Unlike the assert() macro, abort_unless() is not disabled by defining the preprocessor macro NDEBUG.

abort_unless.h

#define abort_unless(expr) do {                                         \
    if (!(expr)) {                                                      \
      fprintf(stderr, "%s:%u (%s): Assertion `%s' failed.\n",           \
                   __FILE__, __LINE__, __func__, __STRING(expr));       \
      fflush(stderr);                                                   \
      abort();                                                          \
    }                                                                   \
  } while (0)