Ho to write logs in Docker container

Question

I made most simple C application which just printing a message:

#include <stdio.h>
#include <unistd.h>
int main(){
    int i = 0;
    while(1) {
        sleep(1);
        printf("sleeping ... %d\n", i++);
    }
}

Then, made a simplest possible Dockerfile for it:

FROM fedora

USER root
WORKDIR /root

ADD sleep-play .

CMD ["/root/sleep-play"]

Then, run that container as simple as it could:

sudo docker build -t sleeping .
sudo docker run --name sleeping -d sleeping

Finally I'm expecting to see my stdout output, but nothing happens:

sudo docker logs -f sleeping
## nothing shown!

From here:

... By default, docker logs shows the command’s STDOUT and STDERR.

What I'm doing wrong?

What happend when you try to start the container in the foreground without "-d"? — Hannes, Dec 20 '17 at 16:47
@hannes91 exactly the same, just nothing! But the executable going well in host environment. — Ivan Talalaev, Dec 20 '17 at 16:50

score 1 · Accepted Answer · answered Dec 20 '17 at 17:04

I'm not quite sure, my best guess is that printf is buffered and maybe the buffer will not flush automaticly here.

Try to flush it manually:

fflush(stdout)

See also this related Stackoverflow entry for flushing manual printf statements: write() to stdout and printf output not interleaved?

score 1 · Answer 2 · answered Dec 20 '17 at 17:23

I reproduced it and figured out that the sleep command is your problem. At the moment i dont know the exactly cause of this but without the sleep command it runs.

Maybe this article can help you: https://github.com/moby/moby/issues/2838#issuecomment-194054757

Ho to write logs in Docker container

2 Answers2