关于野指针的一些问题与总结

待我称王封你为后i 2022-08-10 03:56 248阅读 0赞

void  Test( void )  
\{  
 char  \* str  =  ( char  \* ) malloc( 100 );  
strcpy(str, “hello”);  
free(str);  
 if (str  !=  NULL)  
\{  
strcpy(str, “world”);  
printf(str);  
\}

![复制代码][copycode.gif]

请问运行Test函数会有什么样的结果？  
答：篡改动态内存区的内容，后果难以预料，非常危险。因为free(str);之后，str成为野指针，if(str != NULL)语句不起作用。

1.何为野指针？

野指针指指向一个已删除的对象或未申请访问受限内存区域的[指针][Link 1]。与空指针不同，野指针无法通过简单地判断是否为[NULL][]避免，而只能通过养成良好的编程习惯来尽力减少。对野指针进行操作很容易造成程序错误。

解释一下：

malloc 和 free 是在系统的堆上分配空间。
    malloc是申请，意思就是告诉系统，我要用一块RAM，给我用了别人就不要用了。
    free是释放，意思是告诉系统，给我的这块RAM我用完了，不再用了，系统可以把它干别的了。free()释放的是指针指向的内存。并不是说，free()解除了原来指向这块内存的指针与这块内存地址之间的联系，联系依然存在，这就可能导致误用出错，所以需要在free之后置NULL，这样就解除关联了。举个例子就是，A跟妻子离婚了，但A和她曾经是夫妻，曾经关系很亲密，这种关系不会随着离婚而改变，这是客观的，离婚了即free,A不能再去骚扰前妻，否则就是违法，A要是再想干那啥，A会坐牢的！！！

free之后，系统还没有拿这块RAM干别的事之前，这块RAM的内容可能是不会变的，依然可以读出原来的内容，因为你的指针a还是指向这块RAM。但要注意，这块RAM已经不属于你了，读一下内容无所谓，如果往里面写就很危险了。

## **2.成因** ##

### 通俗说法： ###

###  ###

如果程序定义了一个指针，就必须要立即让它指向一个我们设定的空间或者把它设为NULL，如果没有这么做，那么这个指针里的内容是不可预知的，即不知道它 指向内存中的哪个空间（即野指针），它有可能指向的是一个空白的内存区域，可能指向的是已经受保护的区域，甚至可能指向系统的关键内存，如果是那样就糟 了，也许我们后面不小心对指针进行操作就有可能让系统出现紊乱，死机了。所以我们必须设定一个空间让指针指向它，或者把指针设为NULL，这是怎么样的一 个原理呢，如果是建立一个与指针相同类型的空间，实际上是在内存中的空白区域中开辟了这么一个受保护的内存空间，然后用指针来指向它，那么指针里的地址就 是这个受保护空间的地址了，而不是不可预知的啦，然后我们就可以通过指针对这个空间进行相应的操作了；如果我们把指针设为NULL，我们在头文件定义中的 \#define NULL 0 可以知道，其实NULL就是表示0，那么我们让指针＝NULL，实际上就是让指针＝0，如此，指针里的地址（机器数）就被初始化为0了，而内存中地址为0 的内存空间……不用多说也能想象吧，这个地址是特定的，那么也就不是不可预知的在内存中乱指一气的野指针了。

还应该注意的是，free和delete只是把指针所指的 内存给释放掉，但并没有把指针本身干掉。指针p被free以后其地址仍然不变（非NULL），只是该地址对应的内存是垃圾，p成了“野指针”。如果此时不 把p设置为NULL，会让人误以为p是个合法的指针。用free或delete释放了内存之后，就应立即将指针设置为NULL，防止产生“野指针”。内存 被释放了，并不表示指针会消亡或者成了NULL指针。（而且，指针消亡了，也并不表示它所指的内存会被自动释放。）

### 指针变量未初始化 ###

##  ##

任何 [指针变量][Link 2]刚被创建时不会自动成为NULL指针，它的缺省值是随机的，它会乱指一气。所以，指针变量在创建的同时应当被初始化，要么将指针设置为NULL，要么让它指向合法的内存。

