Re: [Qemu-devel] [PATCH] os-posix: Log to logfile in case of daemonize

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/02/2016 13:12, Dimitris Aragiorgis wrote:
> Besides that, when one executes a daemon, shell redirection is
> hardly, if ever, used. More so if the daemon already has a logfile
> option.
> 
> So, we decided to give it a go and find the least painful way to
> log the stderr of a QEMU process to a logfile.
> 
> To our understanding, the logfile (-D option) is used only for
> messages generated by qemu_log()/qemu_log_mask(). The current
> situation however is that fprintf(stderr, ...) is used in various
> places throughout the codebase for logging/debug purposes.

Right, mostly through error_report.

Actually I like your approach (log to -D if daemonize is used).  I
just was not sure of the best way to implement it.

Perhaps when the logfile is opened you can replace the straight fopen with

    qemu_logfile = fopen(...);
    if (daemonized) {
        dup2(fileno(qemu_logfile), STDERR_FILENO);
        fclose(qemu_logfile);
        qemu_logfile = stderr;
    }

Then the logfile will never be closed by qemu_log_close, and stderr
will always be sent to it.  Does this look sane?

Paolo