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.

Answer 1

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.