### 指针释放后之后未置空 ###

##  ##

有时 [指针][Link 1]在free或delete后未赋值 NULL，便会使人以为是合法的。别看free和delete的名字（尤其是delete），它们只是把指针所指的内存给释放掉，但并没有把指针本身干掉。此时指针指向的就是“垃圾”内存。释放后的指针应立即将指针置为NULL，防止产生“野指针”。

### 指针操作超越变量作用域 ###

##  ##

不要返回指向栈内存的指针或引用，因为栈内存在函数结束时会被释放。示例程序如下：

<table style="background:none;margin:0px;padding:0px;border:0px;width:790px;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;border-collapse:collapse;border-spacing:0px;"> 
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    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
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      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">A&nbsp;*p;</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">&nbsp;&nbsp;</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,102,153);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:700;vertical-align:baseline;float:none;">void</code>&nbsp; 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">Test(</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,102,153);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:700;vertical-align:baseline;float:none;">void</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">)&nbsp;{ </code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">A&nbsp;a;</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">p&nbsp;=&nbsp;&amp;a;&nbsp;</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,130,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">//&nbsp;注意&nbsp;a&nbsp;的生命期&nbsp;，只在这个函数Test中，而不是整个class&nbsp;B</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">&nbsp;&nbsp;</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">}</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">&nbsp;&nbsp;</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,102,153);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:700;vertical-align:baseline;float:none;">void</code>&nbsp; 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">Test1()&nbsp;{ </code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">&nbsp;&nbsp;</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">p-&gt;Func();&nbsp;</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,130,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">//&nbsp;p&nbsp;是“野指针”</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">&nbsp;&nbsp;</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">}</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">};</code> 
     </div> 
    </div></td> 
  </tr> 
 </tbody> 
</table>

函数 Test1 在执行语句 p->Func()时，p 的值还是 a 的地址，对象 a 的内容已经被清除，所以 p 就成了“野指针” 。

### 3.规避 ###

在养成这些习惯的情况下，野指针的危害是可以降低的：

[http://blog.csdn.net/netanimals/article/details/6530787][http_blog.csdn.net_netanimals_article_details_6530787]

如：int\* pInteger=NULL;//指针定义处

...

if(pInteger != NULL)

free(pInteger);

pInteger=NULL;//指针释放之后并不为空，要设置其为空

pInteger=(int\*)malloc(10\*sizeof(int));

if(pInteger != NULL)

printf("内存申请成功/n！");

exit(0);

...

### 初始化时置 NULL ###

指针变量一定要 [初始化][Link 3]为NULL，因为任何指针变量刚被创建时不会自动成为NULL指针，它的 [缺省值][Link 4]是随机的。

### 释放时置 NULL ###

当指针p指向的内存空间释放时，没有设置指针p的值为NULL。delete和free只是把内存空间释放了，但是并没有将指针p的值赋为NULL。通常判断一个指针是否合法，都是使用if语句测试该指针是否为NULL。例如：

