Ada Programming/Pragmas/No Return

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Ada Lovelace 1838.jpg

pragma No_Return (local_name);

Description[edit]

No Return is a representation pragma for procedures, which states that a procedure will never return normally; that is, it will raise an exception, loop endlessly, or terminate the program (e.g. by means of an imported function like C’s exit or by triggering a hardware reset).

On the call site, this enables detection of dead code and suppression of warnings about missing return statements or missing assignment to variables.

The compiler ensures that a non-returning procedure will indeed not return by raising Program_Error if it would otherwise.

Example[edit]

procedure P ( … );
pragma No_Return (P);
procedure Q (out x: … ) is
begin
  if Cond
  then P ( … );
       Some_Thing_Else; -- This is dead code--and due to No_Return probably a compiler warning!
  else x := … ;
  end if;
  -- No warning about a missing assignment to x here
end Q; 

Portability[edit]

The pragma No_Return is standard in the language since Ada 2005. Some compilers (e.g. GNAT and AdaMagic) already recognised No_Return as implementation defined pragma before[1].

See also[edit]

Wikibook[edit]

Ada Reference Manual[edit]

References[edit]

  1. AI95-00329 in Appendix: From: Tucker Taft, Sent: Tuesday, March 4, 2003 11:13 AM