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### Signal handler requirements

A signal handler must meet the requirements for a handler. A value `h` of a signal handler class should work correctly in the expression `h(ec, n)`, where `ec` is an lvalue of type `const error_code` and `n` is an lvalue of type `const int`.

##### Examples

A free function as a signal handler:

    void signal_handler(
        const boost::system::error_code& ec,
        int signal_number)
    {
      ...
    }

A signal handler function object:

    struct signal_handler
    {
      ...
      void operator()(
          const boost::system::error_code& ec,
          int signal_number)
      {
        ...
      }
      ...
    };

A non-static class member function adapted to a signal handler using `bind()`:

    void my_class::signal_handler(
        const boost::system::error_code& ec,
        int signal_number)
    {
      ...
    }
    ...
    my_signal_set.async_wait(
        boost::bind(&my_class::signal_handler,
          this, boost::asio::placeholders::error,
          boost::asio::placeholders::signal_number));