linux内存中的__init和__exit宏

南宫2874

本文翻译整理自：<wbr style="line-height:25px"><a rel="nofollow" href="http://www.faqs.org/docs/kernel/x277.html" style="color:rgb(207,121,28); line-height:25px; text-decoration:none">http://www.faqs.org/docs/kernel/x277.html</a><div style="line-height:25px"><div style="line-height:25px">This demonstrates a feature of kernel 2.2 and later. Notice the change in the definitions of the init and cleanup functions.</div><div style="line-height:25px"><span style="color:#993300; line-height:25px">__init</span><span style="color:#003366; line-height:25px">和</span><span style="color:#993300; line-height:25px">__exit</span><span style="color:#003366; line-height:25px">这两种修饰是在</span><span style="color:#0000ff; line-height:25px">android</span><span style="color:#003366; line-height:25px">的内核2.2中才引入的。它们主要用于释放函数的代码所占的内存。</span></div><div style="line-height:25px">The __init macro causes the init function to be discarded and its memory freed once the init function finishes for built-in drivers, but not loadable modules. If you think about when the init function is invoked, this makes perfectsense.</div><div style="line-height:25px"><span style="color:#003366; line-height:25px">在一个函数前添加</span><span style="line-height:25px; color:rgb(153,51,0)">__init</span><span style="color:#003366; line-height:25px">修饰的话，那么在这个函数首次执行完成后，该函数的代码所占的内存，将被释放掉</span>。<span style="line-height:25px; color:rgb(153,51,0)">__init</span><span style="color:#003366; line-height:25px">主要用于驱动的初始话。另外，在</span><span style="color:#0000ff; line-height:25px">loadable</span><span style="color:#003366; line-height:25px">的模块中函数，使用</span><span style="line-height:25px; color:rgb(153,51,0)">__init</span><span style="color:#003366; line-height:25px">，将无效。</span></div><div style="line-height:25px">There is also an __initdata which works similarly to __init but for init variables rather than functions.</div><div style="line-height:25px"><span style="color:#993300; line-height:25px">__initdata</span><span style="color:#000080; line-height:25px">和</span><span style="line-height:25px; color:rgb(153,51,0)">__init</span><span style="color:#000080; line-height:25px">相似，只是前者用于变量，后者用于函数。</span></div><div style="line-height:25px">The __exit macro causes the omission of the function when the module is built into the kernel, and like __init, has no effect for loadable modules. Again, if you consider when the cleanup function runs, this makes complete sense;built-in drivers don't need a cleanup function, while loadable modules do.</div><div style="line-height:25px"><span style="color:#000080; line-height:25px">在一个函数前添加__</span><span style="color:#800000; line-height:25px">exit</span><span style="color:#000080; line-height:25px">修饰的话，那么在the module is built into the kernel后，该函数所占的内存，将被释放掉。另外它和__init一样，在loadable的模块中函数将无效。</span></div><div style="line-height:25px">These macros are defined in linux/init.h and serve to free up kernel memory. When you boot your kernel and see something like Freeing unused kernel memory: 236k freed, this is precisely what the kernel is freeing.</div></div><div style="line-height:25px"><span style="line-height:25px">示例1</span>：</div><div style="line-height:25px"><div style="line-height:25px">Example 2-5. hello-3.c</div><div style="line-height:25px"><br style="line-height:25px"></div><div style="line-height:25px"><span style="color:#808080; line-height:25px">/* hello-3.c - Illustrating the __init, __initdata and __exit macros.</span></div><div style="line-height:25px"><span style="color:#808080; line-height:25px">*/</span></div><div style="line-height:25px">#include &lt;linux/module.h&gt;   /* Needed by all modules */</div><div style="line-height:25px">#include &lt;linux/kernel.h&gt;   /* Needed for KERN_ALERT */</div><div style="line-height:25px">#include &lt;linux/init.h&gt;    /* Needed for the macros */</div><div style="line-height:25px"><span style="color:#993300; line-height:25px">static int</span><span style="color:#0000ff; line-height:25px">hello3_data __initdata = 3;</span></div><div style="line-height:25px"><span style="color:#993300; line-height:25px">static int</span><span style="color:#0000ff; line-height:25px"></span><span style="color:#ff00ff; line-height:25px">__init</span><span style="color:#0000ff; line-height:25px">hello_3_init(void)</span></div><div style="line-height:25px"><span style="color:#0000ff; line-height:25px">{</span></div><div style="line-height:25px"><span style="color:#0000ff; line-height:25px"> printk(KERN_ALERT "Hello, world %d\n", hello3_data);</span></div><div style="line-height:25px"><span style="color:#0000ff; line-height:25px"> return 0;</span></div><div style="line-height:25px"><span style="color:#0000ff; line-height:25px">}</span></div><div style="line-height:25px"><span style="color:#993300; line-height:25px">static void</span><span style="color:#0000ff; line-height:25px"></span><span style="color:#ff00ff; line-height:25px">__exit</span><span style="color:#0000ff; line-height:25px">hello_3_exit(void)</span></div><div style="line-height:25px"><span style="color:#0000ff; line-height:25px">{</span></div><div style="line-height:25px"><span style="color:#0000ff; line-height:25px"> printk(KERN_ALERT "Goodbye, world 3\n");</span></div><div style="line-height:25px"><span style="color:#0000ff; line-height:25px">}</span></div><div style="line-height:25px"><span style="color:#0000ff; line-height:25px">module_init(</span><span style="color:#ff6600; line-height:25px">hello_3_init)</span><span style="color:#0000ff; line-height:25px">;</span></div><div style="line-height:25px"><span style="color:#0000ff; line-height:25px">module_exit(</span><span style="color:#ff6600; line-height:25px">hello_3_exit)</span><span style="color:#0000ff; line-height:25px">;</span></div><div style="line-height:25px"><span style="color:#0000ff; line-height:25px"><br style="line-height:25px"></span></div><div style="line-height:25px">By the way, you may see the directive "__initfunction()" in drivers written for Linux 2.2 kernels:</div><div style="line-height:25px"><span style="color:#3366ff; line-height:25px">__initfunction(int init_module(void))</span></div><div style="line-height:25px"><span style="color:#3366ff; line-height:25px">{</span></div><div style="line-height:25px"><span style="color:#3366ff; line-height:25px"> printk(KERN_ALERT "Hi there.\n");</span></div><div style="line-height:25px"><span style="color:#3366ff; line-height:25px"> return 0;</span></div><div style="line-height:25px"><span style="color:#3366ff; line-height:25px">}</span></div><div style="line-height:25px"><span style="color:#000080; line-height:25px">This macro served the same purpose as __init, but is now very</span><span style="color:#ff6600; line-height:25px">deprecated</span><span style="color:#000080; line-height:25px">infavor of</span><span style="color:#ff9900; line-height:25px">__init</span><span style="color:#000080; line-height:25px">. I only mention it because you might see it modern kernels. As of 2.4.18, there are 38 references to __initfunction(), and of 2.4.20,there are 37 references. However, don't use it in your own code.</span></div></div></wbr>