error in index constraints with initial value

error in index constraints with initial value

[Thomas Haeckel]

Hi,

I've compiled the following procedure with gnat version 2.8.1 and also
3.14p.

1:with Text_IO; use Text_IO;
2:
3:procedure Index_Constraint_Test is
4:
5:type VAR_LINE(LENGTH:INTEGER:=5) is
6:record
7:   IMM:STRING(1..LENGTH);
8:end record;
9:
10:NULL_LINE: VAR_LINE(0);
11:TWO_LINE: VAR_LINE(2);
12:XXXX_LINE: VAR_LINE;
13:
14:begin
15:   Put_Line("NULL_LINE="&NULL_LINE.IMM&"#");
16:   Put_Line("TWO_LINE="&TWO_LINE.IMM&"#");
17:   Put_Line("XXXX_LINE="&XXXX_LINE.IMM&"#");
18:end;

I got following runtime error with both gnat versions:
  raised STORAGE_ERROR : object too large
This is caused by the variable instantiation XXXX_LINE in line 12.
After the example in LRM 3.6.1(15) this should be allowed.
Also an old Aionix-Ada83-compiler works fine.
Why has the initialization of the index constraint in line 5 no effect
apparently ?
Is this effect a "feature" of gnat or Ada95 ?
How can I do an null-array (length 0, no component) definition at
compile time, to assign an arraysize while runtime ?

Thanks,
     Thomas

Re: error in index constraints with initial value

[Marin David Condic]

The problem is likely to be in:

12:XXXX_LINE: VAR_LINE;

The GNAT compiler allocates for the worst possible case so that if you
assign to XXXX_LINE, it doesn't have to reallocate memory. This is not
really so much a feature of Ada95 as it is a feature of GNAT. IIRC, the old
DEC Ada83 compiler would not have a problem with this and would do the
memory allocation on assignment. Its an implementation decision that is
allowable by the standard.

You could use an access type to VAR_LINE and the "new" allocator to get
around this.

MDC
--
Marin David Condic
Senior Software Engineer
Pace Micro Technology Americas    www.pacemicro.com
Enabling the digital revolution
e-Mail:    marin.condic@pacemicro.com


"Thomas Haeckel" <thomas_haeckel@web.de> wrote in message
news:21b0043b.0204040925.4654c421@posting.google.com...
> Hi,
>
> I've compiled the following procedure with gnat version 2.8.1 and also
> 3.14p.
>
> 1:with Text_IO; use Text_IO;
> 2:
> 3:procedure Index_Constraint_Test is
> 4:
> 5:type VAR_LINE(LENGTH:INTEGER:=5) is
> 6:record
> 7:   IMM:STRING(1..LENGTH);
> 8:end record;
> 9:
> 10:NULL_LINE: VAR_LINE(0);
> 11:TWO_LINE: VAR_LINE(2);
> 12:XXXX_LINE: VAR_LINE;
> 13:
> 14:begin
> 15:   Put_Line("NULL_LINE="&NULL_LINE.IMM&"#");
> 16:   Put_Line("TWO_LINE="&TWO_LINE.IMM&"#");
> 17:   Put_Line("XXXX_LINE="&XXXX_LINE.IMM&"#");
> 18:end;
>
> I got following runtime error with both gnat versions:
>   raised STORAGE_ERROR : object too large
> This is caused by the variable instantiation XXXX_LINE in line 12.
> After the example in LRM 3.6.1(15) this should be allowed.
> Also an old Aionix-Ada83-compiler works fine.
> Why has the initialization of the index constraint in line 5 no effect
> apparently ?
> Is this effect a "feature" of gnat or Ada95 ?
> How can I do an null-array (length 0, no component) definition at
> compile time, to assign an arraysize while runtime ?
>
> Thanks,
>      Thomas

Re: error in index constraints with initial value

[Toshitaka Kumano]

Hi,

Thomas Haeckel wrote:

> 5:type VAR_LINE(LENGTH:INTEGER:=5) is

Try to change "Interger" to more range-constraint subtype.
Integer'Range can make "Storage_Error" for most machines.

-- 
Toshitaka Kumano

Re: error in index constraints with initial value

[Frank J. Lhota]

The problem with your definition of XXXX_LINE is that you do not constrain
the record variant LENGTH. LENGTH will default to 5, but since it is not
constrained, XXXX_LINE can be assigned a VAR_LINE value of any length, even
LENGTH => Integer'Last! Of course, this requires more space than is
possible, resulting in the blow-up.

There are a couple of ways to handle this properly. If XXXX_LINE really
needs to hold VAR_LINE records of widely varying and arbitrarily long
lengths, you should declare XXXX_LINE as an access value and allocate these
VAR_LINE records from the heap.

Another possible solution is available is we can assume that there is a
reasonable upper limit on the length of a VAR_LINE. We can then use a
subtype to limit the value of LENGTH, so that VAR_LINE has a reasonable
maximum size, e.g.,

    subtype Var_Line_Len is Integer range 0 .. 1023;
    type VAR_LINE( LENGTH : Var_Line_Len := 5 ) is
    record
       IMM:STRING(1..LENGTH);
    end record;

With this declaration, VAR_LINE will never have a length greater than
Var_Line_Len'Last. With this change, your program should compile.