CMock's ExpectWithArray: Pitfalls & Solutions

by Alex Johnson 46 views

Let's dive into a peculiar behavior of ExpectWithArray in CMock that can lead to unexpected test results. Specifically, we'll address the issue of how ExpectWithArray handles multiple calls within a test case when using the same buffer but with different content. Understanding this behavior and its implications is crucial for writing robust and reliable unit tests.

The ExpectWithArray Dilemma

The core problem arises from the way ExpectWithArray appears to store expected values. Instead of creating a unique copy of the array content for each call, it seems to only store a pointer to the array. This optimization, while potentially saving memory, introduces a critical flaw: when you call ExpectWithArray multiple times with the same buffer but different data, all expectations end up referencing the last set of data stored in that buffer.

Consider the following scenario:

uint8_t expected[2];

expected[0] = 0x01;
expected[1] = 0x02;
spi_write_ExpectWithArray(&expected, 2);

expected[0] = 0x03;
expected[1] = 0x04;
spi_write_ExpectWithArray(&expected, 2);

In this example, the intention is to set up two distinct expectations for the spi_write function. The first expectation is for the function to be called with an array containing {0x01, 0x02}, and the second expectation is for an array containing {0x03, 0x04}. However, because ExpectWithArray only stores a pointer, both expectations will ultimately compare against the final content of the expected array, which is {0x03, 0x04}. This leads to the first expectation failing, as it's compared against the wrong data.

Why This Matters

This behavior can lead to several problems in your unit tests:

  • False Positives/Negatives: Tests might pass or fail incorrectly, masking real bugs or reporting nonexistent ones.
  • Debugging Nightmares: Tracking down the root cause of these failures can be incredibly difficult, as the issue isn't immediately obvious from the test code.
  • Reduced Confidence: The reliability of your unit tests is undermined, making it harder to trust the results and leading to uncertainty about the correctness of your code.

Deep Dive: Why Does This Happen?

To understand why ExpectWithArray behaves this way, let's consider how CMock typically handles expectations. When you use ExpectAndReturn, for instance, CMock stores the expected return value internally. This ensures that each expectation is independent and isolated.

However, with arrays, copying the entire content for each expectation could be memory-intensive, especially for large arrays. The decision to store only a pointer was likely made as an optimization to reduce memory usage. Unfortunately, this optimization comes at the cost of introducing the aforementioned issue.

Implications and Scenarios

The implications of this behavior extend to any situation where you're using ExpectWithArray multiple times within a single test case with the same buffer. Here are some common scenarios where this issue might arise:

  • Looping Through Data: If you're iterating through a set of data and using ExpectWithArray to check the data processed in each iteration, you'll likely encounter this problem.
  • Testing State Machines: When testing state machines, you might have different expected outputs based on the current state. If you're reusing the same buffer for these outputs, you'll run into this issue.
  • Complex Data Structures: When dealing with complex data structures that contain arrays, you might inadvertently modify the array content between expectation calls.

Solutions and Workarounds

Fortunately, there are several ways to work around this limitation of ExpectWithArray and ensure your tests are accurate and reliable.

1. Unique Buffers for Each Expectation

The most straightforward solution is to use a separate buffer for each call to ExpectWithArray. This ensures that each expectation has its own independent copy of the data.

uint8_t expected1[2];
uint8_t expected2[2];

expected1[0] = 0x01;
expected1[1] = 0x02;
spi_write_ExpectWithArray(&expected1, 2);

expected2[0] = 0x03;
expected2[1] = 0x04;
spi_write_ExpectWithArray(&expected2, 2);

This approach guarantees that each expectation compares against the correct data. However, it can become cumbersome if you have many expectations or large arrays, as it requires allocating and managing multiple buffers.

2. Copy the Array Content

Another approach is to create a copy of the array content within the expectation itself. This can be achieved by wrapping the ExpectWithArray call in a custom function or macro that performs the copy.

void spi_write_ExpectWithArrayAndCopy(uint8_t* expected, int size) {
    uint8_t* copied_expected = (uint8_t*)malloc(size);
    memcpy(copied_expected, expected, size);
    spi_write_ExpectWithArray(copied_expected, size);
    CMock_memcleanup(); // Important: Free the memory after the expectation is checked
}

uint8_t expected[2];

expected[0] = 0x01;
expected[1] = 0x02;
spi_write_ExpectWithArrayAndCopy(expected, 2);

expected[0] = 0x03;
expected[1] = 0x04;
spi_write_ExpectWithArrayAndCopy(expected, 2);

In this example, the spi_write_ExpectWithArrayAndCopy function allocates memory for a copy of the array, copies the content, and then calls spi_write_ExpectWithArray with the copied data. Important: It also uses CMock_memcleanup() to ensure that the allocated memory is freed after the expectation is checked. This prevents memory leaks.

3. Modify CMock (Advanced)

For advanced users, it's possible to modify CMock itself to store a copy of the array content. This would involve modifying the CMock code generation templates to create a custom version of ExpectWithArray that performs the copy. This approach requires a deep understanding of CMock's internals and is not recommended for beginners. You'll need to carefully consider the memory management implications of this approach.

4. Consider Alternatives

Depending on the complexity of your tests and the nature of the data you're working with, you might consider alternative approaches to testing, such as:

  • Higher-Level Tests: Instead of focusing on individual function calls, you could write higher-level tests that verify the overall behavior of your system. This might reduce the need for detailed expectations on array content.
  • Custom Assertions: You could write custom assertion functions that compare the expected and actual array content directly. This gives you more control over the comparison process and avoids the limitations of ExpectWithArray.

Choosing the Right Solution

The best solution for you will depend on your specific needs and constraints. Here's a summary to help you choose:

  • Unique Buffers: Simplest solution, but can be cumbersome for many expectations or large arrays.
  • Copy Array Content: More complex, but avoids the limitations of ExpectWithArray and allows you to reuse buffers. Remember to manage memory carefully.
  • Modify CMock: Most complex, but provides a permanent fix for the issue. Requires deep understanding of CMock.
  • Alternatives: Consider if different testing strategies could simplify your tests and reduce the need for ExpectWithArray.

Best Practices and Recommendations

To avoid the pitfalls of ExpectWithArray, follow these best practices:

  • Understand the Limitations: Be aware of how ExpectWithArray handles array data and the potential for issues when reusing buffers.
  • Choose the Right Solution: Select the solution that best fits your needs and constraints.
  • Write Clear Tests: Make sure your tests are easy to understand and maintain. Use comments to explain the purpose of each expectation.
  • Test Thoroughly: Test your code thoroughly to ensure that it behaves as expected in all scenarios.
  • Document Your Approach: Document your chosen solution and the reasoning behind it. This will help others understand and maintain your tests in the future.

By understanding the behavior of ExpectWithArray and following these best practices, you can write robust and reliable unit tests that give you confidence in the correctness of your code.

Conclusion

The ExpectWithArray function in CMock, while useful, has a quirk that can lead to unexpected behavior when used multiple times with the same buffer but different data. By understanding this limitation and employing the workarounds discussed, you can ensure your unit tests remain accurate and reliable. Remember to choose the solution that best fits your specific needs and always prioritize clear, well-documented tests.

For more information on CMock and unit testing, check out the official ThrowTheSwitch website. It's a great resource for learning more about CMock and other embedded development tools.