path: root/lib/tools/doc/src/cover_chapter.xml

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<chapter>
  <header>
    <copyright>
      <year>2001</year><year>2013</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>

    <title>cover</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>cover_chapter.xml</file>
  </header>

  <section>
    <title>Introduction</title>
    <p>The module <c>cover</c> provides a set of functions for coverage
      analysis of Erlang programs, counting how many times each
      <seealso marker="#lines">executable line</seealso> is executed.</p>
    <p>Coverage analysis can be used to verify test cases, making sure all
      relevant code is covered, and may be helpful when looking for
      bottlenecks in the code.</p>
  </section>

  <section>
    <title>Getting Started With Cover</title>

    <section>
      <title>Example</title>
      <p>Assume that a test case for the following program should be
        verified:</p>
      <code type="none">
-module(channel).
-behaviour(gen_server).

-export([start_link/0,stop/0]).
-export([alloc/0,free/1]). % client interface
-export([init/1,handle_call/3,terminate/2]). % callback functions

start_link() ->
    gen_server:start_link({local,channel},channel,[],[]).

stop() ->
    gen_server:call(channel,stop).

%%%-Client interface functions-------------------------------------------

alloc() ->
    gen_server:call(channel,alloc).

free(Channel) ->
    gen_server:call(channel,{free,Channel}).

%%%-gen_server callback functions----------------------------------------

init(_Arg) ->
    {ok,channels()}.

handle_call(stop,Client,Channels) ->
    {stop,normal,ok,Channels};

handle_call(alloc,Client,Channels) ->
    {Ch,Channels2} = alloc(Channels),
    {reply,{ok,Ch},Channels2};

handle_call({free,Channel},Client,Channels) ->
    Channels2 = free(Channel,Channels),
    {reply,ok,Channels2}.

terminate(_Reason,Channels) ->
    ok.

%%%-Internal functions---------------------------------------------------

channels() ->
    [ch1,ch2,ch3].

alloc([Channel|Channels]) ->
    {Channel,Channels};
alloc([]) ->
    false.

free(Channel,Channels) ->
    [Channel|Channels].</code>
      <p>The test case is implemented as follows:</p>
      <code type="none">
-module(test).
-export([s/0]).

s() ->
    {ok,Pid} = channel:start_link(),
    {ok,Ch1} = channel:alloc(),
    ok = channel:free(Ch1),
    ok = channel:stop().</code>
    </section>

    <section>
      <title>Preparation</title>
      <p>First of all, Cover must be started. This spawns a process which
        owns the Cover database where all coverage data will be stored.</p>
      <pre>
1> <input>cover:start().</input>
{ok,&lt;0.30.0>}</pre>
      <p>To include other nodes in the coverage analysis, use
        <c>start/1</c>. All cover compiled modules will then be loaded
        on all nodes, and data from all nodes will be summed up when
        analysing. For simplicity this example only involves the
        current node.</p>
      <p>Before any analysis can take place, the involved modules must be
        <em>Cover compiled</em>. This means that some extra information is
        added to the module before it is compiled into a binary which then
        is <seealso marker="#loading">loaded</seealso>. The source file of
        the module is not affected and no <c>.beam</c> file is created.</p>
      <pre>
2> <input>cover:compile_module(channel).</input>
{ok,channel}</pre>
      <p>Each time a function in the Cover compiled module <c>channel</c>
        is called, information about the call will be added to the Cover
        database. Run the test case:</p>
      <pre>
3> <input>test:s().</input>
ok</pre>
      <p>Cover analysis is performed by examining the contents of the Cover
        database. The output is determined by two parameters, <c>Level</c>
        and <c>Analysis</c>. <c>Analysis</c> is either <c>coverage</c> or
        <c>calls</c> and determines the type of the analysis. <c>Level</c>
        is either <c>module</c>, <c>function</c>, <c>clause</c>, or
        <c>line</c> and determines the level of the analysis.</p>
    </section>

