C语言-指针二三事

有同事今天问我一个指针的问题，没深究原因，下班回来静下心来分析分析，写下这篇文章，如有任何疑问请留言.

问题一

要实现一个函数，我们姑且理解成函数内部 malloc申请一块内存，然后针对这块内存做一系列操作，操作这部分姑且不用实现，我们要返回申请的这片内存的地址.

version 1

如果你拿到这个问题就立刻编写代码的话极有可能这么写:

/*    Name: 二级指针    Copyright: 52coder.net    Author: 52coder    Date: 03/06/17 17:24    Description: pointer */#include <stdio.h>#include <stdlib.h>/*获取malloc内存的地址*/int getaddress(int *address){    int size = 64;    address = malloc(size *sizeof(int));    printf("Address of malloc memory is:%p\n",address);    return 0;}int main(){    int *pointer = NULL;    getaddress(pointer);    printf("Address of pointer is:%p\n",pointer);    return 0;}

那么结果可能比较让人失望：

Address of malloc memory is:0x100500270
Address of pointer is:0x0
Program ended with exit code: 0

pointer 的值居然是0x0，换言之我们没有将所申请内存的地址带出来.

version 2

/*    Name: 二级指针    Copyright: 52coder.net    Author: 52coder    Date: 03/06/17 17:24    Description: pointer */#include <stdio.h>#include <stdlib.h>/*获取malloc内存的地址*/int getaddress(int **address){    int * tmp = NULL;    int size = 64;    tmp = malloc(size *sizeof(int));    *address = tmp;    printf("Address of malloc memory is:%p\n",tmp);    return 0;}int main(){    int *pointer = NULL;    getaddress(&pointer);    printf("Address of pointer is:%p\n",pointer);    return 0;}

输出结果终于符合我们预期：
Address of malloc memory is:0x100202350
Address of pointer is:0x100202350
Program ended with exit code: 0

pointer的值就是我们在函数中所申请的内存地址.
那么写道这里你是否会有疑问，函数getaddress中是否真正需要tmp中间变量呢？

version 3

/*    Name: 二级指针    Copyright: 52coder.net    Author: 52coder    Date: 03/06/17 17:24    Description: pointer */#include <stdio.h>#include <stdlib.h>/*获取malloc内存的地址*/int getaddress(int **address){    int size = 64;    *address = malloc(size *sizeof(int));    printf("Address of malloc memory is:%p\n",address);    return 0;}int main(){    int *pointer = NULL;    getaddress(&pointer);    printf("Address of pointer is:%p\n",pointer);    return 0;}

运行结果居然是：
Address of malloc memory is:0x7fff5fbff7b0
Address of pointer is:0x100300000
Program ended with exit code: 0
那么为什么会是这样呢？
问题原因很简单，就是在函数getaddress中我们打印*address才是申请内存的地址.

问题二

这个同样是一道考验指针的问题，大致要实现如下内容，结构体中包含二级指针，通过二级指针 一级指针都可以访问新申请的内存.

/*    Name: 二级指针    Copyright: 52coder.net    Author: 52coder    Date: 03/06/17 17:24    Description: pointer */#include <stdio.h>#include <stdlib.h>typedef struct student{    int age;    int height;}stu;typedef struct school{    int num;    stu **boys;}grade;int main(){    int i = 0;    stu *tmp = NULL;    grade oneclass;    //定义班级男生个数    oneclass.num = 55;    grade *one = &oneclass;    //申请内存    tmp = malloc(oneclass.num * sizeof(stu));    one->boys = &tmp;    //打印三种操作方法对应的地址    for (i = 0; i < 55; i++)    {        printf("tmp + i =%p &tmp[i] = %p *one->boys+i =%p\n",                tmp+i, &tmp[i], *(one->boys)+i);    }    //设置55个元素的age    for (i=0; i<55; i++)    {        tmp[i].age = i;    }    //设置55个元素的height    for (i=0; i<55; i++)    {        tmp[i].height = 55-i;    }    for (i=0; i<55; i++)    {        printf("%d ", (*(one->boys))[i].age);    }    for (i=0; i<55; i++)    {        printf("%d ", (*one->boys)[i].height);    }    return 0;}

下面代码是一段验证程序

/*    Name: 二级指针    Copyright: 52coder.net    Author: 52coder    Date: 03/06/17 17:24    Description: pointer*/#include <stdio.h>#include <stdlib.h>typedef struct student{    int age;    int height;}stu;typedef struct school{    int num;    stu **boys;}grade;int main(){    int i = 0;    stu *tmp = NULL;    grade oneclass;    //定义班级男生个数    oneclass.num = 55;    grade *one = &oneclass;    //申请内存    tmp = malloc(oneclass.num * sizeof(stu));    one->boys = &tmp;    printf("\n");    //验证地址与值的关系    printf("&tmp = %p,one->boys = %p\n",&tmp,one->boys);    printf("tmp = %p,*(one->boys) = %p\n",tmp,*(one->boys));    printf("tmp + 10 = %p,*(one->boys) + 10 = %p\n",tmp+10,*(one->boys)+10);    return 0;}