#include<stdio.h>
int main()
{
    int var = 0;
    printf("var value = %d\n", var);
    printf("var address = %p\n", &var);
    return 0;
}

int main()
{
    char* pChar;
    short* pShort;
    int* pInt;
    float* pFloat;
    double* pDouble;
    return 0;   
}

#include <stdio.h>
int main()
{
    int var = 0;
    int another = 0;
    int* pVar = NULL;
    printf("1. var = %d\n", var);
    printf("1. pVar = %p\n", pVar);
    pVar = &var;  // 使用指針保存變量的地址
    *pVar = 100;  // *pVar 等價(jià)于 var ， var = 100;
    printf("2. var = %d\n", var);
    printf("2. pVar = %p\n", pVar);
    pVar = &another;  // 改變了 pVar 的指向，使得 pVar 保存 another 的地址
    *pVar = 1000;     // another = 1000;
    printf("3. another = %d\n", another);
    printf("3. pVar = %p\n", pVar);
    printf("4. add ==> %d\n", var + another + *pVar);   // 100 + 1000 + 1000  ==> 2100
    return 0;
}

#include <stdio.h>
int main()
{
    int i = 10;
    float f = 10;
    int* pi = &f;    // WARNING
    float* pf = &f;  // OK
    printf("pi = %p, pf = %p\n", pi, pf);
    printf("*pi = %d, *pf = %f\n", *pi, *pf);
    pi = i;   // WARNING
    *pi = 110;  // OOPS
    printf("pi = %p, *pi = %d\n", pi, *pi);
    return 0;
}

#include <stdio.h>
void func(int* p)
{
    *p = 100;   // 修改內(nèi)存中 4 字節(jié)的數(shù)據(jù)，即：修改一個(gè)整型變量的值
}
void swap(int* pa, int* pb)
{
    int t = 0;
    t = *pa;
    *pa = *pb;
    *pb = t;
}
int main()
{
    int var = 0;
    int a = 1, b = 2;
    printf("1. var = %d\n", var);
    func( &var );
    printf("2. var = %d\n", var);
    printf("3. a = %d, b = %d\n", a, b);
    swap(&a, &b);
    printf("4. a = %d, b = %d\n", a, b);
    return 0;
}

#include <stdio.h>
int calculate(int n, long long* pa, long long* pm)
{
    int ret = 1;
    if( (1 <= n) && (n <= 20) )
    {
        int i = 0;
        *pa = 0;
        *pm = 1;
        for(i=1; i<=n; i++)
        {
            *pa = *pa + i;
            *pm = *pm * i;
        }
    }
    else
    {
        ret = 0;
    }
    return ret;
}
int main()
{
    long long ar = 0;
    long long mr = 0;
    if( calculate(5, &ar, &mr) )
        printf("ar = %lld, mr = %lld\n", ar, mr);
    return 0;
}

#include <stdio.h>
int main()
{
    int a[] = {1, 2, 3, 4, 0};
    int* p = a;  // a 的類型為 int*, &a[0] ==> int*
    int (*pa) [5] = &a;
    printf("%p, %p\n", p, a);
    p++;
    *p = 100;  // a[1] = 100;
    printf("%d, %d\n", *p, a[1]);
    printf("%p, %p\n", &a, a);
    p = pa;   // WARNING  !!!!
    p = a;
    while( *p )
    {
        printf("%d\n", *p);
        p++;
    }
    return 0;
}

#include <stdio.h>
int main()
{
    int a[] = {1, 2, 3, 4, 5};
    int* p = a;
    int i = 0;
    // a[i] <==> *(a+i) <==> *(p+i) <==> p[i]
    for(i=0; i<5; i++)
    {
        printf("%d, %d\n", a[i], *(a + i));
    }
    printf("\n");
    for(i=0; i<5; i++)
    {
        printf("%d, %d\n", a[i], p[i]);
    }
    printf("\n");
    for(i=0; i<5; i++)
    {
        printf("%d, %d\n", p[i], *(p + i));
    }
    printf("\n");
    printf("a = %p, p = %p\n", a, p);
    printf("&a = %p, &p = %p\n", &a, &p);
    return 0;
}

#include <stdio.h>
int main()
{
    int a[] = {1, 2, 3};
    int* p = a;
    int v = *p++;
    char* s = NULL;
    printf("%p\n", "D.T.Software");
    printf("%p\n", "D.T.Software");
    printf("v = %d, *p = %d\n", v, *p);
    printf("First = %c\n", *"D.T.Software");
    s = "D.T.Software";
    while( *s ) printf("%c", *s++);
    printf("\n");
    return 0;
}

