Linux中EXPORT_SYMBOL的用法
EXPORT_SYMBOL标签内定义的函数对全部内核代码公开，不用修改内核代码就可以在您的内核模块中直接调用。您还可以手工修改内核源代码来导出另外的函数，用于重新编译并加载新内核后的测试。
Linux symbol export method:
[1] If we want export the symbol in a module, just use the EXPORT_SYMBOL(xxxx) in the C or H file.
And compile the module by adding the compile flag -DEXPORT_SYMTAB.
Then we can use the xxxx in the other module.
[2] If we want export some symbol in Kernel that is not in a module such as xxxx in the /arch/ppc/fec.c.
Firstly, define the xxxx in the fec.c;
Secondly, make a new file which contain the "extern" define the xxxx(for example, extern int xxxx);
Lastly, in the ppc_ksyms.c we includes the new file, and add the EXPORT_SYMBOL(xxxx).
Then we can use the xxxx.
使用时注意事项：
在使用EXPORT_SYMBOL 的.c文件中 需要 #include <linux/module.h> 文件。
// 先写函数
func_a ()
{
}
//再使用EXPORT_SYMBOL
EXPORT_SYMBOL(func_a);
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参考：http://apps.hi.baidu.com/share/detail/17895919
http://dev.firnow.com/course/3_program/c++/cppjs/200873/129970.html