Settings and calibration in one page

The code

Embedded C for Cortex-M. It compiles. There are 4 real bugs in it: the kind that pass a bench test and fail in the field.

#define CFG_PAGE  255u
#define CFG_ADDR  0x0807F800u          /* settings: first 1 KB of the page */
#define CAL_ADDR  0x0807FC00u          /* factory calibration: second 1 KB */

typedef struct {
    uint32_t magic;
    uint16_t setpoint[600];
    uint16_t hysteresis;
    uint16_t flags;
    uint32_t crc;
} cfg_t;

bool cfg_save(const cfg_t *c)
{
    HAL_FLASH_Unlock();
    FLASH_EraseInitTypeDef e = { .TypeErase = FLASH_TYPEERASE_PAGES,
                                 .Banks = FLASH_BANK_1, .Page = CFG_PAGE, .NbPages = 1 };
    uint32_t err;
    HAL_FLASHEx_Erase(&e, &err);
    const uint8_t *src = (const uint8_t *)c;
    for (uint32_t i = 0; i < sizeof(cfg_t); i += 8) {
        HAL_FLASH_Program(FLASH_TYPEPROGRAM_DOUBLEWORD, CFG_ADDR + i,
                          *(const uint64_t *)(src + i));
    }
    HAL_FLASH_Lock();
    return true;
}

What an interviewer expects

• Point to each line you'd stop in a code review.
• Say what actually breaks: the failure mode, not just the rule it violates.
• Fix it, then defend the fix when asked the follow-up.

The bugs, reference fixes and the follow-up question are in the practice app, after you've tried it.

Companies that test this

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