Real-time
- C++ course
- Expert
- Lesson 35/47
- 5 min
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.
Practice 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;.
Starter code
#include <iostream>
// Define the runTask function here
int main() {
// Write a loop calling runTask three times and print count
return 0;
}Sample solution
#include <iostream>
void runTask() {
}
int main() {
int count = 0;
for (int i = 0; i < 3; ++i) {
runTask();
count++;
}
std::cout << count;
return 0;
}Expected output
3
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the loop comment
for (;;) { /* ____ loop */ runTask(); }for (;;) { /* real-time loop */ runTask(); }Explanation: The infinite loop is the skeleton of a real-time system.
2 Match each term to its role
| Term | Meaning |
|---|---|
hard real-time |
Deadline critical |
soft real-time |
Deadline flexible |
determinism |
Fixed time upper bound |
jitter |
Time variability |
3 What does hard real-time mean?
// airbag control- Deadline is flexible
- Always the fastest
- Deadline is critical, missing it fails the system Correct answer
- Requires an operating system
Explanation: In hard real-time, missing a deadline counts as system failure.