It does not, it's actually arbitrary code running during the dynamic loading process, i.e. before _start.
replyBut what if I have a C++ dynamic library? Does it call constructors for global variables before _start function in the main program starts?
reply_start takes care of calling the global initializers and register the atexit callback for the finalizers.
reply(In practice _start calls __libc_start_main, a libc function that handles all of that).