assert

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) 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__ .

Parameters

Return value

(none)

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");

The implementation of assert in Microsoft CRT does not conform to C99 and later revisions, because its underlying function ( _wassert ) takes neither __func__ nor an equivalent replacement.

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;
}

--- Output with NDEBUG not defined: ---
a.out: main.cpp:10: main: Assertion `x >= 0.0' failed.
 
--- Output with NDEBUG defined: ---
sqrt(x) = -nan

References

See also