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Subject: What does -gnato do?
From: "Peter C. Chapin" <pchapin@sover.net>
Organization: Kelsey Mountain Software
Message-ID: <Xns982BBFF651E2pchapinsovernet@198.186.192.137>
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Date: 26 Aug 2006 22:52:09 GMT
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Date: 2006-08-26T22:52:09+00:00
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I'm using GNAT GPL 2006. In the User's Guide there is a description of 
the -gnato option. This description includes the following example:

---> begin quote <---
X1 : Integer := Integer'Last;
X2 : Integer range 1 .. 5 := 5;
...
X1 := X1 + 1;
X2 := X2 + 1;

Here the first addition results in a value that is outside the base 
range of Integer, and hence requires an overflow check for detection of 
the constraint error. Thus the first assignment to X1 raises a 
Constraint_Error exception only if `-gnato' is set. 
---> end quote <---

Now consider the following program:

---> begin program <---
with Ada.Integer_Text_IO; use Ada.Integer_Text_IO;

procedure Check is
   X : Integer := Integer'Last;
begin
   X := X + 1;
   Put(X);
end Check;
---> end program <---

When I build this program *without* the -gnato option I get the 
following warning:

C:\home\Languages\Ada>gnatmake check.adb
gcc -c check.adb
check.adb:6:11: warning: value not in range of type "Standard.Integer"
check.adb:6:11: warning: "Constraint_Error" will be raised at run time
gnatbind -x check.ali
gnatlink check.ali

And when I run the result I get

C:\home\Languages\Ada>check

raised CONSTRAINT_ERROR : check.adb:6 overflow check failed


So it seems that overflow checking is enabled even without -gnato 
despite the fact that this contradicts the documentation. I conclude 
that the default has changed and the documentation is out of date. Is 
that true or am I misunderstanding something?

Note that when I include the -gnato option in the compilation of my 
sample program the effect is the same. The -gnato option doesn't change 
anything in this case.

Peter