<assert.h> provides the assert macro for runtime checking during development. It evaluates a condition and terminates the program with a diagnostic message showing the file, line, and failed condition if it's false.

Assertions are stripped entirely when compiled with -DNDEBUG, so they cost nothing in release builds. Never put side effects in assertions—they won't execute in production.

This example uses assertions to check preconditions, which can be completely removed in release builds.

// compile: gcc -o assertexample assertexample.c
// run: ./assertexample
// description: assertions fire on invalid input; -DNDEBUG disables them entirely
 
#include <assert.h>
#include <stdio.h>
 
int divide(int a, int b) {
    assert(b != 0);
    return a / b;
}
 
int main() {
    printf("%d\n", divide(10, 2));
    printf("%d\n", divide(10, 0));  // assertion fires
    return 0;
}

Runtime assertions:

• assert(condition): if condition is false, print diagnostic and abort:
  • Message format: Assertion failed: (condition), file filename, line N
  • Writes to stderr
  • Terminates with abort()

Disabling assertions:

• NDEBUG: preprocessor symbol; define before including <assert.h> to disable all assertions
• Compile with -DNDEBUG to remove assertions
• In release builds, assertions cost nothing (zero overhead)

Usage patterns:

• Check preconditions on entry (e.g., pointer not NULL)
• Check invariants that must hold (e.g., array index in bounds)
• Never put side effects in assertions (e.g., assert(i++ < 10): the increment won't happen in release)
• Use for debugging; for user error handling use exceptions or error codes