Assertions

The assert.h header provides a simple diagnostic mechanism. Assertions can be performed during compile-time (_Static_assert keyword) and run-time (assert macro).

Compile-time

The _Static_assert keyword evaluates an expression at compile-time.

Code

#include <assert.h> // "static_assert".
#include <stdio.h> // "puts".

int main(void)
{
	static_assert (2 + 2 == 4, "Mathematics works."); // Compilation success.

	static_assert (sizeof (signed long long int) < sizeof (signed char), "Primitive data type sizes inverted."); // Compilation failure.

	puts("Execution success.");

	return 0;
}

Compilation

Standard error stream:

In file included from main.c:1:0:
main.c: In function ‘main’:
main.c:7:2: error: static assertion failed: "Primitive data type sizes inverted."
  static_assert (sizeof (signed long long int) < sizeof (signed char), "Primitive data type sizes inverted."); // Compilation failure.
  ^

Run-time

The assert macro evaluates an expression at run-time.

Debug build

Code

#include <assert.h> // "assert".
#include <stdio.h> // "puts".

int main(void)
{
	assert (2 + 2 == 4); // Compilation success.

	assert (sizeof (signed long long int) < sizeof (signed char)); // Compilation success, but execution failure.

	puts("Execution success.");

	return 0;
}

Execution

Standard error stream:

main: main.c:7: main: Assertion `sizeof (signed long long int) < sizeof (signed char)' failed.
Aborted (core dumped)

Release build

Assertions can be disabled for release builds by defining a macro by the name NDEBUG before the inclusion of the assert header.

Code

#define NDEBUG
#include <assert.h> // "assert".
#include <stdio.h> // "puts".

int main(void)
{
	assert (2 + 2 == 4); // Compilation success.

	assert (sizeof (signed long long int) < sizeof (signed char)); // Compilation success.

	puts("Execution success.");

	return 0;
}

Execution

Standard output stream:

Execution success.