The <assert.h> header defines the assert() macro. It refers to the macro NDEBUG which is not defined in the header. If NDEBUG is defined as a macro name before the inclusion of this header, the assert() macro is defined simply as:
#define assert(ignore)((void) 0)
Otherwise, the macro behaves as described in assert(3C).
The assert() macro is redefined according to the current state of NDEBUG each time <assert.h> is included.
The assert() macro is implemented as a macro, not as a function. If the macro definition is suppressed in order to access an actual function, the behavior is undefined.
See attributes(5) for descriptions of the following attributes: