Lesson 10

Unit tests with boundary cases

Write small C tests that assert boundary behavior directly, instead of printing output for a human to inspect.

tests
assertions
boundaries

Good Code

tests/review_label_test.c

#include <assert.h>
#include <string.h>

static void test_review_label_fits_buffer(void)
{
    char buffer[16];

    // Test data names the boundary condition under review.
    assert(format_review_label(buffer, sizeof buffer, 42) == 0);
    assert(strcmp(buffer, "review-42") == 0);
}

int main(void)
{
    test_review_label_fits_buffer();
    return 0;
}

Bad Code

tests/review_label_test.c

#include <stdio.h>

int main(void)
{
    char buffer[16];
    format_review_label(buffer, sizeof buffer, 42);

    // Printing output asks humans to judge the test result.
    printf("%s\n", buffer);
    return 0;
}

Review Notes

What to review

Good Code

The good version turns the expected label into assertions. The test fails automatically when the formatter returns an error or writes the wrong text.

Bad Code

The bad version prints the result and exits successfully. CI will pass even if the output is wrong, unless a human reads the log.

Takeaways

A C test should fail by itself when the code breaks, without asking a reviewer to read terminal output.