assert in C

assert (function macro) aborts the program if the user-specified condition is not true. May be disabled for release builds.

It is defined in header <assert.h>, much like this:

#ifdef NDEBUG
#define assert(condition) ((void)0)
#else
#define assert(condition) /*implementation defined*/
#endif                                   (until C23)

#ifdef NDEBUG
#define assert(...) ((void)0)
#else
#define assert(...) /*implementation defined*/
#endif                             (since C23)

The definition of the macro assert depends on another macro, NDEBUG, which is not defined by the standard library.

If NDEBUG is defined as a macro name at the point in the source code where <assert.h> is included, then assert does nothing.

If NDEBUG is not defined, then assert checks if its argument (until C23), or the expression synthesized from __VA_ARGS__ (since C23) (which must have scalar type, otherwise, the behavior is undefined) compares equal to zero. If it does, assert outputs implementation-specific diagnostic information on the standard error output and calls abort(). The diagnostic information is required to include the text of expression, as well as the values of the predefined variable __func__ and of (since C99) the predefined macros __FILE__ and __LINE__.

Notes

There is no standardized interface to add an additional message to assert errors. A portable way to include one is to use a comma operator, or use && with a string literal:

assert(("There are five lights", 2 + 2 == 5));
assert(2 + 2 == 5 && "There are five lights");

Even though the change of assert in C23 (N2829) is not a formal defect report, the C committee recommends implementations to backport the change to old modes.

Example

#include <stdio.h>
// uncomment to disable assert()
// #define NDEBUG
#include <assert.h>
#include <math.h>

#define TEST(...) __VA_ARGS__

int main(void)
{
    double x = -1.0;
    assert(x >= 0.0);
    printf("sqrt(x) = %f\n", sqrt(x));

    assert(TEST(x >= 0.0));

    return 0;
}