C指针原理教程之垃圾回收-内存泄露

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一、内存泄露

1、正常的链表操作

下面程序建立一个10元素的链表，输出它们的节点，每个节点是一个员工的工号和年龄。最后删除每个节点，释放列表。

dp@dp:~/memorytest % cat 1.c

#include <stdlib.h>
#include <stdio.h>
//code:myhaspl@myhaspl.com
//author:myhaspl
//date:2014-01-10
typedef struct listnode mynode; 
struct listnode{
  mynode *next;
  int number;
  int age;
  };
mynode *addnode(mynode *prevnd,int number,int age){
  mynode *ndtemp=(mynode*)malloc(sizeof(mynode));
  prevnd->next=ndtemp;
  ndtemp->number=number;
  ndtemp->age=age;
  ndtemp->next=NULL;
  return ndtemp;
}
mynode *initlist(){
  mynode *temp=(mynode*)malloc(sizeof(mynode));  
  temp->number=0;
  temp->age=0;
  temp->next=NULL;
  return temp;
}
int main(){
  mynode *mylist=initlist();
  mynode *mytempnd=mylist;
  int i=0;f悬挂指针
  for(i=0;i<10;i++){
    mytempnd=addnode(mytempnd,i,20+i);
  }
  //下面是正常的链表操作
  //先输出链表元素
  for (mytempnd=mylist->next;mytempnd!=NULL;mytempnd=mytempnd->next){
    printf("id:%d,age:%d\n",mytempnd->number,mytempnd->age);
  }
  //然后删除链表中的所有元素
  mynode* oldtmpnd;
  for (mytempnd=mylist->next;mytempnd!=NULL;){
    printf("delete id:%d\n",mytempnd->number);
    oldtmpnd=mytempnd;
    mytempnd=mytempnd->next;
    free(oldtmpnd);
  }
  free(mylist);
    return 0;  
}

下面是程序运行效果

dp@dp:~/memorytest % gcc 1.c -o mytest

dp@dp:~/memorytest % ./mytest

id:0,age:20

id:1,age:21

id:2,age:22

id:3,age:23

id:4,age:24

id:5,age:25

id:6,age:26

id:7,age:27

id:8,age:28

id:9,age:29

delete id:0

delete id:1

delete id:2

delete id:3

delete id:4

delete id:5

delete id:6

delete id:7

delete id:8

delete id:9

dp@dp:~/memorytest %

下面演示了垃圾的形成，这是内存泄露的一种方式，即在链表中，某些节点与链表中的其它节点失去联系，导致无法删除，下面故意让第4个结点的next指针指向null，失去与后面6个元素的联系。

dp@dp:~/memorytest % cat 1.c


#include <stdlib.h>

#include <stdio.h>

//code:myhaspl@myhaspl.com

//author:myhaspl

//date:2014-01-10

typedef struct listnode mynode; 

struct listnode{

mynode *next;

int number;

int age;

};

mynode *addnode(mynode *prevnd,int number,int age){

mynode *ndtemp=(mynode*)malloc(sizeof(mynode));

prevnd->next=ndtemp;

ndtemp->number=number;

ndtemp->age=age;

ndtemp->next=NULL;

return ndtemp;

}

mynode *initlist(){

mynode *temp=(mynode*)malloc(sizeof(mynode));

temp->number=0;

temp->age=0;

temp->next=NULL;

return temp;

}

int main(){

mynode *mylist=initlist();

mynode *mytempnd=mylist;

int i=0;

for(i=0;i<10;i++){

mytempnd=addnode(mytempnd,i,20+i);

}

//下面是正常的链表操作

//先输出链表元素

for (mytempnd=mylist->next;mytempnd!=NULL;mytempnd=mytempnd->next){

printf("id:%d,age:%d\n",mytempnd->number,mytempnd->age);

}

//然后删除链表中的所有元素

for (mytempnd=mylist->next;mytempnd!=NULL;mytempnd=mytempnd->next){

printf("delete id:%d\n",mytempnd->number);

free(mytempnd);

}

free(mylist);

