Re: surprise data from Ada.Sequential_IO

["Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>]

On 3/22/21 6:13 PM, John Perry wrote:
> Hello all
> 
> I was using gnat 2020 CE on a Linux machine the other day, and wanted to write out data for three different types T1, T2, T3 to a file. I instantiated a Sequential_IO package for each; call them P1, P2, P3.
> 
> The following worked as expected (sorry for the abbreviations but I think it will be clear):
> 
>>> P1.Create; P1.Write; P1.Close; P2.Open(Append_File); P2.Write; P2.Close;

This is asking for trouble,

> 
> However, this:
> 
>>> P3.Open(Append_File); P3.Write; P3.Close;
> 
> ...wrote a few bytes of junk between T2's data and T3's data.

which you got.

When you open a P3.File_Type with mode Append_File, it expects the file to 
contain an integral number of values of T3 already. What happens if it doesn't 
appears to be undefined.

Either create a record with components of the the three types, instantiate 
Sequential_IO for that, and write all three values at once, or use streams for 
heterogeneous I/O.

-- 
Jeff Carter
"One day he told me he was a gynecologist. He
couldn't speak no foreign languages."
Take the Money and Run
147