Why won't my program output when redirected to a file?

content.md

The problem

• Output not being output when redirecting output to a file

The cause

• When stdout points to a file, it gets buffered by libc into a 4k buffer
• Terminal output is line buffered, stderr is always unbuffered

The fix

• fflush in the program - what if we don't have the source?
• stdbuf -oL command - forces line buffering mode
• unbuffer command - part of expect, runs the command in a pty

See more

• http://www.pixelbeat.org/programming/stdio_buffering/
• http://unix.stackexchange.com/questions/25372/turn-off-buffering-in-pipe

test-fixed.c

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

int main() {
	for(int i = 0;; ++i) {
		printf("%d\n", i);
		fflush(stdout);
		sleep(1);
	}
}

test.c

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

int main() {
	for(int i = 0;; ++i) {
		printf("%d\n", i);
		sleep(1);
	}
}