Heap use in a message handler

The code

Embedded C for Cortex-M. It compiles. There are 3 real bugs in it: the kind that pass a bench test and fail in the field.

void on_mqtt_msg(const uint8_t *payload, size_t len)
{
    char *json = malloc(len);
    memcpy(json, payload, len);
    parse_json(json);

    if (!config_is_valid()) {
        log_warn("bad config");
        return;
    }

    apply_config();
    free(json);
}

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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