什么叫回调函数？

回调函数是应用程序提供给Windows系统DLL或其它DLL调用的函数，一般用于截获消息、获取系统信息或处理异步事件。应用程序把回调函数的地址指针告诉DLL，而DLL在适当的时候会调用该函数。回调函数必须遵守事先规定好的参数格式和传递方式，否则DLL一调用它就会引起程序或系统的崩溃。通常情况下，回调函数采用标准WindowsAPI的调用方式，即__stdcall，当然，DLL编制者可以自己定义调用方式，但客户程序也必须遵守相同的规定。在__stdcall方式下，函数的参数按从右到左的顺序压入堆栈，除了明确指明是指针或引用外，参数都按值传递，函数返回之前自己负责把参数从堆栈中弹出。

生动的描述

你到一个商店买东西，刚好你要的东西没有货，于是你在店员那里留下了你的电话，过了几天店里有货了，店员就打了你的电话，然后你接到电话后就到店里去取了货。在这个例子里，你的电话号码就叫回调函数，你把电话留给店员就叫登记回调函数，店里后来有货了叫做触发了回调关联的事件，店员给你打电话叫做调用回调函数，你到店里去取货叫做响应回调事件。（by常溪玲）

专业的解释

In computer programming, a callback is a reference to executable code, or a piece of executable code, that is passed as an argument to other code. This allows a lower-level software layer to call a subroutine (or function) defined in a higher-level layer.

