Ada, GNAT.Ctrl_C "expected type Handler_Type defined at g-ctrl_c.ads:45"

Question 1

Install_Handler(Handler => MyHandler'Unrestricted_Access);

Unlike some languages, to get an access or "reference" or "pointer" to a procedure, you have to be explicit by using the 'Access attribute. It's a bit like using &func in C to get a pointer to a function, but C allows you to omit the &. Ada doesn't allow for that kind of shorthand, in part because Ada doesn't require parentheses when calling a procedure/function with no parameters, which means that using the name alone would be syntactically ambiguous.

In many cases, using 'Access would be good enough. The reason it isn't enough here, is because of Ada's accessibility rules. Suppose you write something like

procedure Spam is
  Number_Of_Vikings : Integer := 0;
  procedure MyHandler is
  begin
    Put_Line("YOU WILL NEVER END MY SPAM");
    Number_Of_Vikings := Number_Of_Vikings + 1;
  end MyHandler;
begin
  Install_Handler(Handler => MyHandler);
  --while True loop
  for I in 1 .. 10 loop
    Put_Line("SPAM SPAM SPAM SPAM SPAM SPAM");
  end loop;
end Spam;

In theory, Install_Handler could store the Handler parameter in a global, which means that (in theory) something else could use the access-to-procedure global to call MyHandler after the Spam procedure has returned. Then, MyHandler would try to do something with Number_Of_Vikings. But that variable would no longer exist, since it belongs to the Spam procedure which is no longer running. The result could be disastrous. (JavaScript has closures so it would keep the variable around even after Spam exits, and Java and C++11 also have limited forms of this, but Ada doesn't have anything equivalent.) Therefore, Ada has rules to prevent this sort of thing from happening. To get around the rules, you can use 'Unrestricted_Access (which is a GNAT attribute, not defined by the language), which tells the compiler "I know what I'm doing, don't worry about it".

Question 2

If the handler was moved to the "library level" (i.e. not embedded in your test program), it would be sufficient to add 'Access to the reference to it:

my_handler.ads:

procedure My_Handler;

my_handler.adb:

with Ada.Text_IO;
procedure My_Handler is
begin
   Ada.Text_IO.Put_Line ("YOU WILL NEVER END MY SPAM");
end My_Handler;

spam.adb:

with Ada.Text_IO;
with GNAT.Ctrl_C;
with My_Handler;
procedure Spam is
begin
   GNAT.Ctrl_C.Install_Handler (Handler => My_Handler'Access);
   loop
      Ada.Text_IO.Put_Line("SPAM SPAM SPAM SPAM SPAM SPAM");
   end loop;
end Spam;