Gem #18: Warnings in GNAT

by Bob Duff—AdaCore

Let's get started…

The recent "gems" about pragma No_Return talked a lot about compile-time checks that generate warnings. Indeed, GNAT is capable of generating lots of useful warnings. Here is some advice about how to get the most out of these warnings.

What's the difference between a "warning" and an "error"? First, errors are generally violations of the Ada language rules as specified in the Ada Reference Manual; warnings are GNAT specific. Thus, other Ada compilers might not warn about the same things that GNAT does. Second, warnings are typically conservative; that is, some warnings will be false alarms, and the programmer needs to study the code to see if the warning is a real problem.

Some warnings are given by default, whereas some are given only if a switch enables them. Use the -gnatwa switch to turn on (almost) all warnings.

Warnings are useless if you don't do something about them. If you give your team-member some code that causes warnings, how are they supposed to know if they represent real problems? Pretty soon people will ignore warnings, and they will scatter themselves all about the code. Use the -gnatwae switch to turn on (almost) all warnings, and to treat warnings as errors. This will force you to get a clean (no warnings or errors) compilation.

But some warnings are false alarms. Use pragma Warnings (Off) to suppress false alarms. It's best to be as specific as possible: narrow down to a single line of code, and a single warning message. And use a comment to explain why the warning is a false alarm, if it's not obvious. The following, compiled with -gnatwae:

   package body Warnings_Example is

      procedure Mumble (X : Integer) is
      begin
         null;
      end Mumble;

   end Warnings_Example;

will cause GNAT to complain:

warnings_example.adb:5:22: warning: formal parameter "X" is not referenced

But the following will compile cleanly:

   package body Warnings_Example is

      pragma Warnings (Off, "formal parameter ""X"" is not referenced");
      procedure Mumble (X : Integer) is
      pragma Warnings (On, "formal parameter ""X"" is not referenced");
      --  X is ignored here, because blah blah blah...
      begin
         null;
      end Mumble;

   end Warnings_Example;

Here we've suppressed the specific warning message on a specific line.

If you get many warnings of a specific type, and it's not feasible to fix them all, then suppress that type of message, so the good warnings won't get buried beneath a pile of bogus ones. The -gnatwaeF switch will silence the warning on the first version of Mumble above: the F means suppress warnings on unreferenced formal parameters, and would be a good idea if you have lots of those.

In summary, I suggest turning on as many warnings as makes sense for your project. Then whenever you see a warning message, look at the code and decide if it's real. If so, fix the code. If it's a false alarm, suppress the warning. Either way, make the warning disappear before checking your code into your configuration management system.

The details of the switches and pragmas can be found in the GNAT Reference Manual and User Guide.


Last Updated: 10/13/2017
Posted on: 11/19/2007