Why isn't '\b' working in the following piece of C code?

Question

Following is my code : -

#include <stdio.h>
#include <unistd.h>

int main()
{
    int i;
    printf("Continue in...");
    for (i = 10; i > 0 ; --i)
    {
        printf("%d",i);
        sleep(1);
        printf("\b");
    }
}

I am trying to make a countdown timer in C, so that only the value of i changes on the STDOUT, and the words "Continue in..." stay as it is on the shell(i.e at the same location of screen).

But the code above produces nothing for 10 seconds, rather just prints the string "Continue in..." after 10th second.

I read this answer which says that the behaviour of \b is terminal dependent. My question is what should I edit in my code to make its output independent from the output device.

behavior of `\b` depends on what is managing your output: terminal, console, serial line etc... — LPs, May 12 '17 at 13:39
Possible duplicate of [Usage of \b and \r in C](http://stackoverflow.com/questions/17236242/usage-of-b-and-r-in-c) — gsamaras, May 12 '17 at 13:40

score 3 · Accepted Answer · answered May 12 '17 at 13:40

The output of the printf function is typically line buffered. That means you won't see any output until a newline is printed.

If you call fflush(stdout), that will flush the output buffer so you can see the results.

printf("Continue in...");
fflush(stdout);
for (i = 10; i > 0 ; --i)
{
    printf("%d",i);
    fflush(stdout);
    sleep(1);
    printf("\b");
    if  (i == 10) printf("\b  \b");   // for 10 you need to erase 2 characters
}

score 0 · Answer 2 · answered May 12 '17 at 13:50

0

Alternately, call setvbuf(stdout, NULL, _IONBF, 0) at the start.

answered May 12 '17 at 13:50

Mischa

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score -1 · Answer 3 · edited May 12 '17 at 13:42

-1

You have to add \n in printf("%d",i); after %d . As buffer doesn't gets flushed with new line or until it is full.

edited May 12 '17 at 13:42

LPs

16,045
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answered May 12 '17 at 13:41

user7345878

492
2
7
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Buffering of `stdout` is implementation-specific. – Andrew Henle May 12 '17 at 14:06

Why isn't '\b' working in the following piece of C code?

3 Answers3