120. Copy Semantics-II

Question.7

A developer passes a large config struct by value to an ISR callback:

struct MotorConfig {
   float kp, ki, kd;
   int limits[4];
   uint32_t flags;
}; // ~32 bytes

void motor_isr(MotorConfig cfg) {  // By value!
   apply_pid(cfg);
}

Why is pass-by-value dangerous in an ISR?

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ISRs cannot receive parameters
By-value parameters are not volatile
The copy constructor may allocate heap memory
Copying 32 bytes onto the ISR stack consumes precious stack space and takes non-deterministic time -- both are critical constraints in ISR context

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