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Keywords

auto break case char
const continue default do
double else enum extern
float for goto if
int long register return
short signed sizeof static
struct switch typedef union
unsigned void volatile while

refer: https://www.programiz.com/c-programming/list-all-keywords-c-language

typedef

typedef unsigned char BYTE;
BYTE b1, b2;

typedef struct {
    const unsigned char *json;
    size_t position;
} error;
error e1, e2;

typedef int * IntPtr;
IntPtr p1, p2, p3; // the type of p1, p2 and p3 are all the int *

enum

enum color {
    RED,
    YELLOW,
    GREEN,
    BLUE,
    PINK
};

enum color newColor = BLUE;

typedef enum {
    RED,
    YELLOW,
    GREEN,
    BLUE,
    PINK
} color;

color newColor = BLUE;

enum color {
    RED,
    YELLOW,
    GREEN,
    BLUE,
    PINK
};

typedef enum color color;
color newColor = BLUE;
enum color oldColor = GREEN;

Other examples

enum boolean {false, true};
enum boolean check;

union

union 占用内存大小取决于size最大的元素

union [union tag] {
    member definition;
    member definition;
    ...
    member definition;
} [one or more union variables];

example:

union Data {
    int i;
    float f;
    char str[20];
} data;
// access data with data.i, data.f, data.str
// sizeof(data) == 20

volatile

volatile防止因编译器优化导致变量读取错误的情况的发生。不加该关键词情况下，编译器可能会对某变量进行存储优化，将第一次存储的数据存入缓存，而后从缓存读取数据，但实际上该变量可能在以下情况发生了更改：

  1. 信号处理程序
  2. 硬件中断处理程序
  3. 硬件交互(与内存映射IO相关)
  4. 多线程中的其它线程修改了变量

添加关键词volatile后，编译器便知道这个变量是不稳定的，就会保证每次都从变量的原始地址读取数据。

volatile int i = 0;
volatile float * j = 1.0;

refer: C/C++ volatile

register

It's a hint to the compiler that the variable will be heavily used and that you recommend it be kept in a processor register if possible.

Most modern compilers do that automatically, and are better at picking them than us humans.

refer: “register” keyword in C?

简单点说，register告诉编译器，该变量会经常被用到，建议将其保存至CPU的寄存器中，以便快速读取。当然只是建议，编译器会依据情况看是否放入寄存器，而且现代编译器大多会自动选择将某变量存入内存还是寄存器。所以貌似用到的情况并不多见。

register int i = 0;

int j = 10;
register int * k = &j;

关于register：

  1. 如果对register变量使用取地址符&，会引发编译错误，因为访问寄存器地址是无效的
  2. register关键词可用于指针，此时使用&则不会报错
  3. register是存储类，C语言不允许多个存储类关键词施加到一个变量，编译如下所示代码是会报错的
  4. register只能作用于局部block，不能作用于全局变量
  5. 理论上没有限制register变量个数，但编译器会选择性的存放至寄存器或内存中
int i = 10;
register static int * a = &i;

refer: Understanding “register” keyword in C