On UNIX, script agents use one of the different agent binaries that are shipped with VCS. The agent binaries are located at:
See Script based agent binaries.
You can implement entry points using C++ or scripts. If you are implementing even one entry point in c++ then you must implement the VCSAgStartup function. If you do not implement any entry points in C++, then you do not need to implement the VCSAgStartup function since the default implementation of VCSAgStartup is present in the script agent binary provided by VCS as mentioned above.