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强符号和弱符号.html
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<h1><a href="https://chukeer.github.io/强符号和弱符号.html"> 强符号和弱符号 </a></h1>
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<p>之前在<a href="http://littlewhite.us/archives/240">extern “C” 用法详解</a>中已经提到过符号的概念，它是编译器对变量和函数的一种标记，编译器对C和C++代码在生产符号时规则也是不一样的，符号除了本身名字的区别外，还有强符号和弱符号之分</p>
<p>我们先看一段简单的代码</p>
<div class="highlight"><pre><span></span><span class="cm">/* test.c */</span><span class="w"></span>
void<span class="w"> </span>hello<span class="o">()</span><span class="err">;</span><span class="w"></span>
int<span class="w"> </span>main<span class="o">()</span><span class="w"></span>
<span class="err">{</span><span class="w"></span>
<span class="w"> </span>hello<span class="o">()</span><span class="err">;</span><span class="w"></span>
<span class="w"> </span><span class="kr">return</span><span class="w"> </span><span class="m">0</span><span class="err">;</span><span class="w"></span>
<span class="err">}</span><span class="w"></span>
</pre></div>
<p>很显然，这段代码是没法链接通过的，它会报错<code>undefined reference to hello</code>，说的是hello未定义，因为这里我们只声明了函数hello，而没有定义它。但是我们把代码稍作修改如下</p>
<div class="highlight"><pre><span></span>__attribute__((weak)) void hello();
int main()
{
hello();
return 0;
}
</pre></div>
<p>这时你会发现，编译链接都可通过，但是运行会报错，因为这时我们将hello声明为了弱符号，在链接时弱符号会被链接器当做0，执行一个地址为0的函数当然会报错，改为如下代码就不会报错了，只是它没有任何输出</p>
<div class="highlight"><pre><span></span>__attribute__((weak)) void hello();
int main()
{
if(hello)
hello();
return 0;
}
</pre></div>
<p>编译器认为，<font color="red"><strong>函数和初始化了的全局变量为强符号，未初始化的全局变量为弱符号</strong></font>，链接器在处理强符号和弱符号时有如下规则</p>
<ol>
<li>不同目标文件中，不允许有同名的强符号</li>
<li>如果一个符号在某个目标文件中是强符号，在其它目标文件中为弱符号，选择强符号</li>
<li>如果一个符号在所有目标文件中都是弱符号，选择占用空间最大的，比如目标文件A中有double global_var，文件B中有int global_var，double占用8字节，大于int的4字节，A和B链接后，符号global占8字节</li>
</ol>
<p>对此我们可以简单的验证一下，有如下两个文件</p>
<div class="highlight"><pre><span></span><span class="cm">/* 1.c */</span><span class="w"></span>
char<span class="w"> </span>global_var<span class="err">;</span><span class="w"></span>
int<span class="w"> </span>main<span class="o">()</span><span class="w"></span>
<span class="err">{</span><span class="w"></span>
<span class="w"> </span><span class="kr">return</span><span class="w"> </span><span class="m">0</span><span class="err">;</span><span class="w"></span>
<span class="err">}</span><span class="w"></span>
<span class="cm">/* 2.c */</span><span class="w"></span>
int<span class="w"> </span>global_var<span class="err">;</span><span class="w"></span>
</pre></div>
<p>全局变量global_var在两个文件中都没有初始化，因此都是弱符号，执行编译命令<code>gcc 1.c 2.c</code>，用readelf查看符号表<code>readelf -s a.out</code>，为了查看方便我们只输出最后几行</p>
<div class="highlight"><pre><span></span><span class="n">Num</span><span class="o">:</span> <span class="n">Value</span> <span class="n">Size</span> <span class="n">Type</span> <span class="n">Bind</span> <span class="n">Vis</span> <span class="n">Ndx</span> <span class="n">Name</span>
<span class="mi">62</span><span class="o">:</span> <span class="mi">0000000000600818</span> <span class="mi">4</span> <span class="n">OBJECT</span> <span class="n">GLOBAL</span> <span class="n">DEFAULT</span> <span class="mi">25</span> <span class="n">global_var</span>
<span class="mi">63</span><span class="o">:</span> <span class="mi">0000000000400474</span> <span class="mi">11</span> <span class="n">FUNC</span> <span class="n">GLOBAL</span> <span class="n">DEFAULT</span> <span class="mi">13</span> <span class="n">main</span>
<span class="mi">64</span><span class="o">:</span> <span class="mi">0000000000400358</span> <span class="mi">0</span> <span class="n">FUNC</span> <span class="n">GLOBAL</span> <span class="n">DEFAULT</span> <span class="mi">11</span> <span class="n">_init</span>
</pre></div>
<p>这里符号global_var占用的size是4，说明链接器选择的是占用空间更大的int global_var，我们再稍作修改，将1.c中的全局变量初始化，如下</p>
<div class="highlight"><pre><span></span><span class="cm">/* 1.c */</span><span class="w"></span>
