Re: Slightly OT: non-buffered stdin in Java

On Thu, Jun 8, 2023 at 10:55 PM Aryeh Friedman <aryeh.friedman@gmail.com>
wrote:

> On Thu, Jun 8, 2023 at 10:32 PM Paul Procacci <pprocacci@gmail.com> wrote:
> >
> >
> >
> > On Thu, Jun 8, 2023 at 9:22 PM Aryeh Friedman <aryeh.friedman@gmail.com>
> wrote:
> >>
> >> On Thu, Jun 8, 2023 at 6:37 PM Dr. Nikolaus Klepp <dr.klepp@gmx.at>
> wrote:
> >> >
> >> > Anno domini 2023 Thu, 8 Jun 17:22:38 -0400
> >> >  Aryeh Friedman scripsit:
> >> > > On Thu, Jun 8, 2023 at 2:39 PM Dr. Nikolaus Klepp <dr.klepp@gmx.at>
> wrote:
> >> > > >
> >> > > > Anno domini 2023 Thu, 8 Jun 14:01:19 -0400
> >> > > >  Aryeh Friedman scripsit:
> >> > > > > Under Java stdin (System.in) is a buffered stream not sent to
> the
> >> > > > > application until return is pressed.  But, Java can read from
> >> > > > > files/sockets and other generic InputStreams unbuffered.   So I
> was
> >> > > > > wondering if there is a command that will make stdin go to a
> file so
> >> > > > > that Java can open that file and read it unbuffered?
> >> > > > >
> >> > > > > I know I can do something like cat ->file but that makes it
> hard to
> >> > > > > sync stdout and stderr (both are unbuffered in Java) with the
> file
> >> > > > > version of stdin
> >> > > > >
> >> > > >
> >> > > > "stdbuf" might be what you look for:
> >> > > >
> >> > > > https://man.freebsd.org/cgi/man.cgi?query=stdbuf
> >> > >
> >> > > Will likely need to play with it more but stdbuf -i 0 -o 0 cat -|cat
> >> > > didn't produce the expected immediate echo I still had to hit return
> >> > >
> >> >
> >> > Your console is linebuffered, so "cat" receives lines. IIRC "cat"
> disables linebuffer on input by itself, so you should use someting else for
> testing.
> >> >
> >> > Nik
> >> >
> >>
> >> I am pretty convinced by the following test it is not working as
> advertised:
> >>
> >> aryehl@neomarx:~/Desktop % cat foo.c
> >> #include <stdio.h>
> >> #include <fcntl.h>
> >> #include <unistd.h>
> >>
> >> int main()
> >> {
> >>     int in=fcntl(STDIN_FILENO, F_DUPFD, 0);
> >>     int out=fcntl(STDOUT_FILENO, F_DUPFD, 0);
> >>     char c=0;
> >>
> >>     do {
> >>         read(in,&c,1);
> >>         write(out,&c,1);
> >>     } while(c!=EOF);
> >> }
> >> aryehl@neomarx:~/Desktop % !cc
> >> cc foo.c
> >> aryehl@neomarx:~/Desktop % stdbuf -i 0 -o 0 ./a.out
> >> this is not echoing!
> >> this is not echoing!
> >> neither is this
> >> neither is this
> >> ^C
> >> aryehl@neomarx:~/Desktop %
> >>
> >> --
> >> Aryeh M. Friedman, Lead Developer, http://www.PetiteCloud.org
> >>
> >
> > stdbuf only works for stdio buffering of which read(2) and write(2)
> aren't.
>
> I also tried it with System.in.read() in Java and it was also buffered
> but according to the openjdk source it appears that this is on
> purpose.
>
>
>
> --
> Aryeh M. Friedman, Lead Developer, http://www.PetiteCloud.org
>


When you just `./a.out` you're being bitten by the tty.
You can manipulate that terminal to do what you want.  Start reading
termios(4).

Without a tty you will only get a character at a time with the following:

#include <unistd.h>

int main(int argc, char **argv)
{
  char ch;
  for(;;){
    ssize_t r = read(0,&ch,1);
    if(!r) break;
    write(1, &ch, 1);
  }
  _exit(0);
}


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