C指针原理教程之垃圾回收-内存泄露
时间：2021-04-26 11:05:34|栏目：C代码|点击： 次
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 %
