printf newline has line buffer?

Question

Language: C, OS: Linux

Code:

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>

int main(void)
{
    fork();
    printf("hello world\n");
    fork();
    printf("bye\n");
    return 0;
}

Output:

hello world
bye
hello world
bye
hello world
bye
hello world
bye

According to this and this, printf() buffers output until a newline is encountered.

So why does we have 4 "hello world" in this case? (instead of 2 "hello world")

Edit: Sorry all, but like @GregHewgill said, I running this program from an environment where the output cannot be directly to the terminal, when I check it again on my computer, it just run as expected.

Line buffering is only one *possible* strategy `printf` can implement. The only sure way to flush an output stream is `fflush`. — StoryTeller - Unslander Monica, Mar 27 '17 at 17:24
"So why does we have 4 "hello world" in this case?" --> What output were you expecting? — chux - Reinstate Monica, Mar 27 '17 at 17:26
@chux, They are expecting the output to have been sent to disk before the second fork, and thus only get two instances of the message — ikegami, Mar 27 '17 at 17:27
Can't reproduce on Ubuntu 12 (gcc 4.6.3) unless I redirect the output (e.g. `a | cat`). This switches from line buffering to block buffering. — ikegami, Mar 27 '17 at 17:30
Are you running this from a shell command line, or are you running it in some environment where the output may not be *directly* to a terminal device? — Greg Hewgill, Mar 27 '17 at 17:31
Sorry all, but like @GregHewgill said, I running this program from an environment where the output cannot be directly to the terminal, when I check it again on my computer, it just run as expected. — lzutao, Mar 27 '17 at 18:00

score 2 · Answer 1 · answered Mar 27 '17 at 17:31

According to this and this, printf() buffers output until a newline is encountered.

Printing a newline usually flushes only if the output goes to a terminal device. For example:

$ ./a.out >out_file

will not flush the buffer even with the newline character. So, your expectation is flawed.

The only right way to get "desired" output (2 hello world and 4 bye) is to either disable buffering totally using setbuf:

setbuf(stdout, 0);

or use fflush:

fflush(stdout);

after each printf call to flush explicitly.

It should be sufficient to ensure that the stream is line-buffered, which can be done via `setvbuf()`. It should not be necessary to disable buffering altogether, though perhaps that would be safer. — John Bollinger, Mar 27 '17 at 17:40

printf newline has line buffer?

1 Answers1