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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.ada
Subject: Re: Determining size of Standard_Input (when file contents are piped
 into a program).
Date: Sun, 06 Jan 2019 19:05:31 -0800
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Niklas Holsti <niklas.holsti@tidorum.invalid> writes:
> On 19-01-06 13:02 , Bojan Bozovic wrote:
>> Is there platform independent way to do this? I was unable to find
>> anything in RM under Ada.text_IO.
>
> I would say that this is impossible in principle. On some systems, other 
> concurrent programs can write more data into the input file as your 
> program is reading the file, so the input file can grow during the 
> execution of the program, and may not even have any fixed upper bound on 
> its size (apart from the total space available in the system).
>
> IMO the only robust way is to read all of standard input and store it 
> within the program, or in a temporary file, and then process the stored 
> data.

I mostly agree, but it could be possible in some limited
circumstances.

For example, if a program on Linux has its standard input redirected
from a file, the system still keeps track of the identity of the
file (/proc/$PID/fd/0 is a symbolic link to the file).  You can
then determine the size of that file.

This is all very system-specific and not generally useful --
though some programs might use it as a heuristic to estimate how
much memory to allocate (only an estimate, since the size of the
file might change while the program is running).

And if input is not a regular file, no meaningful size information
will be available.

Ada doesn't provide a portable way to do this.

-- 
Keith Thompson (The_Other_Keith) kst@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"