Buffer flushing not working for printf function

Question

Coming from here: https://stackoverflow.com/a/1716621/1461017

I want to print separate points (dots, "."), one at a time on the same line, under control of a for loop:

for (i=0; i<100; i++)
{
    printf(".");
    sleep(1);
}
printf("\n");

But the output only goes to screen after the final "\n", and I want each one to display immediately upon printing, so as to serve as progress indicators. I expected that fflush()ing the standard output on each loop iteration would produce that result, but this does not work (results are only shown on screen after the execution of the for loop):

for (i=0; i<100; i++)
{
    printf(".");
    sleep(1);
    fflush(stdout);
}
printf("\n");

I have furthermore tested all of these approaches, both in and out of the loop, with no success:

fflush(stdout);
fflush(NULL);
setbuf(stdout,NULL);

What is going on here, and how can I solve it?

Further Data:

Ubuntu Linux v16 as operating system.
Code::Blocks as IDE.
Problem only happens when running on Bash command line. Working OK inside the IDE.
Tested on Ubuntu Linux (same that runs the IDE) and FreeBSD (via cross compiling with clang).

try `sleep(1); fflush(stdout);` --> `fflush(stdout);sleep(1);` If this does not help, I suspect your compiler is non-compliant. — chux - Reinstate Monica, Apr 05 '19 at 15:29
@chux I think it's not compiler but the IDE console. Eclipse has these issues for years. — Eugene Sh., Apr 05 '19 at 15:32
@EugeneSh. Good point. Sopalajo de Arrierez, Try code outside IDE. — chux - Reinstate Monica, Apr 05 '19 at 15:35
To add on @EugeneSh. IMHO IDEs try to optimise their console output by only managing lines not characters. — Meher Khiari, Apr 05 '19 at 15:47
@AndrewHenle , I was emphasizing (now I see my question have been changed) the words that I consider are the core of the issue, so the text become more readable. Deleting my emphasizing could end by discouraging me on keeping this habit, that I think pretends to help others reading me. — Sopalajo de Arrierez, Apr 05 '19 at 16:08
@EugeneSh. : "tested" when bold, like the original bolded words on my question, were helping to make a quick read of the text. Simple "plain" and boring texts are not so confortable to read. — Sopalajo de Arrierez, Apr 05 '19 at 16:10
So, @EugeneSh. , maybe you could point some link to the correct emphasizing rules ? — Sopalajo de Arrierez, Apr 05 '19 at 16:16
@chux : (correcting myself) the problem only happens when running on `Bash` command line. Tested on Ubuntu Linux (same that runs the IDE) and FreeBSD v10 (via cross compiling with `clang`). Updated the question to reflect. — Sopalajo de Arrierez, Apr 05 '19 at 16:28
@SopalajodeArrierez It sounds like your _program_ is performing as desired. It is the shell (bash) that is buffering output. — chux - Reinstate Monica, Apr 05 '19 at 16:32
@chux: confirmed that `fflush(stdout);sleep(1);` makes no difference. Tested on both BSD and Linux. — Sopalajo de Arrierez, Apr 05 '19 at 16:34

score 0 · Answer 1 · answered Apr 05 '19 at 21:33

Well... a stupid issue, but I think I should write it down anyway, because someone could have it too:

The problem was: on the command-line testing, I appended some | more in order to stop the screen dumping. So the stdout buffer was altered by the piping | command.

Say my program binary file was named dotting. I called it by typing:

dotting | more

I hope this ridiculous case could help anyone in a future.

Buffer flushing not working for printf function

1 Answers1