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6.4 Hints on writing a test case

There may be useful existing procedures already written for your test in the ‘lib’ directory of the DejaGnu distribution. See section DejaGnu Builtins.

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. See section Debug Log.

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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