assert
- C course
- Intermediate
- Lesson 56/116
- 5 min
Learn debug checks with assert.
The assert macro
The assert macro checks a condition; if false, it stops the program and prints an error. Usage: assert(ptr != NULL);. You need <assert.h>. When the condition is false, it calls abort with the source file and line number.
assert is a debugging tool; it catches unexpected states. If the NDEBUG macro is defined, assert does nothing; so production builds can disable it. It's not for error handling but for programmer errors (precondition violations).
assert and function calls
Don't put a function with side effects inside assert. Example: assert(allocate() != NULL); — with NDEBUG, allocate is never called, behavior changes. The right way: call first, then check.
assert's message includes file, line and the condition; you don't need an extra message. C99 added static_assert for compile-time checks, also usable as _Static_assert. For runtime checks you still use assert.
Practice Add a precondition with assert
Include <stdio.h> and <assert.h>. Write int divide(int a, int b) that calls assert(b != 0); and returns a / b;. In main, print printf("%d\n", divide(10, 2));.
Starter code
#include <stdio.h>
// Include assert.h
// Write divide: precondition and division
int main(void) {
// Call divide(10, 2) and print
return 0;
}Sample solution
#include <stdio.h>
#include <assert.h>
int divide(int a, int b) {
assert(b != 0);
return a / b;
}
int main(void) {
printf("%d\n", divide(10, 2));
return 0;
}Expected output
5
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which macro disables assert?
- DEBUG
- NDEBUG Correct answer
- RELEASE
- ASSERT
Explanation: With NDEBUG defined, assert does nothing.
2 Complete the assert call
#include <assert.h>
void set_age(int age) {
____(age >= 0);
}#include <assert.h>
void set_age(int age) {
assert(age >= 0);
}Explanation: assert(age >= 0) catches negative ages.
3 Match assert usage patterns
| Term | Meaning |
|---|---|
assert(ptr != NULL) |
Precondition check |
NDEBUG |
Disables assert |
abort |
assert calls it |
assert.h |
Required header |