The header <assert.h> defines the assert macro and refers to another macro.
NDEBUG
which is not defined by <assert.h>. If NDEBUG is defined as a macro name at the point in the source file where <assert.h> is included, the assert macro is defined simply as
#define assert(ignore) ((void)0)
The assert macro is implemented as a macro, not as an actual function. If the macro definition is suppressed in order to access an actual function, the behavior is undefined.
#include <assert.h>
void
assert (int
expression );
The assert macro puts diagnostics into programs. When it is executed, if expression is false (that is, it compares equal to zero), the assert macro writes information about the particular call that failed (including the text of the argument, the name of the source file, and the source line number–the latter are respectively the values of the preprocessing macros __FILE__ and __LINE__) on the standard error file in an implementation-defined format. It then calls the abort function.
The assert macro returns no value.
Section 10.4.1, the abort function
The assert macro calls the library function _assert.