//下面是形成内存泄露第一种情况-垃圾的演示

//生成并输出链表，这个与前面相同

mylist=initlist();

mytempnd=mylist;

i=0;

for(i=0;i<10;i++){

mytempnd=addnode(mytempnd,i,20+i);

}

for (mytempnd=mylist->next;mytempnd!=NULL;mytempnd=mytempnd->next){

printf("id:%d,age:%d\n",mytempnd->number,mytempnd->age);

}

//删除链表，我们故意留下后面6个链表节点无法删除，导致后面6个链表节点形成垃圾

int j=0;

for (mytempnd=mylist->next;mytempnd!=NULL;mytempnd=mytempnd->next){

if (++j>3){

mytempnd->next=NULL;

break;

}

}

for (mytempnd=mylist->next;mytempnd!=NULL;mytempnd=mytempnd->next){

printf("delete id:%d\n",mytempnd->number);

free(mytempnd);

j++; 

}

    return 0;

}

下面是程序运行效果

dp@dp:~/memorytest % gcc 1.c -o mytest

dp@dp:~/memorytest % ./mytest

id:0,age:20

id:1,age:21

id:2,age:22

id:3,age:23

id:4,age:24

id:5,age:25

id:6,age:26

id:7,age:27

id:8,age:28

id:9,age:29

delete id:0

delete id:1

delete id:2

delete id:3

delete id:4

delete id:5

delete id:6

delete id:7

delete id:8

delete id:9

id:0,age:20

id:1,age:21

id:2,age:22

id:3,age:23

id:4,age:24

id:5,age:25

id:6,age:26

id:7,age:27

id:8,age:28

id:9,age:29

delete id:0

delete id:1

delete id:2

delete id:3

dp@dp:~/memorytest %

3、悬挂指针

一个指针不为空，但是指向一个无效的地址或耒知对象的地址，则这样的指针称为悬挂指针。

dp@dp:~/memorytest % cat 2.c

#include <stdio.h>

#include <stdlib.h>

//code:myhaspl@myhaspl.com

//author:myhaspl

//date:2014-01-10

typedef struct listnode mynode;

struct listnode{

mynode *next;

int number;

int age;

};

mynode *addnode(mynode *prevnd,int number,int age){

mynode *ndtemp=(mynode*)malloc(sizeof(mynode));

prevnd->next=ndtemp;

ndtemp->number=number;

ndtemp->age=age;

ndtemp->next=NULL;

return ndtemp;

}

mynode *initlist(){

mynode *temp=(mynode*)malloc(sizeof(mynode));

temp->number=0;

temp->age=0;

temp->next=NULL;

return temp;

}

int main(){

mynode *mylist=initlist();

mynode *mytempnd=mylist;

int i=0;

for(i=0;i<10;i++){

mytempnd=addnode(mytempnd,i,20+i);

}

//下面是正常的链表操作

//先输出链表元素

for (mytempnd=mylist->next;mytempnd!=NULL;mytempnd=mytempnd->next){

printf("id:%d,age:%d\n",mytempnd->number,mytempnd->age);

}

//然后删除链表中的所有元素

mynode* oldtmpnd;

for (mytempnd=mylist->next;mytempnd!=NULL;){

printf("delete id:%d\n",mytempnd->number);

oldtmpnd=mytempnd;

mytempnd=mytempnd->next;

free(oldtmpnd);

}

free(mylist);

//下面是形成内存泄露第二种情况-悬挂指针的演示

//生成并输出链表，这个与前面相同

mylist=initlist();

mytempnd=mylist;

i=0;

for(i=0;i<10;i++){

mytempnd=addnode(mytempnd,i,20+i);

}

for (mytempnd=mylist->next;mytempnd!=NULL;mytempnd=mytempnd->next){

printf("id:%d,age:%d\n",mytempnd->number,mytempnd->age);

}

//我们故意删除链表后面的4个节点，但是让第6个元素的next指向的地址无效，

//仍指向已经删除的第7个节点，导致悬挂指针

printf ("-------------------------\n");

int j=0;

for (mytempnd=mylist->next;mytempnd!=NULL;){

oldtmpnd=mytempnd;

mytempnd=mytempnd->next;

if (++j>6){

printf("delete id:%d\n",oldtmpnd->number);

free(oldtmpnd);

