of the test. Common Test provides simple access to the OTP Cover tool for this purpose. Common Test handles all necessary communication with the Cover tool (starting, compiling, analysing, etc). All the Common Test user needs to do is to specify the extent of the code coverage analysis.

To specify what modules should be included in the code coverage test, you provide a cover specification file. Using this file you can point out specific modules or specify directories that contain modules which should all be included in the analysis. You can also, in the same fashion, specify modules that should be excluded from the analysis.

If you are testing a distributed Erlang application, it is likely that code you want included in the code coverage analysis gets executed on an Erlang node other than the one Common Test is running on. If this is the case you need to specify these other nodes in the cover specification file or add them dynamically to the code coverage set of nodes. See the ct_cover page in the reference manual for details on the latter.

In the cover specification file you can also specify your required level of the code coverage analysis; details or overview. In detailed mode, you get a coverage overview page, showing you per module and total coverage percentages, as well as one HTML file printed for each module included in the analysis that shows exactly what parts of the code have been executed during the test. In overview mode, only the code coverage overview page gets printed.

Note: Currently, for Common Test to be able to print code coverage HTML files for the modules included in the analysis, the source code files of these modules must be located in the same directory as the corresponding .beam files. This is a limitation that will be removed later.

You can choose to export and import code coverage data between tests. If you specify the name of an export file in the cover specification file, Common Test will export collected coverage data to this file at the end of the test. You may similarly specify that previously exported data should be imported and included in the analysis for a test (you can specify multiple import files). This way it is possible to analyse total code coverage without necessarily running all tests at once. Note that even if you run separate tests in one test run, code coverage data will not be passed on from one test to another unless you specify an export file for Common Test to use for this purpose.

To activate the code coverage support, you simply specify the name of the cover specification file as you start Common Test. This you do either by using the -cover flag with ct_run. Example:

$ ct_run -dir $TESTOBJS/db -cover $TESTOBJS/db/config/db.coverspec

You may also pass the cover specification file name in a call to ct:run_test/1, by adding a {cover,CoverSpec} tuple to the Opts argument. Also, you can of course enable code coverage in your test specifications (read more in the chapter about using test specifications).

By default the Cover tool is automatically stopped when the tests are completed. This causes the original (non cover compiled) modules to be loaded back in to the test node. If a process at this point is still running old code of any of the modules that are cover compiled, meaning that it has not done any fully qualified function call after the cover compilation, the process will now be killed. To avoid this it is possible to set the value of the cover_stop option to false. This means that the modules will stay cover compiled, and it is therefore only recommended if the erlang node(s) under test is terminated after the test is completed or if cover can be manually stopped.

The option can be set by using the -cover_stop flag with ct_run, by adding {cover_stop,true|false} to the Opts argument to ct:run_test/1, or by adding a cover_stop term in your test specification (see chapter about test specifications).

These are the terms allowed in a cover specification file:

      %% List of Nodes on which cover will be active during test.
      %% Nodes = [atom()]
      {nodes, Nodes}.       

      %% Files with previously exported cover data to include in analysis.
      %% CoverDataFiles = [string()]
      {import, CoverDataFiles}.

      %% Cover data file to export from this session.
      %% CoverDataFile = string()
      {export, CoverDataFile}.

      %% Cover analysis level.
      %% Level = details | overview
      {level, Level}.       

      %% Directories to include in cover.
      %% Dirs = [string()]
      {incl_dirs, Dirs}.

      %% Directories, including subdirectories, to include.
      {incl_dirs_r, Dirs}.

      %% Specific modules to include in cover.
      %% Mods = [atom()]
      {incl_mods, Mods}.

      %% Directories to exclude in cover.
      {excl_dirs, Dirs}.

      %% Directories, including subdirectories, to exclude.
      {excl_dirs_r, Dirs}.

      %% Specific modules to exclude in cover.
      {excl_mods, Mods}.

      %% Cross cover compilation
      %% Tag = atom(), an identifier for a test run
      %% Mod = [atom()], modules to compile for accumulated analysis
      {cross,[{Tag,Mods}]}.
    

The incl_dirs_r and excl_dirs_r terms tell Common Test to search the given directories recursively and include or exclude any module found during the search. The incl_dirs and excl_dirs terms result in a non-recursive search for modules (i.e. only modules found in the given directories are included or excluded).

Note: Directories containing Erlang modules that are to be included in a code coverage test must exist in the code server path, or the cover tool will fail to recompile the modules. (It is not sufficient to specify these directories in the cover specification file for Common Test).