输入限定符
类型限定符是描述有关类型的其他语义的关键字。它们是类型签名的组成部分。它们既可以出现在声明的最顶层（直接影响标识符），也可以出现在子级别（仅与指针相关，影响指向的值）：
| 关键词 | 备注 |
|---|---|
const |
防止声明对象的突变（通过出现在最顶层）或防止指向值的突变（通过出现在指针子类型旁边）。 |
volatile |
通知编译器声明的对象（在最顶层）或指向的值（在指针子类型中）可能由于外部条件而改变其值，而不仅仅是程序控制流的结果。 |
restrict |
优化提示，仅与指针相关。声明意图在指针的生命周期中，不会使用其他指针来访问同一个指向的对象。 |
关于存储类说明符（static，extern，auto，register），类型修饰符（signed，unsigned，short，long）和类型说明符（int，char，double 等）的类型限定符的顺序未强制执行，但是好的做法是将它们按上述顺序排列：
static const volatile unsigned long int a = 5; /* good practice */
unsigned volatile long static int const b = 5; /* bad practice */
顶级资格
/* "a" cannot be mutated by the program but can change as a result of external conditions */
const volatile int a = 5;
/* the const applies to array elements, i.e. "a[0]" cannot be mutated */
const int arr[] = { 1, 2, 3 };
/* for the lifetime of "ptr", no other pointer could point to the same "int" object */
int *restrict ptr;
指针子类型资格
/* "s1" can be mutated, but "*s1" cannot */
const char *s1 = "Hello";
/* neither "s2" (because of top-level const) nor "*s2" can be mutated */
const char *const s2 = "World";
/* "*p" may change its value as a result of external conditions, "**p" and "p" cannot */
char *volatile *p;
/* "q", "*q" and "**q" may change their values as a result of external conditions */
volatile char *volatile *volatile q;