Re: Weird Bug in Get_Line

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

On 04/26/2017 09:38 AM, Brian Kolden wrote:
> I came across an interesting issue in the Get_Line function. When piping in a
> file (using the linux cammand '<') GNAT "raises ADA.IO_EXCEPTIONS.END_ERROR :
> a-tigeli.adb:96".
>
> However, the interesting issue is this only happens for a very small number
> of line lengths. I've noticed the exception is only thrown on lines with
> length 500, 501, 1000, 1001. Lines bigger or smaller don't seem to raise the
> issue, even other multiples of 500. This only is effected when using a file
> as input, copying and pasting the line into the terminal does not seem to
> trigger it.

And also

> I forgot to mention the line has to be the very last line, otherwise there
> would be no EOF character to read. No newline character should be there, vim
> auto inserts one by default, for example. I was also able to trigger it on
> Redhat 5.11.0.9 running GNAT 4.1.2.

According to the ARM (A.10), Text_IO operates on "text files" as defined there. 
Part of that definition is that "the end of a line is marked by a _line 
terminator_" and "the end of a file is marked by the combination of a line 
terminator immediately followed by a page terminator and then a _file 
terminator_" [A.10(7)]. So technically, an implementation of Text_IO is not 
required to work on files that aren't valid Text_IO text files, and the error is 
that your file is not a valid Text_IO text file.

However, such an implementation of Text_IO is pretty useless on most systems, 
and compiler writers usually try hard to make sure that Text_IO works on foreign 
files. AdaCore certainly does. Still, if you produce files without a final line 
terminator, it's usually a good idea to find out why and fix that.

-- 
Jeff Carter
"Hold your temper. Count ten.... Now let 'er go.
You got a good aim."
Never Give a Sucker an Even Break
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