Improving test coverage

It is difficult to write test suites that fully cover programs if you have no way of determining how much of the code is exercised. lprof removes the guesswork by showing, on a line-by-line basis, which lines of code are executed. That allows the tester to know exactly what has been tested. It also makes it easier to refine and improve tests.

Suppose you want to measure how well a given test suite tests a program. First, compile and link the program with -ql so that profiling information will be saved. Then, run the program with the tests to get the profiling data. By looking at the summary output, you can see how much of the code is exercised.

   Coverage Data Source: test.cnt
   Date of Coverage Data Source: Wed Mar  2 11:11:58 1994
   Object: prog
   

   percent   lines   total function
   covered  covered  lines name
   

     91.5      97     106  compile
    100.0      18      18  step
    100.0      73      73  advance
    100.0       4       4  getrnge
     42.9      12      28  main
    100.0      29      29  execute
    100.0      19      19  succeed
     42.9       3       7  putdata
      0.0       0      19  regerr
    100.0      21      21  fgetl
   

     85.2     276     324  TOTAL

lprof Summary Output for a Test Suite

Now you can examine individual functions that do not have 100% coverage to find ways of improving the tests.

The rest of this section consists of three examples that show why certain functions may not have 100% coverage. The first example demonstrates how to uncover an option that is usually missed because it is not documented. The second example shows how to uncover a function that is never called. The third example examines code that is never executed because of an error condition that is difficult to produce. Each section also explains how to resolve the problem of lack of coverage.