Test doubles
- C course
- Advanced
- Lesson 69/83
- 5 min
Learn test doubles and stubs.
Test doubles
Test doubles are stand-ins for real dependencies: stub, mock and fake. The most common is a stub; it returns a fixed value.
Example: time() returns a different value on every call, which makes it hard to test. By writing long stub_time(void) { return 1000; } you get a deterministic value. The test then produces the same result on every machine. Stubs separate the outside world (clock, random number, network) from the test.
Types of doubles
A stub returns a fixed value; a mock also records which calls were made; a fake provides a simplified but working implementation (e.g. an in-memory database).
In this lesson we only use a stub: long stub_time(void). In main you write long t = stub_time(); and printf("zaman: %ld\n", t);. The output is always "zaman: 1000"; this is the definition of a deterministic test. Testing with the real time would give different results every hour.
Practice Write stub_time
Write long stub_time(void) that returns 1000. In main compute long t = stub_time(); and print printf("zaman: %ld\n", t);.
Starter code
#include <stdio.h>
// Write long stub_time(void) that returns 1000
// main: call stub_time and print the time
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
long stub_time(void) {
return 1000;
}
int main(void) {
long t = stub_time();
printf("zaman: %ld\n", t);
return 0;
}Expected output
zaman: 1000
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 stub_time body
long stub_time(void) { return ____; }long stub_time(void) { return 1000; }Explanation: The stub returns a fixed 1000; the test is deterministic.
2 Match double types
| Term | Meaning |
|---|---|
stub |
Fixed value |
mock |
Call record |
fake |
Simple impl |
real |
Real dependency |
3 When is stub_time useful?
- For deterministic tests Correct answer
- To allocate memory
- To close files
- To send signals
Explanation: A stub gives a fixed value for deterministic tests.