🚀 go-pugleaf

In article 6di@engnews1.Eng.Sun.COM, axl@zedat.fu-berlin.de (Axel Thimm)
writes:

>Hello,
>
>I have had a look in the April draft, but I think that there is a small
>ambiguity about eof() methods in streams.
>Is the eof-flag raised, when the end of the stream is reached, or when
>an attempt to move past this end is made?

It is deliberately not specified in detail, the same situation as with
C stdio, and for the same reasons.

If the eof flag has been set, it means that the end of the file has been
seen and that no more characters are available. It doesn't necessarily
mean that you have attempted to read beyond the end yet. If the flag is
not set, it doesn't mean that more characters are available.

The reason for the looseness in the definition is to allow for different
kinds of input devices. From a pipe or keyboard, you don't know whether
eof has been reached until you get a positive indication of eof. If no
more characters are currently available, you don't know whether you are
at eof until the next input item shows up (a character or an EOF). A
test like
	if( cin.eof() ) ...
could wait indefinitely under such circumstances.

Standard Pascal, by contrast, requires predictive I/O. The file variable
always contains the next available data or EOF. That is a nice model of
I/O, but causes a problem for devices like keyboards and pipes: the
program waits during file opening until input is available. (The program
hangs during intialization until the user types something at the keyboard.)
Consequently, most Pascal implementations don't follow the standard for
console input, but use "lazy I/O" instead.

---
Steve Clamage, stephen.clamage@eng.sun.com
---
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