    <section>
      <title>Coverage Analysis</title>
      <p>Analysis of type <c>coverage</c> is used to find out how much of
        the code has been executed and how much has not been executed.
        Coverage is represented by a tuple <c>{Cov,NotCov}</c>, where
        <c>Cov</c> is the number of executable lines that have been executed
        at least once and <c>NotCov</c> is the number of executable lines
        that have not been executed.</p>
      <p>If the analysis is made on module level, the result is given for
        the entire module as a tuple <c>{Module,{Cov,NotCov}}</c>:</p>
      <pre>
4> <input>cover:analyse(channel,coverage,module).</input>
{ok,{channel,{14,1}}}</pre>
      <p>For <c>channel</c>, the result shows that 14 lines in the module
        are covered but one line is not covered.</p>
      <p>If the analysis is made on function level, the result is given as
        a list of tuples <c>{Function,{Cov,NotCov}}</c>, one for each
        function in the module. A function is specified by its module name,
        function name and arity:</p>
      <pre>
5> <input>cover:analyse(channel,coverage,function).</input>
{ok,[{{channel,start_link,0},{1,0}},
     {{channel,stop,0},{1,0}},
     {{channel,alloc,0},{1,0}},
     {{channel,free,1},{1,0}},
     {{channel,init,1},{1,0}},
     {{channel,handle_call,3},{5,0}},
     {{channel,terminate,2},{1,0}},
     {{channel,channels,0},{1,0}},
     {{channel,alloc,1},{1,1}},
     {{channel,free,2},{1,0}}]}</pre>
      <p>For <c>channel</c>, the result shows that the uncovered line is in
        the function <c>channel:alloc/1</c>.</p>
      <p>If the analysis is made on clause level, the result is given as
        a list of tuples <c>{Clause,{Cov,NotCov}}</c>, one for each
        function clause in the module. A clause is specified by its module
        name, function name, arity and position within the function
        definition:</p>
      <pre>
6> <input>cover:analyse(channel,coverage,clause).</input>
{ok,[{{channel,start_link,0,1},{1,0}},
     {{channel,stop,0,1},{1,0}},
     {{channel,alloc,0,1},{1,0}},
     {{channel,free,1,1},{1,0}},
     {{channel,init,1,1},{1,0}},
     {{channel,handle_call,3,1},{1,0}},
     {{channel,handle_call,3,2},{2,0}},
     {{channel,handle_call,3,3},{2,0}},
     {{channel,terminate,2,1},{1,0}},
     {{channel,channels,0,1},{1,0}},
     {{channel,alloc,1,1},{1,0}},
     {{channel,alloc,1,2},{0,1}},
     {{channel,free,2,1},{1,0}}]}</pre>
      <p>For <c>channel</c>, the result shows that the uncovered line is in
        the second clause of <c>channel:alloc/1</c>.</p>
      <p>Finally, if the analysis is made on line level, the result is given
        as a list of tuples <c>{Line,{Cov,NotCov}}</c>, one for each
        executable line in the source code. A line is specified by its
        module name and line number.</p>
      <pre>
7> <input>cover:analyse(channel,coverage,line).</input>
{ok,[{{channel,9},{1,0}},
     {{channel,12},{1,0}},
     {{channel,17},{1,0}},
     {{channel,20},{1,0}},
     {{channel,25},{1,0}},
     {{channel,28},{1,0}},
     {{channel,31},{1,0}},
     {{channel,32},{1,0}},
     {{channel,35},{1,0}},
     {{channel,36},{1,0}},
     {{channel,39},{1,0}},
     {{channel,44},{1,0}},
     {{channel,47},{1,0}},
     {{channel,49},{0,1}},
     {{channel,52},{1,0}}]}</pre>
      <p>For <c>channel</c>, the result shows that the uncovered line is
        line number 49.</p>
    </section>