对于使用 free 的情况，常常定义一个宏或者函数 xfree 来代替 free 置空指针：

<table style="background:none;margin:0px;padding:0px;border:0px;width:790px;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;border-collapse:collapse;border-spacing:0px;"> 
 <tbody style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;"> 
  <tr style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;"> 
   <td style="background:none;margin:0px;padding:2px 10px;border:1px solid rgb(230,230,230);width:auto;height:22px;text-align:left;color:rgb(175,175,175);line-height:22px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:12px;font-style:normal;font-weight:400;vertical-align:baseline;float:none;"> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      1 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      2 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      3 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      4 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      5 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      6 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      7 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      8 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      9 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      10 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      11 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      12 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      13 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      14 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      15 
    </div> 
    <div style="background:rgb(255,255,255);border-width:0px 3px 0px 0px;margin:0px;padding:0px .5em 0px 1em;width:auto;height:auto;text-align:right;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;border-right-color:rgb(108,226,108);border-right-style:solid;float:none;white-space:pre;">
      16 
    </div></td> 
   <td style="background:none;margin:0px;padding:2px 10px;border:1px solid rgb(230,230,230);width:755px;height:22px;text-align:left;line-height:22px;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:12px;font-style:normal;font-weight:400;vertical-align:baseline;float:none;"> 
    <div style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;"> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:#808080;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">#define&nbsp;xfree(x)&nbsp;free(x);&nbsp;x&nbsp;=&nbsp;NULL;</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,130,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">//&nbsp;在&nbsp;C++&nbsp;中应使用&nbsp;nullptr&nbsp;指代空指针</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,130,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">//&nbsp;一些平台上的&nbsp;C/C++&nbsp;已经预先添加了&nbsp;xfree&nbsp;拓展，如&nbsp;GNU&nbsp;的&nbsp;</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;">
       &nbsp; 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">libiberty</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">xfree(p);</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,130,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">//&nbsp;用函数实现，例如&nbsp;GitHub&nbsp;上的&nbsp;AOSC-Dev/Anthon-Starter&nbsp;#9：</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,102,153);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:700;vertical-align:baseline;float:none;">static</code>&nbsp; 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,102,153);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:700;vertical-align:baseline;float:none;">inline</code>&nbsp; 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,102,153);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:700;vertical-align:baseline;float:none;">void</code>&nbsp; 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">*Xfree(</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,102,153);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:700;vertical-align:baseline;float:none;">void</code>&nbsp; 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">*ptr)&nbsp;{ </code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(255,20,147);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:700;vertical-align:baseline;float:none;">free</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">(ptr);</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:#808080;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">#ifdef&nbsp;__cplusplus</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,102,153);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:700;vertical-align:baseline;float:none;">return</code>&nbsp; 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,102,153);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:700;vertical-align:baseline;float:none;">nullptr</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">;</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:#808080;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">#else</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,102,153);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:700;vertical-align:baseline;float:none;">return</code>&nbsp; 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">NULL;</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:#808080;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">#endif</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">}</code> 
     </div> 
     <div style="background:rgb(255,255,255);margin:0px;padding:0px 1em;border:0px;width:auto;height:auto;text-align:left;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;white-space:pre;"> 
      <code style="background:none;margin:0px;padding:0px;border:0px;width:auto;height:auto;text-align:left;color:rgb(0,0,0);line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;font-style:normal;font-weight:400;vertical-align:baseline;float:none;">q=Xfree(q);</code> 
     </div> 
    </div></td> 
  </tr> 
 </tbody> 
</table>

所以动态分配内存后，如果使用完这个动态分配的内存空间后，必须习惯性地使用delete操作符取释放它。

关于函数使用需要注意的一些地方：  
  
A、申请了内存空间后，必须检查是否分配成功。  
  
B、当不需要再使用申请的内存时，记得释放；释放后应该把指向这块内存的指针指向NULL，防止程序后面不小心使用了它。  
  
C、这两个函数应该是配对。如果申请后不释放就是内存泄露；如果无故释放那就是什么也没有做。释放只能一次，如果释放两次及两次以上会  
  
出现错误（释放空指针例外，释放空指针其实也等于啥也没做，所以释放空指针释放多少次都没有问题）。  
  
D、虽然malloc()函数的类型是(void \*),任何类型的指针都可以转换成(void \*),但是最好还是在前面进行强制类型转换，因为这样可以躲过一  
  
些编译器的检查。

## 4.相关链接 ##

[http://www.cnblogs.com/viviwind/archive/2012/08/14/2638810.html][http_www.cnblogs.com_viviwind_archive_2012_08_14_2638810.html]

[http://blog.chinaunix.net/uid-24227137-id-3270110.html][http_blog.chinaunix.net_uid-24227137-id-3270110.html]

[copycode.gif]: /images/20220731/41e5876675834ee7bccebf374d8065c1.png
[Link 1]: http://baike.baidu.com/view/159417.htm
[NULL]: http://baike.baidu.com/view/329484.htm
[Link 2]: http://baike.baidu.com/view/379211.htm
[http_blog.csdn.net_netanimals_article_details_6530787]: http://blog.csdn.net/netanimals/article/details/6530787
[Link 3]: http://baike.baidu.com/view/872.htm
[Link 4]: http://baike.baidu.com/view/116467.htm
[http_www.cnblogs.com_viviwind_archive_2012_08_14_2638810.html]: http://www.cnblogs.com/viviwind/archive/2012/08/14/2638810.html
[http_blog.chinaunix.net_uid-24227137-id-3270110.html]: http://blog.chinaunix.net/uid-24227137-id-3270110.html