assert.h

assert.h is a header file in the C standard library. It defines the C preprocessor macro assert and implements runtime assertion in C.

assert.h is defined in ANSI C as part of the C standard library. Additionally, assert.h is available in C++.

Use

The assert macro implements runtime assertion. If the expression within fails, the assert macro will print a message to stderr and call abort(). The message printed includes the source filename and the source line number and (since C99) the name of the function the assert statement is in, and the expression itself. In ANSI C, the expression in the assert macro must be a signed integer; in C99, the expression may be of any scalar type. Two common uses of the assert macro are to assert that a pointer is not null and to ensure that an array index is in-bounds.

An example program, and the output (using GCC on Linux):

#include <assert.h>
int main()
{
    int a = 1;
    assert(a > 6);
    return 0;
}

program: source.c:5: main: Assertion 'a > 6' failed.
Aborted (core dumpted)

The value of the assert macro changes depending on the definition of another macro, NDEBUG. If NDEBUG is defined as a macro name, the assert macro is defined as #define assert(ignore) ((void)0), so the assertion does not happen. Notice that this does not execute the expression, relying on side-effects of the assert expression is a common mistake.

The assert macro does not include an error message. However the comma operator can be used to add it to the printed expression, as in assert(("Not Orwellian", 2 + 2 == 5));.

In the C++ language <cassert> may be used instead of <assert.h>.

static_assert

The static_assert macro, added in C++11, serves a similar purpose to the assert macro, but it runs at compile-time rather than execution of the program. In C++17, this assertion failure message was made optional, and the subsequent message is omitted if not specified. Originally this was done with template hacks (which are impossible in C), but all modern compilers provide direct support for this.

In C11, the _Static_assert declaration was added to provide the same abilities for C. In C23, _Static_assert was renamed to static_assert and the string literal argument was made optional. Gnulib defines static_assert for platforms that do not use C11 and does not require assert.h to be included.

References

Bibliography

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• Swaminathan, Jeganathan (2017). Mastering C++ Programming. Birmingham: Packt. ISBN 9781786461629.