fork() not giving desired result

anshu33331 · 07-29-2009, 12:08 PM

main()
{
printf("LINUX");
fork();
printf("UNIX");
}

expected result is LINUX UNIX UNIX.
actual result: LINUX UNIX LINUX UNIX

But if i write
printf("LINUX\n"); in line 3

it is giving expected output.

can anybody give the justified answer..

paulsm4 · 07-29-2009, 12:56 PM

It's called "buffering".

"Printf()" doesn't necessarily write to the screen as soon as you call it; the I/O was "buffered" until later in the program (as it happens, until *after* you called "fork()" in the first program).

catkin · 07-29-2009, 01:08 PM

Quote:

Originally Posted by anshu33331

main()
{
printf("LINUX");
fork();
printf("UNIX");
}

expected result is LINUX UNIX UNIX.
actual result: LINUX UNIX LINUX UNIX

But if i write
printf("LINUX\n"); in line 3

it is giving expected output.

can anybody give the justified answer..

It's caused by buffering. Lines are only written to the terminal when a newline is encountered or when the process exits. The parent writes LINUX to buffer, forks (giving the child a copy of its unflushed buffer containing LINUX), writes UNIX to buffer and then exits, flushing the buffer containing LINUX UNIX to terminal. The child writes UNIX to the buffer already containing LINUX then exits, flushing the buffer containing LINUX UNIX to terminal. See http://www.linuxforums.org/forum/147741-post6.html for a fuller explanation including how to avoid the "problem".

Best

Charles

fantas · 07-29-2009, 01:37 PM

using

Code:

fprintf(stderr, "LINUX");

instead of

Code:

printf("LINUX");

should give you the expected results, as printing to stderr is by default unbuffered.

David1357 · 07-29-2009, 01:48 PM

Quote:

Originally Posted by anshu33331

can anybody give the justified answer..

This has been answered to death, but here's some sample code:

Code:

[user@machine:~]:cat blah.c
#include <stdio.h>

int main(void)
{
    fprintf(stderr, "LINUX\n");
    fork();
    fprintf(stderr, "UNIX\n");
}

[user@machine:~]:./blah
LINUX
UNIX
UNIX