Question

I have two shared libraries: mylib and loglib (names are changed).
Both have destructor functions (extension of gcc).
The destructor function of mylib requires functions of loglib. This way:

mylib.c from libmy.so:

void __attribute__ ((destructor)) mylib_destructor()
{
    loglib_write_log("destructor");
}

loglib.c from liblog.so:

void loglib_write_log( const char* txt )
{
   fprintf( log_file, "%s\n", txt );
}

void __attribute__ ((destructor)) loglib_destructor()
{
    if( log_file )
    {
        fclose( log_file );
        log_file = NULL;
    } 
}

As you can see, problems occurs if loglib_destructor() is called before mylib_destructor():
fprintf will get NULL pointer parameter.

I can not change loglib.c.

How can I ensure that mylib_destructor is called before destructors of other libraries?

I don’t want to set highest priority for mylib_destructor, because users of mylib may want to use even higher priority for own destructors.

Answer 1

From the gcc docs:

You may provide an optional integer priority to control the order in which constructor and destructor functions are run. A constructor with a smaller priority number runs before a constructor with a larger priority number; the opposite relationship holds for destructors. So, if you have a constructor that allocates a resource and a destructor that deallocates the same resource, both functions typically have the same priority. The priorities for constructor and destructor functions are the same as those specified for namespace-scope C++ objects (see C++ Attributes).

Have you tried this?

It doesn’t have to be the highest priority for mylib, just large enough. If it’s documented, users of your library can rely on it.