How to guarantee the stdout buffer between systemd and journald is flushed when program quits?

[zerkms@xxxxxxxxx (Ivan Kurnosov)]

Hi,

I originally asked this question on the stackoverflow (
https://stackoverflow.com/q/50400145/251311) but I feel I wouldn't get an
answer there.

The simplified example that demonstrates a problem is

    #include <stdio.h>

    int main(void)
    {
        printf("foo\n");
        fflush(stdout);
        getchar();
        return 0;
    }

(it's not programming language specific though - the original application I
discovered the problem with is in Go).

If it's compiled and is run by the following unit:

    [Unit]
    Description=description
    After=network.target

    [Service]
    ExecStart=/path/to/binary

    [Install]
    WantedBy=multi-user.target

then on `systemctl restart` `journald` would in most cases not get the
`foo\n` output, while `syslog` successfully writes it down to the
`/var/log/syslog`.

Here is the example of the `journald` output for the service:

    May 18 08:30:38 hostname systemd[1]: Stopped servicename
    May 18 08:30:38 hostname systemd[1]: Started servicename
    May 18 08:30:38 hostname systemd[1]: Stopped servicename
    May 18 08:30:38 hostname systemd[1]: Started servicename
    May 18 08:30:38 hostname servicename[7701]: foo
    May 18 08:30:41 hostname systemd[1]: Stopped servicename
    May 18 08:30:41 hostname systemd[1]: Started servicename
    May 18 08:30:46 hostname systemd[1]: Stopped servicename
    May 18 08:30:46 hostname systemd[1]: Started servicename

and then the corresponding part of the `/var/log/syslog`:

    May 18 08:30:38 hostname systemd[1]: Stopped servicename.
    May 18 08:30:38 hostname systemd[1]: Started servicename.
    May 18 08:30:38 hostname servicename[7682]: foo
    May 18 08:30:38 hostname systemd[1]: Stopped servicename.
    May 18 08:30:38 hostname systemd[1]: Started servicename.
    May 18 08:30:38 hostname servicename[7701]: foo
    May 18 08:30:41 hostname systemd[1]: Stopped servicename.
    May 18 08:30:41 hostname systemd[1]: Started servicename.
    May 18 08:30:41 hostname servicename[7720]: foo
    May 18 08:30:46 hostname systemd[1]: Stopped servicename.
    May 18 08:30:46 hostname systemd[1]: Started servicename.
    May 18 08:30:46 hostname servicename[7739]: foo

What would be a way to **guarantee** journald received the `stdout`, if any?

-- 
With best regards, Ivan Kurnosov
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