Boost
  C++ Libraries
  
  ...one of the most highly
  regarded and expertly designed C++ library projects in the
  world.
 — Herb Sutter and Andrei
  Alexandrescu, C++
  Coding Standards
boost::process::extend::on_exec_setup — This handler is invoked if the fork succeeded. The required signature is void(Exec &), where Exec is a template parameter. 
// In header: <boost/process/extend.hpp> unspecified on_exec_setup;