Defined in header <assert.h>
#ifdef NDEBUG
#define assert(condition) ((void)0)
#define assert(condition) /*implementation defined*/

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 (which must have scalar type) 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__.


condition - expression of scalar type

Return value



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:

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

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.


#include <stdio.h>
// uncomment to disable assert()
// #define NDEBUG
#include <assert.h>
#include <math.h>
int main(void)
    double x = -1.0;
    assert(x >= 0.0);
    printf("sqrt(x) = %f\n", sqrt(x));   
    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


  • C17 standard (ISO/IEC 9899:2018):
    • The assert macro (p: 135)
  • C11 standard (ISO/IEC 9899:2011):
    • The assert macro (p: 186-187)
  • C99 standard (ISO/IEC 9899:1999):
    • The assert macro (p: 169)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • The assert macro

See also

causes abnormal program termination (without cleaning up)
C++ documentation for assert

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