[bonzini]

It does not, it's actually arbitrary code running during the dynamic loading process, i.e. before _start.

[Panzerschrek]

But what if I have a C++ dynamic library? Does it call constructors for global variables before _start function in the main program starts?

[bonzini]

_start takes care of calling the global initializers and register the atexit callback for the finalizers.

(In practice _start calls __libc_start_main, a libc function that handles all of that).