Library constructor and destructor functifunctions

Libraries should export initialization and cleanup routines using the gcc __attribute__((constructor)) and __attribute__((destructor)) function attributes. See the gcc info pages for information on these. Constructor routines are executed before dlopen returns (or before main() is started if the library is loaded at load time). Destructor routines are executed before dlclose returns (or after exit() or completion of main() if the library is loaded at load time). The C prototypes for these functions are:

  void __attribute__ ((constructor)) my_init(void);
  void __attribute__ ((destructor)) my_fini(void);

Shared libraries must not be compiled with the gcc arguments ``-nostartfiles'' or ``-nostdlib''. If those arguments are used, the constructor/destructor routines will not be executed (unless special measures are taken).

这里说明了void __attribute__ ((constructor)) my_init(void);是在dlopen返回之前或是main函数之前调用的，这两个调用点的却别是由于库是在应用程序启动前加载还是库用到的时候加载。

动态库的加载时机：

1 程序运行前加载：某个应用程序运行前，先加载依赖的库，然后运行应用程序，此时，void __attribute__ ((constructor)) my_init(void);是在main函数运行前被调用的。

2 程序运行中用到某库才去加载：此时void __attribute__ ((constructor)) my_init(void);是在dlopen返回前调用的。这种时机库的加载要用到dlopen等函数。