The gd_vc_handler() callback is using g_malloc0() to allocate the CharDriverState struct. As a result the logfd field is getting initialized to 0, instead of -1 when no logfile is requested.
The result is that when running $ qemu-system-i386 -nodefaults -chardev vc,id=mon0 -mon chardev=mon0 qemu duplicates all monitor output to stdout as well as the GTK window. Not using qemu_chr_alloc() was already a bug, but harmless until this commit commit d0d7708ba29cbcc343364a46bff981e0ff88366f Author: Daniel P. Berrange <berra...@redhat.com> Date: Mon Jan 11 12:44:41 2016 +0000 qemu-char: add logfile facility to all chardev backends which exposed the problem as a behaviour regression Reported-by: Hervé Poussineau <hpous...@reactos.org> Signed-off-by: Daniel P. Berrange <berra...@redhat.com> --- ui/gtk.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/ui/gtk.c b/ui/gtk.c index ce7018e..c8dbd5c 100644 --- a/ui/gtk.c +++ b/ui/gtk.c @@ -1598,11 +1598,16 @@ static void gd_vc_chr_set_echo(CharDriverState *chr, bool echo) static int nb_vcs; static CharDriverState *vcs[MAX_VCS]; -static CharDriverState *gd_vc_handler(ChardevVC *unused, Error **errp) +static CharDriverState *gd_vc_handler(ChardevVC *vc, Error **errp) { + ChardevCommon *common = qapi_ChardevVC_base(vc); CharDriverState *chr; - chr = g_malloc0(sizeof(*chr)); + chr = qemu_chr_alloc(common, errp); + if (!chr) { + return NULL; + } + chr->chr_write = gd_vc_chr_write; chr->chr_set_echo = gd_vc_chr_set_echo; -- 2.5.0