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4.6 Writing a test case

The easiest way to prepare a new test case is to base it on an existing one for a similar situation. There are two major categories of tests: batch-oriented and interactive. Batch-oriented tests are usually easier to write.

The GCC tests are a good example of batch-oriented tests. All GCC tests consist primarily of a call to a single common procedure, since all the tests either have no output, or only have a few warning messages when successfully compiled. Any non-warning output constitutes a test failure. All the C code needed is kept in the test directory. The test driver, written in Tcl, need only get a listing of all the C files in the directory, and compile them all using a generic procedure. This procedure and a few others supporting for these tests are kept in the library module lib/c-torture.exp of the GCC testsuite. Most tests of this kind use very few Expect features, and are coded almost purely in Tcl.

Writing the complete suite of C tests, then, consisted of these steps:

Testing interactive programs is intrinsically more complex. Tests for most interactive programs require some trial and error before they are complete.

However, some interactive programs can be tested in a simple fashion reminiscent of batch tests. For example, prior to the creation of DejaGnu, the GDB distribution already included a wide-ranging testing procedure. This procedure was very robust, and had already undergone much more debugging and error checking than many recent DejaGnu test cases. Accordingly, the best approach was simply to encapsulate the existing GDB tests, for reporting purposes. Thereafter, new GDB tests built up a family of Tcl procedures specialized for GDB testing.

4.6.1 Hints on writing a test case

It is safest to write patterns that match all the output generated by the tested program; this is called closure. If a pattern does not match the entire output, any output that remains will be examined by the next expect command. In this situation, the precise boundary that determines which expect command sees what is very sensitive to timing between the Expect task and the task running the tested tool. As a result, the test may sometimes appear to work, but is likely to have unpredictable results. (This problem is particularly likely for interactive tools, but can also affect batch tools—especially for tests that take a long time to finish.) The best way to ensure closure is to use the -re option for the expect command to write the pattern as a full regular expressions; then you can match the end of output using a $. It is also a good idea to write patterns that match all available output by using .*\ after the text of interest; this will also match any intervening blank lines. Sometimes an alternative is to match end of line using \r or \n, but this is usually too dependent on terminal settings.

Always escape punctuation, such as ( or ", in your patterns; for example, write \(. If you forget to escape punctuation, you will usually see an error message like:

extra characters after close-quote

If you have trouble understanding why a pattern does not match the program output, try using the --debug option to runtest, and examine the debug log carefully.

Be careful not to neglect output generated by setup rather than by the interesting parts of a test case. For example, while testing GDB, I issue a send set height 0\n command. The purpose is simply to make sure GDB never calls a paging program. The set height command in GDB does not generate any output; but running any command makes GDB issue a new (gdb) prompt. If there were no expect command to match this prompt, the output (gdb) begins the text seen by the next expect command—which might make that pattern fail to match.

To preserve basic sanity, I also recommended that no test ever pass if there was any kind of problem in the test case. To take an extreme case, tests that pass even when the tool will not spawn are misleading. Ideally, a test in this sort of situation should not fail either. Instead, print an error message by calling one of the DejaGnu procedures error or warning.


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