Real-time

Learn real-time C++ and timing constraints.

Real-time

A real-time system requires operations to complete within a specific deadline. The key is not speed but determinism: how long each task takes must be known in advance.

There are two kinds of real-time: hard real-time (missing a deadline fails the system — airbag control, for example) and soft real-time (missing a deadline degrades quality but the system still runs — video playback, for example). Embedded systems usually carry real-time constraints.

Determinism

In real-time systems determinism is being able to know a task's duration in advance. Operations with uncertain duration like dynamic memory allocation, cache misses, garbage collection are avoided. The task duration is computed for the worst case.

Example: an embedded loop is while (true) { runTask(); }. Each runTask call's duration must be known in advance; otherwise the deadline may be missed. jitter (time variability) is kept small; this means the task duration is the same each time.

Exercise Write a real-time loop

Define void runTask() (an empty body is fine). In main define int count = 0;, write for (int i = 0; i < 3; ++i) { runTask(); count++; } and write std::cout << count;.

Show example solution

#include <iostream>

void runTask() {
}

int main() {
    int count = 0;
    for (int i = 0; i < 3; ++i) {
        runTask();
        count++;
    }
    std::cout << count;
    return 0;
}

Test yourself

Complete the loop comment

for (;;) { /* ____ loop */ runTask(); }

Show answer

for (;;) { /* real-time loop */ runTask(); }

Match each term to its role

hard real-time

Deadline critical
soft real-time

Deadline flexible
determinism

Fixed time upper bound
jitter

Time variability
What does hard real-time mean?
```
// airbag control
```
- Deadline is flexible
- Always the fastest
- Deadline is critical, missing it fails the system
- Requires an operating system
Show answer

Deadline is critical, missing it fails the system — In hard real-time, missing a deadline counts as system failure.

Next lesson: Summary - Embedded C++

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