指针，变量从函数取值

结构体

typedef struct _room
{
    int id;
    char name[32];
    int *p;
    int phone;
}room;

room g_room = { 1, "room", NULL, 123};

//指针取值

void get_roomp(room **r)
{
    *r = &g_room;
}
//变量取值
void get_room(room *r)
{
    memcpy(r, &g_room, sizeof(room));
}
void test_struct()
{
    printf("%p, id = %d,%p, name = %s, %p, %p, phone = %d, %p\n",
            &g_room,
            g_room.id,
            &g_room.id,
            g_room.name,
            &g_room.name,
            &g_room.p,
            g_room.phone,
            &g_room.phone);

    room *r;

    get_roomp(&r);
    printf("%p, id = %d,%p, name = %s, %p, %p, phone = %d, %p\n",
            r,
            r->id,
            &r->id,
            r->name,
            &r->name,
            &r->p,
            r->phone,
            &r->phone);

    room rom;
    get_room(&rom);
    printf("%p, id = %d,%p, name = %s, %p, %p, phone = %d, %p\n",
            &rom,
            rom.id,
            &rom.id,
            rom.name,
            &rom.name,
            &rom.p,
            rom.phone,
            &rom.phone);
}

简单变量

int g_val = 1;
//变量取值
void get_val(int *v)
{
    *v = g_val;
    printf("%p,%p\n", v, &v);
}
//指针取值
void get_valp(int **v)
{
    *v = &g_val;
}
void test_val()
{
    printf("%p\n", &g_val);
    /*addr|val
     * | |
     * --|--|
     * &v|v |
     * --|--|
     * | |
     * --|--|
     * | |
     * --|--|
     */
    int v = 0;
    printf("%p\n", &v);
    get_val(&v);
    printf("%p, %d\n", &v, v);
    /*addr|val |val
     * | | | |
     * --|--| |--|
     * &p|p | |*p|
     * --|--| |--|
     * | | | |
     * --|--| |--|
     * | | | |
     * --|--| |--|
     */
    int *p;
    printf("%p\n", p);
    get_valp(&p);
    printf("%p, %d\n", p, *p);
}

指针取值

通过修改指针的指向的地址(*p)，不需要分配内存，*v = &g_val;

void get_valp(int **v)
{
    *v = &g_val;
}

             通过函数获取需要传递二级指针(room **r)

void get_roomp(room **r)
{
    *r = &g_room;
}

变量取值

1. memcpy

void get_room(room *r)
{
    memcpy(r, &g_room, sizeof(room));
}

2. 一个个赋值，比较慢

void get_val(int *v)
{
    *v = g_val;
    printf("%p,%p\n", v, &v);
}