    <section>
      <title>Call Statistics</title>
      <p>Analysis of type <c>calls</c> is used to find out how many times
        something has been called and is represented by an integer
        <c>Calls</c>.</p>
      <p>If the analysis is made on module level, the result is given as a
        tuple <c>{Module,Calls}</c>. Here <c>Calls</c> is the total number
        of calls to functions in the module:</p>
      <pre>
8> <input>cover:analyse(channel,calls,module).</input>
{ok,{channel,12}}</pre>
      <p>For <c>channel</c>, the result shows that a total of twelve calls
        have been made to functions in the module.</p>
      <p>If the analysis is made on function level, the result is given as
        a list of tuples <c>{Function,Calls}</c>. Here <c>Calls</c> is
        the number of calls to each function:</p>
      <pre>
9> <input>cover:analyse(channel,calls,function).</input>
{ok,[{{channel,start_link,0},1},
     {{channel,stop,0},1},
     {{channel,alloc,0},1},
     {{channel,free,1},1},
     {{channel,init,1},1},
     {{channel,handle_call,3},3},
     {{channel,terminate,2},1},
     {{channel,channels,0},1},
     {{channel,alloc,1},1},
     {{channel,free,2},1}]}</pre>
      <p>For <c>channel</c>, the result shows that <c>handle_call/3</c> is
        the most called function in the module (three calls). All other
        functions have been called once.</p>
      <p>If the analysis is made on clause level, the result is given as
        a list of tuples <c>{Clause,Calls}</c>. Here <c>Calls</c> is
        the number of calls to each function clause:</p>
      <pre>
10> <input>cover:analyse(channel,calls,clause).</input>
{ok,[{{channel,start_link,0,1},1},
     {{channel,stop,0,1},1},
     {{channel,alloc,0,1},1},
     {{channel,free,1,1},1},
     {{channel,init,1,1},1},
     {{channel,handle_call,3,1},1},
     {{channel,handle_call,3,2},1},
     {{channel,handle_call,3,3},1},
     {{channel,terminate,2,1},1},
     {{channel,channels,0,1},1},
     {{channel,alloc,1,1},1},
     {{channel,alloc,1,2},0},
     {{channel,free,2,1},1}]}</pre>
      <p>For <c>channel</c>, the result shows that all clauses have been
        called once, except the second clause of <c>channel:alloc/1</c>
        which has not been called at all.</p>
      <p>Finally, if the analysis is made on line level, the result is given
        as a list of tuples <c>{Line,Calls}</c>. Here <c>Calls</c> is 
        the number of times each line has been executed:</p>
      <pre>
11> <input>cover:analyse(channel,calls,line).</input>
{ok,[{{channel,9},1},
     {{channel,12},1},
     {{channel,17},1},
     {{channel,20},1},
     {{channel,25},1},
     {{channel,28},1},
     {{channel,31},1},
     {{channel,32},1},
     {{channel,35},1},
     {{channel,36},1},
     {{channel,39},1},
     {{channel,44},1},
     {{channel,47},1},
     {{channel,49},0},
     {{channel,52},1}]}</pre>
      <p>For <c>channel</c>, the result shows that all lines have been
        executed once, except line number 49 which has not been executed at
        all.</p>
    </section>

    <section>
      <title>Analysis to File</title>
      <p>A line level calls analysis of <c>channel</c> can be written to
        a file using <c>cover:analysis_to_file/1</c>:</p>
      <pre>
12> <input>cover:analyse_to_file(channel).</input>
{ok,"channel.COVER.out"}</pre>
      <p>The function creates a copy of <c>channel.erl</c> where it for
        each executable line is specified how many times that line has been
        executed. The output file is called <c>channel.COVER.out</c>.</p>
      <pre>
File generated from channel.erl by COVER 2001-05-21 at 11:16:38

