关于C语言指针的用法

近来有些时间，在开发过程中调试bug的时候，经常会遇到指针乱用或者，用了不free的现象，我这里写了个例子，把我的个人看法给有同样疑惑的朋友共勉，如果有错的地方，请指正，以一个人person为对象，展开进行的一些描述，需要仔细体会多级指针怎么分配内存的，可能会发现指针的使用还是比较有意思的，如果有什么错误请指出，经过测试，运行能通过，例子如下（如果你看过linux的内核代码，那么你对结构体的指针用法一定不会陌生，这里用的结构体指针有些面向对象的思想）：

/*
* This instance is doing a pointer usage
* I list a person detail to explain the usage.
*/
#include <stdlib.h>
#include <stdio.h>
#define NUMBER_GIRLFIREDN 1
#define ANNUAL_INCOMING 10
#define LENGHT 171
struct basic_info
{
char *name;
int lenght;
char *birthday;
char *qq;
char *address;
char *major;
char *married;
};
struct company
{
char *company_name;
int fundation_time;
char *product;
};
struct work_exp
{
struct company *com;
int num_company;
};
struct university
{
char *university_name;
int history_time;
int num_std;
};
struct study_exp
{
struct university *unt;
int num_university;
};
struct person
{
struct basic_info *bs_info;
struct work_exp *wk_info;
struct study_exp *std_info;
int num_gf;
int incoming;
};//注意看定义结构体指针时候的层次关系
static void print_personal_inforamtion(struct person *p)
{
printf("The detail inforamtin as following:\n\
Name:     %s\n\
Lenght:     %d\n\
Birthday:     %s\n\
QQ:     %s\n\
Address:     %s\n\
Major:     %s\n\
Married:     %s\n\
Girl friend(inclued ex):     %d\n\
Annaul incoming(so poor):     %d\n\
Work company toal number:     %d\n\
The fist company name:     %s\n\
The company fundation time:     %d\n\
The company's product:     %s\n\
The university number:     %d\n\
The university name:     %s\n\
The university history:     %d\n\
The university student number:     %d\n",
p->bs_info->name,
p->bs_info->lenght,
p->bs_info->birthday,
p->bs_info->qq,
p->bs_info->address,
p->bs_info->major,
p->bs_info->married,
p->num_gf,
p->incoming,
p->wk_info->num_company,
p->wk_info->com->company_name,
p->wk_info->com->fundation_time,
p->wk_info->com->product,
p->std_info->num_university,
p->std_info->unt->university_name,
p->std_info->unt->history_time,
p->std_info->unt->num_std);
printf("Upper is my information, if you are have some problem to discuss, please add my QQ.\n");
}
static int person_information_free(struct person *p)//注意看free时候的顺序关系
{
if(p != NULL) {
if(p->bs_info != NULL) {
free(p->bs_info->name);
p->bs_info->name = NULL;
free(p->bs_info->birthday);
p->bs_info->birthday = NULL;
free(p->bs_info->qq);
p->bs_info->qq = NULL;
free(p->bs_info->address);
p->bs_info->address = NULL;
free(p->bs_info->married);
p->bs_info->married = NULL;
free(p->bs_info);
p->bs_info = NULL;
}
if(p->wk_info != NULL) {
if(p->wk_info->com != NULL) {
free(p->wk_info->com->company_name);
p->wk_info->com->company_name = NULL;
free(p->wk_info->com->product);
p->wk_info->com->product = NULL;
free(p->wk_info->com);
p->wk_info->com = NULL;
}
free(p->wk_info);
p->wk_info = NULL;
}
if(p->std_info != NULL) {
if(p->std_info->unt != NULL) {
free(p->std_info->unt->university_name);
p->std_info->unt->university_name = NULL;
free(p->std_info->unt);
p->std_info->unt = NULL;
}
free(p->std_info);
p->std_info = NULL;
}
free(p);
p = NULL;
} else {
printf("The student information have freed\n");
}
return 0;
}
/*Starting.........*/
int main(int argc, char *argv[])
{
struct person *fuxin;
fuxin = malloc(sizeof(struct person));// 这之后会用malloc分配很多内存，注意看层次关系
if (!fuxin) {
printf("Malloc the data for fuxin fail.\n");
goto malloc_fail;
}
fuxin->num_gf = NUMBER_GIRLFIREDN;
fuxin->incoming = ANNUAL_INCOMING;
fuxin->bs_info = malloc(sizeof(struct basic_info));
if (!fuxin->bs_info) {
printf("Malloc the basic infomation for fuxin fail.\n");
goto malloc_fail;
}
fuxin->bs_info->lenght = LENGHT;
fuxin->bs_info->name = malloc(sizeof(char) * 10);
if (!fuxin->bs_info->name) {
printf("Malloc the name for fuxin fail.\n");
goto malloc_fail;
}
strcpy(fuxin->bs_info->name, "Who am I");

