assert testing

Learn unit testing with assert.

assert testing

assert aborts the program when a condition is false; it is defined in <assert.h>. If the condition is true nothing happens; if false a message is written to stderr and abort() is called.

This is the simplest form of unit testing: assert(2 + 2 == 4);. Asserts are active in debug builds; in release they are disabled with the NDEBUG macro. In this lesson we always keep them active; they are the fastest way to check small conditions.

Using assert

You write assert(condition); to check a condition. If true execution continues; if false the program stops. This is ideal for checking invariants.

Example: assert(p != NULL); guarantees the pointer is not null. On failure you see the source file and line; this makes assert a powerful debugging tool. After tests it is common to print a success message with printf: printf("test: tamam\n");.

Exercise First assert test

Call assert(2 + 2 == 4);. Print the success message with printf("test: tamam\n");. Add the <assert.h> header.

Show example solution

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

int main(void) {
    assert(2 + 2 == 4);
    printf("test: tamam\n");
    return 0;
}

Test yourself

Which header defines assert?
- stdlib.h
- stdio.h
- assert.h
- string.h
Show answer

assert.h — answer: assert.h. The assert macro is defined in that header.

assert condition

assert(2 + 2 == ____);

Show answer

assert(2 + 2 == 4);

Match assert concepts

assert

Checks condition
abort

Stops program
NDEBUG

Disables assert
stderr

Error message

Next lesson: Test framework

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