数学的定义

是设计模式里面template method pattern当template method的数量无限趋近于一的时候的极限。（by vczh ）

通俗易懂的说法

想象你的朋友将要离开你家，走的时候，你告诉对方说“你到家后，给我打个电话，这样我才知道你安全到达了。” 这就是字面上的call back

举例说明

<span style="font-family:Courier New"><span class="co2" style="color:rgb(51,153,51)">#include <stdio.h></span><span class="co2" style="color:rgb(51,153,51)">#include <stdlib.h></span> <span class="coMULTI" style="color:rgb(128,128,128); font-style:italic">/* The calling function takes a single callback as a parameter. */</span><span class="kw4" style="color:rgb(153,51,51)">void</span> PrintTwoNumbers<span class="br0" style="color:rgb(0,153,0)">(</span><span class="kw4" style="color:rgb(153,51,51)">int</span> <span class="br0" style="color:rgb(0,153,0)">(</span><span class="sy0" style="color:rgb(51,153,51)">*</span>numberSource<span class="br0" style="color:rgb(0,153,0)">)</span><span class="br0" style="color:rgb(0,153,0)">(</span><span class="kw4" style="color:rgb(153,51,51)">void</span><span class="br0" style="color:rgb(0,153,0)">)</span><span class="br0" style="color:rgb(0,153,0)">)</span> <span class="br0" style="color:rgb(0,153,0)">{</span>    <span class="kw3" style="color:rgb(0,0,102)">printf</span><span class="br0" style="color:rgb(0,153,0)">(</span><span class="st0" style="color:rgb(255,0,0)">"%d and %d<span class="es1" style="color:rgb(0,0,153); font-weight:bold">\n</span>"</span><span class="sy0" style="color:rgb(51,153,51)">,</span> numberSource<span class="br0" style="color:rgb(0,153,0)">(</span><span class="br0" style="color:rgb(0,153,0)">)</span><span class="sy0" style="color:rgb(51,153,51)">,</span> numberSource<span class="br0" style="color:rgb(0,153,0)">(</span><span class="br0" style="color:rgb(0,153,0)">)</span><span class="br0" style="color:rgb(0,153,0)">)</span><span class="sy0" style="color:rgb(51,153,51)">;</span><span class="br0" style="color:rgb(0,153,0)">}</span> <span class="coMULTI" style="color:rgb(128,128,128); font-style:italic">/* A possible callback */</span><span class="kw4" style="color:rgb(153,51,51)">int</span> overNineThousand<span class="br0" style="color:rgb(0,153,0)">(</span><span class="kw4" style="color:rgb(153,51,51)">void</span><span class="br0" style="color:rgb(0,153,0)">)</span> <span class="br0" style="color:rgb(0,153,0)">{</span>    <span class="kw1" style="color:rgb(177,177,0)">return</span> <span class="br0" style="color:rgb(0,153,0)">(</span><span class="kw3" style="color:rgb(0,0,102)">rand</span><span class="br0" style="color:rgb(0,153,0)">(</span><span class="br0" style="color:rgb(0,153,0)">)</span> <span class="sy0" style="color:rgb(51,153,51)">%</span> <span class="nu0" style="color:rgb(0,0,221)">1000</span><span class="br0" style="color:rgb(0,153,0)">)</span> <span class="sy0" style="color:rgb(51,153,51)">+</span> <span class="nu0" style="color:rgb(0,0,221)">9001</span><span class="sy0" style="color:rgb(51,153,51)">;</span><span class="br0" style="color:rgb(0,153,0)">}</span> <span class="coMULTI" style="color:rgb(128,128,128); font-style:italic">/* Another possible callback. */</span><span class="kw4" style="color:rgb(153,51,51)">int</span> meaningOfLife<span class="br0" style="color:rgb(0,153,0)">(</span><span class="kw4" style="color:rgb(153,51,51)">void</span><span class="br0" style="color:rgb(0,153,0)">)</span> <span class="br0" style="color:rgb(0,153,0)">{</span>    <span class="kw1" style="color:rgb(177,177,0)">return</span> <span class="nu0" style="color:rgb(0,0,221)">42</span><span class="sy0" style="color:rgb(51,153,51)">;</span><span class="br0" style="color:rgb(0,153,0)">}</span> <span class="coMULTI" style="color:rgb(128,128,128); font-style:italic">/* Here we call PrintTwoNumbers() with three different callbacks. */</span><span class="kw4" style="color:rgb(153,51,51)">int</span> main<span class="br0" style="color:rgb(0,153,0)">(</span><span class="kw4" style="color:rgb(153,51,51)">void</span><span class="br0" style="color:rgb(0,153,0)">)</span> <span class="br0" style="color:rgb(0,153,0)">{</span>    PrintTwoNumbers<span class="br0" style="color:rgb(0,153,0)">(</span><span class="sy0" style="color:rgb(51,153,51)">&</span><span class="kw3" style="color:rgb(0,0,102)">rand</span><span class="br0" style="color:rgb(0,153,0)">)</span><span class="sy0" style="color:rgb(51,153,51)">;</span>    PrintTwoNumbers<span class="br0" style="color:rgb(0,153,0)">(</span><span class="sy0" style="color:rgb(51,153,51)">&</span>overNineThousand<span class="br0" style="color:rgb(0,153,0)">)</span><span class="sy0" style="color:rgb(51,153,51)">;</span>    PrintTwoNumbers<span class="br0" style="color:rgb(0,153,0)">(</span><span class="sy0" style="color:rgb(51,153,51)">&</span>meaningOfLife<span class="br0" style="color:rgb(0,153,0)">)</span><span class="sy0" style="color:rgb(51,153,51)">;</span>    <span class="kw1" style="color:rgb(177,177,0)">return</span> <span class="nu0" style="color:rgb(0,0,221)">0</span><span class="sy0" style="color:rgb(51,153,51)">;</span><span class="br0" style="color:rgb(0,153,0)">}</span></span>

回调的作用

回调可用于通知机制，例如，有时要在程序中设置一个计时器，每到一定时间，程序会得到相应的通知，但通知机制的实现者对我们的程序一无所知。而此时，就需有一个特定原型的函数指针，用这个指针来进行回调，来通知我们的程序事件已经发生。

看完后，你觉得还有可补充的内容，欢迎给我留言：）

参考资料

http://stackoverflow.com/questions/824234/what-is-a-callback-function?answertab=votes#tab-top

http://en.wikipedia.org/wiki/Callback_(computer_science)

http://www.zhihu.com/question/19801131

http://wmnmtm.blog.163.com/blog/static/3824571420105484116877/

http://stackoverflow.com/questions/9596276/how-to-explain-callbacks-in-plain-english-how-are-they-different-from-calling-o