Crafting Robust CMake Tests: Beyond Exit Codes with PASS_REGULAR_EXPRESSION

Purpose

• Allows you to define regular expressions that the test output (stdout or stderr) must match for the test to be considered passed.
• Controls how test success is determined in CMake.

Functionality

• You can specify multiple regular expressions as a semicolon-separated list.
• The test's exit code is ignored in this scenario. Even if the program exits with a non-zero code (typically indicating an error), the test will still pass as long as the output matches the regular expression.
• At least one of the regular expressions must match for the test to pass.
• When you set PASS_REGULAR_EXPRESSION for a test, CMake checks the test's output against the provided regular expressions.

Configuration

• Used with the set_tests_properties command in your CMakeLists.txt file.

Example

set_tests_properties(mytest PROPERTIES PASS_REGULAR_EXPRESSION "Test Passed;.*All tests successful.*")

In this example, the test mytest will pass if:

• OR the output contains any text followed by "All tests successful" (wildcard .* matches any characters).
• The output contains the literal string "Test Passed" (exact match).

Important Considerations

• Timeouts set with the TIMEOUT property still apply, regardless of PASS_REGULAR_EXPRESSION. If the test exceeds the timeout, it will fail.
• This property overrides the default behavior of relying solely on the test program's exit code.
• Regular expressions can be complex, so ensure you understand their syntax to avoid unintended matches or misinterpretations.

Effective Use Cases

• Matching expected output patterns without relying solely on exit codes (useful for tests that might exit with non-zero but still indicate success).
• Verifying specific messages or logs are generated by the test.

• Use it cautiously, as over-reliance on specific output patterns can make tests less robust to changes in program behavior.
• If your tests primarily rely on exit codes for success/failure indication, PASS_REGULAR_EXPRESSION might be unnecessary.

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Verifying specific messages and ignoring others

set_tests_properties(mytest PROPERTIES PASS_REGULAR_EXPRESSION "Test completed successfully;Calculation result: (.*)")

This example ensures the test output contains "Test completed successfully" and then captures the calculation result in a group using (.*). This allows for flexibility in the actual result value.

Matching expected output format with options

set_tests_properties(compressiontest PROPERTIES PASS_REGULAR_EXPRESSION "Compressed file size: (\\d+) bytes;Compression ratio: (\\d+\.\\d+)")

This example verifies the test output contains information about the compressed file size (captured in group 1) and compression ratio (captured in group 2). The regular expression allows for any number of digits followed by "bytes" and a decimal number followed by "Compression ratio".

Handling expected warnings

set_tests_properties(compiletest PROPERTIES PASS_REGULAR_EXPRESSION "compilation warning:.*")

This example allows the test to pass even if there are compilation warnings (any text followed by "compilation warning:"). This can be helpful when testing code that might trigger warnings but still compiles successfully.

• Be cautious when ignoring potential errors (like warnings) to avoid missing critical issues.
• Choose regular expressions that accurately reflect the expected output format.

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Relying on Exit Code

• You can set custom exit codes within your test program for different outcomes (e.g., 1 for failure, 2 for skipped).
• This is the simplest approach and works well if your tests clearly indicate success/failure through their exit code.
• The default behavior of CMake tests is to consider a test successful if the test program exits with a return code of 0.

Checking Files

• This is useful for validating the output of the test program is written to a specific file.
• You can compare the content with expected values using string manipulation commands like string(EQUAL) or regular expressions if needed.
• CMake provides commands like file(READ) or configure_file to read the contents of files generated by your tests.

Custom Test Fixture

• This offers more flexibility and separation of concerns for complex tests.
• Within the fixture, you can implement additional logic to check test output, files, or other criteria before marking the test as passed or failed.
• Create a custom test fixture (a CMake target that sets up and tears down the testing environment) using the ctest_register_test_setup and ctest_register_test_teardown commands.

• PASS_REGULAR_EXPRESSION offers a balance when exit codes aren't enough and file checking isn't the best fit.
• If you need to verify specific file content or have more complex test logic, checking files or creating a custom fixture are alternatives.
• If your tests provide clear exit codes, relying on them might be sufficient.