embedded-testing-patterns

embedded-testing-patterns

Knowledge Base

Embedded C test patterns using Unity, CMock, Ceedling, and QEMU.

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Pattern 1: Ceedling Project Structure

project/
├── project.yml           # Ceedling configuration
├── src/
│   ├── sensor.c
│   └── sensor.h
├── test/
│   ├── test_sensor.c
│   └── support/          # Test helpers, stubs
└── vendor/               # Unity, CMock (git submodule or gem)

Minimal project.yml:

:project:
  :build_root: build
  :test_file_prefix: test_

:paths:
  :source: ["src/**"]
  :test:   ["test/**"]
  :include: ["src"]

:tools:
  :test_compiler:
    :arguments:
      - -std=c11
      - -DUNIT_TEST

:plugins:
  :load_paths: ["vendor/ceedling/plugins"]
  :enabled: [gcov, xml_tests_report]

:cmock:
  :enforce_strict_ordering: TRUE
  :plugins: [:ignore, :return_thru_ptr, :callback]

Run:

ceedling test:all           # All tests
ceedling test:sensor        # Single module
ceedling gcov:all           # Coverage
ceedling utils:gcov         # HTML coverage report

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Pattern 2: Testable HAL Interface Pattern

The golden rule: drivers never call HAL_I2C_Master_Transmit directly. They call through a pointer.

/* hal_i2c.h */
typedef int (*i2c_write_fn)(uint8_t dev_addr, uint8_t reg_addr,
                             const uint8_t *data, uint8_t len);
typedef int (*i2c_read_fn)(uint8_t dev_addr, uint8_t reg_addr,
                            uint8_t *data, uint8_t len);

typedef struct {
    i2c_write_fn write;
    i2c_read_fn  read;
} i2c_ops_t;

/* sensor.c uses i2c_ops_t, not HAL directly */
static const i2c_ops_t *s_i2c;

int sensor_begin(const i2c_ops_t *i2c_ops) {
    s_i2c = i2c_ops;
    return s_i2c->write(SENSOR_ADDR, REG_RESET, (uint8_t[]){0xB6}, 1);
}

Production: pass a &stm32_i2c_ops that wraps HAL_I2C_Master_Transmit. Test: pass a &mock_i2c_ops backed by function pointers set by CMock.

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Pattern 3: CMock Expect Chaining

Order matters. CMock verifies calls happen in declared order when enforce_strict_ordering: TRUE.