linux dlopen: can a library be "notified" when it is loaded?

Question

Is there a way for a shared library to be "notified" when it is loaded?

In other words, let's say I use dlopen on a shared library, is there a function that is automatically called (if present) on the shared library (e.g. main?)

Answer 1

At least on Linux, and probably on at least some other Unix systems, if the library is dynamically opened a global function named _init, if it exists, will be called by the dynamic linker.

Answer 2

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);

Taken from http://tldp.org/HOWTO/Program-Library-HOWTO/index.html

THat is, you just tack on __attribute__ ((constructor)) to the functions you want to be called when the shared library is loaded. The above docuemtn also notes that the older _ini and _fini functions are considered obsolete.

Answer 3

Yes. When a library is opened, all static construction takes place... so, if you use C++, you can do:

// mylibrary.cpp
namespace
{
    class dynamic_library_load_unload_handler
    {
         public:
              dynamic_library_load_unload_handler(){
                    // Code to execute when the library is loaded
              }
              ~dynamic_library_load_unload_handler(){
                    // Code to execute when the library is unloaded
              }
    } dynamic_library_load_unload_handler_hook;
}

Unlike the __attribute__ ((constructor)) solutions given, this will be portable. Note, though, that if you have multiple objects like this, there is no guarantee with respect to the construction/destruction order.