C语言：结构体中一级指针和二级指针的创建与释放示例

这几天把C语言巩固了一下，作为一门最基本的编程语言，C语言还是相当基础和非常重要的，个人认为C语言还是很有必要学好吃透的。

今天写的话题是结构体结构体中一级指针和二级指针的创建与释放，以一个例子来说明，笔者这里自己敲得代码，虽然用C语言几年了，但是有的东西还是会遗忘。笔者敲代码过程中也是有bug出现的，经过调试也找到了bug所在，不得不说，巩固也是很重要的。而结构体作为C语言一部分也是很重要的，指针作为C语言的半壁江山也保持着其重要地位。学过C++或Java的都知道class（类），个人觉得结构体就像是class的一部分，不过里面不能放实现class的相关函数。代码不是很长，但里面有很多细节的考虑，特别是一级指针和二级指针的内存的申请和释放。

代码的主要表达是：有三个老师，每个老师都有属性名字,年龄，别名，编号，以及有三个学生名字。老师名字（name）的内存分配是在栈区，别名（alisname）和学生名（stuname）的内存分配是在堆区。个人认为C语言的内存四区（栈区，堆区，全局区，代码区）很重要，所以喜欢从内存四区的角度看代码，对理解C语言的本质是重要，也是理解bug所在的重要原因。

下面是示例代码：

// 结构体 示例#include "stdio.h"#include "stdlib.h"#include "string.h"typedef struct Teacher // 定义一个结构体，名称为Teacher{char name[64];int age;char *alisname;//别名char **stuname;int id;}Teacher;int createTeacher(Teacher **pT, int num){int i = 0;int j = 0;int ret = 0;Teacher *tmp = NULL;tmp = (Teacher *)malloc(num * sizeof(Teacher));if (tmp == NULL){ret = -1;printf("malloc err: %d\n",ret);return ret;}memset(tmp, 0, num * sizeof(Teacher));for (i = 0; i < num; i++){char **myp = NULL;//先创建一级指针tmp[i].alisname = (char *)malloc(60);if ( tmp[i].alisname == NULL){ret = -2;printf("malloc tmp[i].alisname err: %d\n",ret);return ret;}//再创建二级指针myp = (char **)malloc(3 * sizeof(char *));if (myp == NULL){ret = -3;return ret;}for (j = 0; j < 3; j++){myp[j] = (char *)malloc(60);if (myp[j] == NULL){ret = -4;return ret;}}tmp[i].stuname = myp;}*pT = tmp;return ret;}int printTeacherandStu(Teacher *p, int num){int i = 0;int j = 0;int ret = 0;Teacher *tmp = NULL;if (p == NULL){ret = -1;printf("p is null : %d\n",ret);return ret;}tmp = p;for (i = 0; i < num; i++){char **myp = NULL;printf("teacher name: %s\n",tmp[i].name);printf(" teacher alisname: %s\n",tmp[i].alisname);printf("  student's name:  ");myp = tmp[i].stuname;for (j = 0; j < 3; j++){printf("%s  ",myp[j]);}printf("\n");}return ret;}void FreeTeacher(Teacher *p, int num){int i = 0;int j = 0;Teacher *tmp = NULL;tmp = p;for(i = 0; i < num; i++){char **myp = NULL;//先释放以及指针if (tmp[i].alisname != NULL){free(tmp[i].alisname);}//再释放二级指针if (tmp[i].stuname != NULL){myp = tmp[i].stuname;for (j = 0; j < 3; j++){if (myp[j] != NULL){free(myp[j]);}}free(myp);tmp[i].stuname = NULL;}}free(tmp);}int main(){int ret = 0;int i = 0;int j = 0;int num = 3;Teacher *pArray = NULL;ret = createTeacher(&pArray, num);if (ret != 0){printf("func createTeacher() err: %d\n",ret);return ret;}for (i = 0; i < num; i++){printf("\nplease input teacher's name:");scanf("%s",pArray[i].name);printf("\n please input teacher's age:");scanf("%d",&(pArray[i].age));printf("\n please input teacher's alsname:");scanf("%s",pArray[i].alisname);for (j = 0; j < 3; j++){printf("\n  please input student name:");scanf("%s",pArray[i].stuname[j]);}}ret = printTeacherandStu(pArray, num);if (ret != 0){ret = -2;printf("func printTeacherandStu() err: %d\n",ret);return ret;}FreeTeacher(pArray, num);return 0;}

具体详细细节就不讨论了，不懂的可以问，下面是运行结果：