Although Common Test was created primarily for 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 with 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, and so on). The Common Test user only needs to specify the extent of the code coverage analysis.

To specify the modules to be included in the code coverage test, provide a cover specification file. With this file you can point out specific modules or specify directories containing modules to be included in the analysis. You can also specify modules to 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 another Erlang node than the one Common Test is running on. If so, you must specify these other nodes in the cover specification file or add them dynamically to the code coverage set of nodes. For details on the latter, see module ct_cover.

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 per module and total coverage percentages. You also get an HTML file printed for each module included in the analysis showing exactly what parts of the code have been executed during the test. In overview mode, only the code coverage overview page is printed.

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 exports collected coverage data to this file at the end of the test. You can similarly specify previously exported data to be imported and included in the analysis for a test (multiple import files can be specified). This way, the total code coverage can be analyzed without necessarily running all tests at once.

To activate the code coverage support, specify the name of the cover specification file as you start Common Test. Do this by using flag -cover with ct_run, for example:

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

You can also pass the cover specification file name in a call to ct:run_test/1, by adding a {cover,CoverSpec} tuple to argument Opts.

You can also enable code coverage in your test specifications (see section Test Specifications in section Running Tests and Analyzing Results).

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 into the test node. If a process at this point still runs 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 is killed. To avoid this, set the value of option cover_stop to false. This means that the modules stay cover compiled. Therefore, this is only recommended if the Erlang nodes under test are terminated after the test is completed, or if cover can be manually stopped.

The option can be set by using flag -cover_stop with ct_run, by adding {cover_stop,true|false} to argument Opts to ct:run_test/1, or by adding a cover_stop term in the test specification (see section Test Specifications in section Running Tests and Analyzing Results).

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

To view the result of a code coverage test, click the button labeled "COVER LOG" in the top-level index page for the test run.

Before Erlang/OTP 17.1, if your test run consisted of multiple tests, cover would be started and stopped for each test within the test run. Separate logs would be available through the "Coverage log" link on the test suite result pages. These links are still available, but now they all point to the same page as the button on the top-level index page. The log contains the accumulated results for the complete test run. For details about this change, see the release notes.

The button takes you to the code coverage overview page. If you have successfully performed a detailed coverage analysis, links to each individual module coverage page are found here.

If cross cover analysis is performed, and there are accumulated coverage results for the current test, the link "Coverdata collected over all tests" takes you to these results.