char<span class="w"> </span>global_var<span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="m">1</span><span class="err">;</span><span class="w"></span>
int<span class="w"> </span>main<span class="o">()</span><span class="w"></span>
<span class="err">{</span><span class="w"></span>
<span class="w"> </span><span class="kr">return</span><span class="w"> </span><span class="m">0</span><span class="err">;</span><span class="w"></span>
<span class="err">}</span><span class="w"></span>
<span class="cm">/* 2.c */</span><span class="w"></span>
int<span class="w"> </span>global_var<span class="err">;</span><span class="w"></span>
</pre></div>
<p>这时1.c中的global_var为强符号，2.c中的global_var为弱符号，同样编译之后用readelf查看符号表<code>readelf -s a.out</code>如下</p>
<div class="highlight"><pre><span></span><span class="n">Num</span><span class="o">:</span> <span class="n">Value</span> <span class="n">Size</span> <span class="n">Type</span> <span class="n">Bind</span> <span class="n">Vis</span> <span class="n">Ndx</span> <span class="n">Name</span>
<span class="mi">62</span><span class="o">:</span> <span class="mi">0000000000600818</span> <span class="mi">1</span> <span class="n">OBJECT</span> <span class="n">GLOBAL</span> <span class="n">DEFAULT</span> <span class="mi">25</span> <span class="n">global_var</span>
<span class="mi">63</span><span class="o">:</span> <span class="mi">0000000000400474</span> <span class="mi">11</span> <span class="n">FUNC</span> <span class="n">GLOBAL</span> <span class="n">DEFAULT</span> <span class="mi">13</span> <span class="n">main</span>
<span class="mi">64</span><span class="o">:</span> <span class="mi">0000000000400358</span> <span class="mi">0</span> <span class="n">FUNC</span> <span class="n">GLOBAL</span> <span class="n">DEFAULT</span> <span class="mi">11</span> <span class="n">_init</span>
</pre></div>
<p>此时符号global_var占用的size是1，说明链接器选择的是强符号</p>
<p>在写代码时应该尽量避免有不同类型的符号，否则会引发非常诡异且不易察觉的错误，为了避免可以采取如下措施：</p>
<ul>
<li>上策：消除所有的全局变量 </li>
<li>中策：将全局变量声明为static类型，并提供接口供访问 </li>
<li>下策：全局变量一定要初始化，哪怕初始化为0 </li>
<li>必备：打开gcc的-fno-common选项，它会禁止有不同类型的符号</li>
</ul>
<p>说了这么多，好像在说应该尽量用强符号，那弱符号有什么用呢，所谓存在即合理，有时候我们甚至需要显示定义弱符号，这对库函数会非常有用，比如库中的弱符号可以被用户自定义的强符号覆盖，从而实现自定义的库版本，或者在使用某些扩展功能时，用户可以定义一个弱符号，当链接了该功能时，功能模块可以正常使用，如果去掉功能模块，程序也可正常链接，只是缺少某些功能而已，比如我们可以通过下面的代码判断程序是否链接了pthread库，从而决定执行什么样的操作</p>
<div class="highlight"><pre><span></span><span class="cm">/* test.c */</span>
<span class="cp">#include</span> <span class="cpf"><stdio.h></span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf"><pthread.h></span><span class="cp"></span>
<span class="n">__attribute__</span><span class="p">((</span><span class="n">weak</span><span class="p">))</span> <span class="kt">int</span> <span class="n">pthread_create</span><span class="p">(</span>
<span class="n">pthread_t</span><span class="o">*</span><span class="p">,</span>
<span class="k">const</span> <span class="n">pthread_attr_t</span><span class="o">*</span><span class="p">,</span>
<span class="kt">void</span><span class="o">*</span><span class="p">(</span><span class="o">*</span><span class="p">)(</span><span class="kt">void</span><span class="o">*</span><span class="p">),</span>
<span class="kt">void</span><span class="o">*</span><span class="p">);</span>
<span class="kt">int</span> <span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="k">if</span> <span class="p">(</span><span class="n">pthread_create</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"This is multi-thread version!</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">else</span>
<span class="p">{</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"This is single-thread version!</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
<p>编译运行结果如下</p>
<div class="highlight"><pre><span></span>$ gcc test.c
$ ./a.out
This is single-thread version!
$ gcc test.c -lpthread
$ a.out
This is multi-thread version!
</pre></div>
<p>EOF</p>
<hr/>
<p>本文参考：</p>
<ol>
<li>《程序员的自我修养》3.5.5章节</li>
<li><a href="http://blog.csdn.net/astrotycoon/article/details/8008629">http://blog.csdn.net/astrotycoon/article/details/8008629</a></li>
</ol>
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