​​assert(expression)​​​，判断表达式 ​​expression​​。

如果注释掉 ​​#define NDEBUG​​，assert 语句被触发。

q@ubuntu:~$ ./a.out 
a.out: assert_test.c:13: main: Assertion `KEY==11' failed.
Aborted (core dumped)
q@ubuntu:~$ cat assert_test.c 
// #define NDEBUG
#include <stdio.h>
#include <assert.h>
#define KEY 10 
#if KEY==10
  #define A "A"
#else
  #define B "B"
#endif

int main()
{
  assert(KEY==11);
  printf("%s\n", A);
}

q@ubuntu:~$ ./a.out 
A
q@ubuntu:~$ cat assert_test.c 
#define NDEBUG
#include <stdio.h>
#include <assert.h>
#define KEY 10 
#if KEY==10
  #define A "A"
#else
  #define B "B"
#endif

int main()
{
  assert(KEY==11);
  printf("%s\n", A);
}