}

}

    return 0;

}

执行程序

dp@dp:~/memorytest % gcc 2.c -o mytest

dp@dp:~/memorytest % ./mytest

id:0,age:20

id:1,age:21

id:2,age:22

id:3,age:23

id:4,age:24

id:5,age:25

id:6,age:26

id:7,age:27

id:8,age:28

id:9,age:29

delete id:0

delete id:1

delete id:2

delete id:3

delete id:4

delete id:5

delete id:6

delete id:7

delete id:8

delete id:9

id:0,age:20

id:1,age:21

id:2,age:22

id:3,age:23

id:4,age:24

id:5,age:25

id:6,age:26

id:7,age:27

id:8,age:28

id:9,age:29

delete id:6

delete id:7

delete id:8

delete id:9

但是注意free函数表示释放，这个释放指的是把这段内存标记成可用状态，或者说，没有人在用这段内存了，也就是意味着如果这段内存如果没有被操作系统重新使用，里面的数据还存在，如果被操作系统分配给其它程序或本程序的其它内存块申请之用，则数据会被清空。

3、下面是形成内存泄露第三种情况-共享的演示，多个指针指向同一个内存，这个内存因为某个指针不再使用的原因删除，导致其它指针指向一个无效地址

dp@dp:~/memorytest % cat 2.c


#include <stdio.h>

#include <stdlib.h>

//code:myhaspl@myhaspl.com

//author:myhaspl

//date:2014-01-10

typedef struct listnode mynode;

struct listnode{

mynode *next;

char *data;

int number;

int age;

};

mynode *addnode(mynode *prevnd,int number,int age,char *data){

mynode *ndtemp=(mynode*)malloc(sizeof(mynode));

prevnd->next=ndtemp;

ndtemp->number=number;

ndtemp->age=age;

ndtemp->data=data;

ndtemp->next=NULL;

return ndtemp;

}

mynode *initlist(){

mynode *temp=(mynode*)malloc(sizeof(mynode));

temp->number=0;

temp->age=0;

temp->data=NULL;

temp->next=NULL;

return temp;

}

int main(){

    //下面是形成内存泄露第三种情况-共享的演示，多个指针指向同一个内存，这个内存因为某个指针不再使用的原因删除，

//生成并输出链表，生成1个链表(共3个元素)，元素的data都指向同一个内存块

mynode *mylist=initlist();

mynode *mytempnd=mylist;

char *mydata=(char *)malloc(100);

const char *strsrc="helloworld";

strcpy(mydata,strsrc);

int i=0;

for(i=0;i<3;i++){

    mytempnd=addnode(mytempnd,i,20+i,mydata);

}

for (mytempnd=mylist->next;mytempnd!=NULL;mytempnd=mytempnd->next){

printf("id:%d,age:%d,data:%s\n",mytempnd->number,mytempnd->age,mytempnd->data);

    }

//下面将导致共享的内存释放，但仍有2个结点指向这个内存，这将导致内存泄露

//我们故意删除最后一个节点，并释放最后一个结点的data指针指向的内存

printf ("-------------------------\n");

mynode *oldtmpnd;

for (mytempnd=mylist->next;mytempnd!=NULL;){

oldtmpnd=mytempnd;

mytempnd=mytempnd->next;

if (mytempnd==NULL){

printf("delete id:%d\n",oldtmpnd->number);

free(oldtmpnd->data);

free(oldtmpnd);

}

}

    return 0;

}

执行程序：

dp@dp:~/memorytest % gcc 2.c -o mytest

2.c: In function 'main':

2.c:37: warning: incompatible implicit declaration of built-in function 'strcpy'

dp@dp:~/memorytest % ./mytest

id:0,age:20,data:helloworld

id:1,age:21,data:helloworld

id:2,age:22,data:helloworld

delete id:2

dp@dp:~/memorytest %

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