Checking Correctness with the assert Macro (Your First Step into Testing)

This lesson covers writing a simple test in C using the assert macro, your first step toward the habit of verifying a program's correctness. It's for anyone searching "C assert usage."

assert(condition) is a macro that abnormally terminates the program and lets you know when a condition is false. It's the basic idea behind test code — automatically checking "does this function's result match what I expect."

The example confirms the expected and actual results match in a single line, assert(add(2, 3) == 5). When the condition is false, the program prints a message and stops.

A common early mistake is not knowing assert can be disabled in a release build. When a macro called NDEBUG is defined, assert's contents don't run at all — so it can't be a substitute for real error handling in production.

In real projects, using assert as a debugging aid during development, and implementing separate, proper error handling for production, is the standard approach.

📖 Reference code

#include <stdio.h>
#include <assert.h>

int add(int a, int b) {
    return a + b;
}

int main(void) {
    assert(add(2, 3) == 5);
    printf("Test passed: add(2, 3) is 5\n");
    printf("Test run complete\n");
    return 0;
}

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).