#include <stdio.h>
int add(int a, int b)
{
    return a + b;
}
int mul(int a, int b)
{
    return a * b;
}
int calculate(int a[], int len, int(*cal)(int, int))
{
    int ret = a[0];
    int i = 0;
    for(i=1; i<len; i++)
    {
        ret = cal(ret, a[i]);
    }
    return ret;
}
int main()
{
    int a[] = {1, 2, 3, 4, 5};
    int (*pFunc) (int, int) = NULL;
    pFunc = add;
    printf("%d\n", pFunc(1, 2));
    printf("%d\n", (*pFunc)(3, 4));
    pFunc = &mul;
    printf("%d\n", pFunc(5, 6));
    printf("%d\n", (*pFunc)(7, 8));
    printf("1 + ... + 5 = %d\n", calculate(a, 5, add));
    printf("1 * ... * 5 = %d\n", calculate(a, 5, mul));
    return 0;
}

#include <stdio.h>
int demo(int arr[], int len)  // int demo(int* arr, int len)
{
    int ret = 0;
    int i = 0;
    printf("demo: sizeof(arr) = %d\n", sizeof(arr));
    while( i < len )
    {
        ret += *arr++;
        i++;
    }
    return ret;
}
int main()
{
    int a[] = {1, 2, 3, 4, 5};
    // int v = *a++;
    printf("return value: %d\n", demo(a, 5));
    return 0;
}

#include <stdio.h>
int main()
{
    char c = 0;
    int i = 0;
    float f = 2.0f;
    double d = 3.0;
    void* p = NULL;
    double* pd = NULL;
    int* pi = NULL;
    /* void* 指針可以保存任意類型的地址 */
    p = &c;
    p = &i;
    p = &f;
    p = &d;
    printf("%p\n", p);
    // void* 類型的指針無法訪問內(nèi)存中的數(shù)據(jù)
    // printf("%f\n", *p);
    /* void* 類型的變量可以直接合法的賦值給其他具體數(shù)據(jù)類型的指針變量 */
    pd = p;
    pi = p;
    // void* 是例外，其他指針類型的變量不能相互賦值
    // pd = pi;
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
int main()
{
    int* p = malloc(4); // 從堆空間申請 4 個(gè)字節(jié)當(dāng)作 int 類型的變量使用
    if( p != NULL )  // 如果申請失敗 p 為 0 ，即：空值
    {
        *p = 100;
        printf("%d\n", *p);
        free(p);
    }
    p = malloc(4 * sizeof(int));
    if( p != NULL )
    {
        int i = 0;
        for(i=0; i<4; i++)
        {
            p[i] = i * 10;
        }
        for(i=0; i<4; i++)
        {
            printf("%d\n", p[i]);
        }
        free(p);
    }
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
int main()
{
    int a = 0;
    int b = 1;
    int* p = &a;
    int** pp = &p;
    **pp = 2;   // a = 2;
    *pp = &b;   // p = &b;
    *p = 3;     // b = 3;
    printf("a = %d, b = %d\n", a, b);
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
int getDouble(double** pp, unsigned n)
{
    int ret = 0;
    double* pd = malloc(sizeof(double) * n);
    if( pd != NULL )
    {
        printf("pd = %p\n", pd);
        *pp = pd;
        ret = 1;
    }
    return ret;
}
int main()
{
    double* p = NULL;
    if( getDouble(&p, 5) )
    {
        printf("p = %p\n", p);
        free(p);
    }
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
int main()
{
    int b[2][2] = {{1, 2}, {3, 4}};
    int (*pnb) [2] = b;  // b 的類型是 int(*)[2]
    *pnb[1] = 30;
    printf("b[0][0] = %d\n", b[0][0]);
    printf("b[0][1] = %d\n", b[0][1]);
    printf("b[1][0] = %d\n", b[1][0]);
    printf("b[1][1] = %d\n", b[1][1]);
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
int* func()
{
    int var = 100;
    return &var;
}
int main()
{
    int* p = func();  // OOPS!!!!
                      // p 指向了不合法的地址，這個(gè)地址處沒有變量存在
                      // p 是一個(gè)野指針，保存不合法地址的指針都是野指針
    printf("*p = %d\n", *p);
    *p = 200;   // 改變 func 函數(shù)中局部變量 var 的值，是不是非常奇怪？？？
    printf("*p = %d\n", *p);
    return 0;
}