static

• 全局：本文件作用域
• 局部：函数作用域。内存地址不会变动

/* loc_stat.c -- using a local static variable */
#include <stdio.h>
void trystat(void);

int main(void) {
    int count;

    for (count = 1; count <= 3; count++) {
        printf("Here comes iteration %d:\n", count);
        trystat();
    }

    return 0;
}

void trystat(void) {
    int fade = 1;
    static int stay = 1; // 内存地址不会变动

    printf("fade = %d and stay = %d\n", fade++, stay++);
}

extern：

使用外部已经定义过的变量，

/* global.c  -- uses an external variable */
#include <stdio.h>
int units = 0; /* an external variable      */

void critic(void);

int main(void) {
    extern int units; /* an optional redeclaration */

    printf("How many pounds to a firkin of butter?\n");
    scanf("%d", &units);
    while (units != 56)
        critic();
    printf("You must have looked it up!\n");

    return 0;
}

void critic(void) {
    /* optional redeclaration omitted */
    printf("No luck, chummy. Try again.\n");
    scanf("%d", &units);
}

const：

关键字声明的对象，其值不能通过赋值或递增，递减来修改

#include <stdio.h>

void change_var_by_pointer(void) {
    const int days[12] = {21, 32, 43, 54, 55, 43};
    // 通过指针改数据
    int* day = days;
    *day = 100;
    printf("days[0] = %d\n", days[0]);
}

int main(int argc, char* argv[]) {
    const int nochange = 0;

    const int nochange1 = 10;

    const int days[12] = {21, 32, 43, 54, 55, 43};
    // days[0] = 100; // err

    const float* pf;         // *pf
    float* const pt;         // pt
    const float* const ptr;  // ptr/*ptr

    float const* pfc1;  // *pfc1
    const float* pfc2;  // *pfc2

    change_var_by_pointer();
    return 0;
}

typedef:

为某一类型自定义类型，给类型定义一个别名

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// 普通类型typedef
typedef unsigned char byte;
typedef char* string;

// struct类型typedef
typedef struct person_s {
    int age;
    string name;
} person_t;

// 函数指针类型typedef
typedef int (*func_t)(int a, int b);

int add(int a, int b) {
    return a + b;
}

void test(func_t func) {  //
    int ret = func(10, 20);
    printf("ret = %d\n", ret);
}
int main(int argc, char* argv[]) {
    // normal type
    unsigned char a = 100;
    byte b = 100;

    char* c = "hello";
    string d = "world";

    // struct
    struct person_s p1;
    person_t p2;

    // function pointer
    test(add);

    // typedef unsigned long           clock_t;       /* clock() */
    // typedef long                    ssize_t;       /* byte count or error */
    // typedef long                    time_t;        /* time() */
    return 0;
}