On 11 April 2018 at 15:11, Sven Van Caekenberghe <[email protected]> wrote:
>
>
>> On 11 Apr 2018, at 11:12, Sven Van Caekenberghe <[email protected]> wrote:
>>
>> How does one modify #atEnd to block ? I suppose you are talking about
>> StdioStream>>#atEnd ?
>>
>> ^ self peek isNil
>>
>> ?
>
> Still the same question, how do you implement a blocking #atEnd for stdin ?
>
> I have seen your stdio.cs which is indeed needed as the current
> StdioStream>>#atEnd is bogus for sure.
>
> But that is still a non-blocking one, right ?
>
> Since there is a peekBuffer in StdioStream, why can't that be used ?
I think you've created a chicken-and-egg problem with this question,
but ignoring that for now:
StdioStream>>peek
"Answer what would be returned if the message next were sent to the
receiver. If the receiver is at the end, answer nil. "
self atEnd ifTrue: [^ nil ].
peekBuffer ifNotNil: [ ^ peekBuffer ].
^ peekBuffer := self next.
So when we first start the program, i.e. the user hasn't entered any
input yet, and #peek is called:
1. #atEnd returns false because Ctrl-D (or similar) hasn't been
entered (assuming it is non-blocking).
2. peekBuffer is nil because we haven't previously called #peek.
3. The system now blocks on "self next".
Just a reminder: for terminal input the end-of-file isn't reached
until the user explicitly enters the end of file key (Ctrl-D).
So, if there is no buffered input (either none has been entered yet,
or all input has been consumed)
#atEnd (after the patch) calls #primAtEnd:.
At the moment, #primAtEnd: ends up calling the libc function feof(),
which is non-blocking and answers the end-of-file flag for the FILE*.
Since the user hasn't entered Ctrl-D, that's false.
If we want to control iteration over the stream and ensure that we
don't need to do a "stream next == nil" check, then #primAtEnd: is
going to have to peek for the next character, and that means waiting
for the user to enter that character.
In c that is typically done using:
atEnd = ungetc(fgetc(fp), fp);
and fgetc() will block until the user enters something.
> I have run your example testAtEnd.st now, and it works/fails as advertised.
:-)
Cheers,
Alistair