****************************************************************************

        |  -module(channel).
        |  -behaviour(gen_server).
        |  
        |  -export([start_link/0,stop/0]).
        |  -export([alloc/0,free/1]). % client interface
        |  -export([init/1,handle_call/3,terminate/2]). % callback functions
        |  
        |  start_link() ->
     1..|      gen_server:start_link({local,channel},channel,[],[]).
        |  
        |  stop() ->
     1..|      gen_server:call(channel,stop).
        |  
        |  %%%-Client interface functions------------------------------------
        |  
        |  alloc() ->
     1..|      gen_server:call(channel,alloc).
        |  
        |  free(Channel) ->
     1..|      gen_server:call(channel,{free,Channel}).
        |  
        |  %%%-gen_server callback functions---------------------------------
        |  
        |  init(_Arg) ->
     1..|      {ok,channels()}.
        |  
        |  handle_call(stop,Client,Channels) ->
     1..|      {stop,normal,ok,Channels};
        |  
        |  handle_call(alloc,Client,Channels) ->
     1..|      {Ch,Channels2} = alloc(Channels),
     1..|      {reply,{ok,Ch},Channels2};
        |  
        |  handle_call({free,Channel},Client,Channels) ->
     1..|      Channels2 = free(Channel,Channels),
     1..|      {reply,ok,Channels2}.
        |  
        |  terminate(_Reason,Channels) ->
     1..|      ok.
        |  
        |  %%%-Internal functions--------------------------------------------
        |  
        |  channels() ->
     1..|      [ch1,ch2,ch3].
        |  
        |  alloc([Channel|Channels]) ->
     1..|      {Channel,Channels};
        |  alloc([]) ->
     0..|      false.
        |  
        |  free(Channel,Channels) ->
     1..|      [Channel|Channels].</pre>
    </section>

    <section>
      <title>Conclusion</title>
      <p>By looking at the results from the analyses, it can be deducted
        that the test case does not cover the case when all channels are
        allocated and <c>test.erl</c> should be extended accordingly.        <br></br>

        Incidentally, when the test case is corrected a bug in <c>channel</c>
        should indeed be discovered.</p>
      <p>When the Cover analysis is ready, Cover is stopped and all Cover
        compiled modules are <seealso marker="#loading">unloaded</seealso>.
        The code for <c>channel</c> is now loaded as usual from a
        <c>.beam</c> file in the current path.</p>
      <pre>
13> <input>code:which(channel).</input>
cover_compiled
14> <input>cover:stop().</input>
ok
15> <input>code:which(channel).</input>
"./channel.beam"</pre>
    </section>
  </section>

  <section>
    <title>Miscellaneous</title>

    <section>
      <title>Performance</title>
      <p>Execution of code in Cover compiled modules is slower and more
        memory consuming than for regularly compiled modules. As the Cover
        database contains information about each executable line in each
        Cover compiled module, performance decreases proportionally to
        the size and number of the Cover compiled modules.</p>
      <p>To improve performance when analysing cover results it is possible
	to do multiple calls to <seealso marker="cover#analyse-1">analyse</seealso>
	and <seealso marker="cover#analyse_to_file-1">analyse_to_file</seealso>
	at once. You can also use the 
	<seealso marker="cover#async_analyse_to_file-1">async_analyse_to_file</seealso> 
	convenience function.
      </p>
    </section>

    <section>
      <marker id="lines"></marker>
      <title>Executable Lines</title>
      <p>Cover uses the concept of <em>executable lines</em>, which is lines
        of code containing an executable expression such as a matching or
        a function call. A blank line or a line containing a comment,
        function head or pattern in a <c>case</c>- or <c>receive</c>
        statement is not executable.</p>
      <p>In the example below, lines number 2,4,6,8 and 11 are executable
        lines:</p>
      <p></p>
      <pre>
1: is_loaded(Module,Compiled) ->
2:   case get_file(Module,Compiled) of
3:     {ok,File} ->
4:       case code:which(Module) of
5:         ?TAG ->
6:           {loaded,File};
7:         _ ->
8:           unloaded
9:       end;
10:    false ->
11:      false
12:  end.</pre>
    </section>

    <section>
      <marker id="loading"></marker>
      <title>Code Loading Mechanism</title>
      <p>When a module is Cover compiled, it is also loaded using the normal
        code loading mechanism of Erlang. This means that if a Cover
        compiled module is re-loaded during a Cover session, for example
        using <c>c(Module)</c>, it will no longer be Cover compiled.</p>
      <p>Use <c>cover:is_compiled/1</c> or <c>code:which/1</c> to see if
        a module is Cover compiled (and still loaded) or not.</p>
      <p>When Cover is stopped, all Cover compiled modules are unloaded.</p>
    </section>
  </section>
</chapter>