fuxin->bs_info->birthday = malloc(sizeof(char) * 10);
if (!fuxin->bs_info->birthday) {
printf("Malloc the birthday information for fuxin fail.\n");
goto malloc_fail;
}
strcpy(fuxin->bs_info->birthday, "1983-8-8");
fuxin->bs_info->qq = malloc(sizeof(char) * 13);
if (!fuxin->bs_info->birthday) {
printf("Malloc the qq information for fuxin fail.\n");
goto malloc_fail;
}
strcpy(fuxin->bs_info->qq, "1320332906");
fuxin->bs_info->address = malloc(sizeof(char) * 100);
if (!fuxin->bs_info->address) {
printf("Malloc the address information for fuxin fail.\n");
goto malloc_fail;
}
strcpy(fuxin->bs_info->address, "Wangjing east road, chao yang, BeiJing");
fuxin->bs_info->major = malloc(sizeof(char) * 10);
if (!fuxin->bs_info->major) {
printf("Malloc the birthday information for fuxin fail.\n");
goto malloc_fail;
}
strcpy(fuxin->bs_info->major, "Computer");
fuxin->bs_info->married = malloc(sizeof(char) * 10);
if (!fuxin->bs_info->married) {
printf("Malloc the married information for fuxin fail.\n");
goto malloc_fail;
}
strcpy(fuxin->bs_info->married, "No married");
fuxin->wk_info = malloc(sizeof(struct work_exp));
if (!fuxin->wk_info) {
printf("Malloc the work information for fuxin fail.\n");
goto malloc_fail;
}
fuxin->wk_info->num_company = 1;
fuxin->wk_info->com = malloc(sizeof(struct company));
if (!fuxin->wk_info->com) {
printf("Malloc the work company information for fuxin fail.\n");
goto malloc_fail;
}
fuxin->wk_info->com->fundation_time = 25;
fuxin->wk_info->com->company_name = malloc(sizeof(char) * 20);
if (!fuxin->wk_info->com->company_name) {
printf("Malloc the work company name information for fuxin fail.\n");
goto malloc_fail;
}
strcpy(fuxin->wk_info->com->company_name, "Motorola INC.");
fuxin->wk_info->com->product = malloc(sizeof(char) * 40);
if (!fuxin->wk_info->com->product) {
printf("Malloc the work company product information for fuxin fail.\n");
goto malloc_fail;
}
strcpy(fuxin->wk_info->com->product, "Mobile phone XT910, MT917");
fuxin->std_info = malloc(sizeof(struct study_exp));
if (!fuxin->std_info) {
printf("Malloc the study exp information for fuxin fail.\n");
goto malloc_fail;
}
fuxin->std_info->num_university = 1;
fuxin->std_info->unt = malloc(sizeof(struct university));
if (!fuxin->std_info->unt) {
printf("Malloc the study university information for fuxin fail.\n");
goto malloc_fail;
}
fuxin->std_info->unt->history_time = 100;
fuxin->std_info->unt->num_std = 10000;
fuxin->std_info->unt->university_name = malloc(sizeof(char) * 20);
if (!fuxin->std_info->unt->university_name) {
printf("Malloc the study university name information for fuxin fail.\n");
goto malloc_fail;
}
strcpy(fuxin->std_info->unt->university_name, "My UNIVERSITY");
malloc_success:
printf("setup personal information success, as following......\n");
printf("=========================================================================================\n");
print_personal_inforamtion(fuxin);
printf("=========================================================================================\n");
malloc_fail:
person_information_free(fuxin); //不用内存的时候，一定要记得free
return 0;
}

run the program as below:

gcc pointer.c

./a.out

运行结果：

=========================================================================================

The detail inforamtin as following:

Name: Who am I

Lenght: 171

Birthday: 1983-8-8

QQ: 1320332906

Address: Wangjing east road, chao yang, BeiJing

Major: Computer

Married: No married

Girl friend(inclued ex): 1

Annaul incoming(so poor): 10

Work company toal number: 1

The fist company name: Motorola INC.

The company fundation time: 25

The company's product: Mobile phone XT910, MT917

The university number: 1

The university name: My UNIVERSITY

The university history: 100

The university student number: 10000

Upper is my information, if you are have some problem to discuss, please add my QQ

=========================================================================================

通过上面这个简单的例子，可以看到其实指针使用不算复杂， 一个原则是不用的时候就free，对于结构体嵌套的情况，要先free内层结构的指针，再free外层结构的指针，虽然例子简单，但是对于看内核代码还是有好处的，如果有条件的话，最好连接上trace32跟踪一下内存，那样理解将会更加深刻。