From: Allison Karlitskaya <allison.karlitskaya@redhat.com>
Date: Tue, 28 Oct 2025 08:41:40 +0000 (+0100)
Subject: udevadm: flush output after each monitor event
X-Git-Tag: v259-rc1~233
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=516df12af9679ab25c39813d6787b0f044a53990;p=thirdparty%2Fsystemd.git

udevadm: flush output after each monitor event

If you're using `udevadm monitor` from a script, without a tty, then
libc defaults to being fully-buffered, and won't flush stdout after
newlines.  This is fine for tools that dump a bunch of data and then
exit immediately.  It's a problem for tools like `udevadm monitor` which
have long pauses: the buffered data can get stuck in the buffer for an
unbounded amount of time.

In the Cockpit project we've been working around this for some time with
`stdbuf` which is a `LD_PRELOAD` hack to change the libc buffering
behaviour, but we'd like to stop doing that.

Let's make sure we flush the buffer after each event.
---

diff --git a/src/udev/udevadm-monitor.c b/src/udev/udevadm-monitor.c
index 95fe65ff9d1..e729a99e95b 100644
--- a/src/udev/udevadm-monitor.c
+++ b/src/udev/udevadm-monitor.c
@@ -55,6 +55,8 @@ static int device_monitor_handler(sd_device_monitor *monitor, sd_device *device,
                 printf("\n");
         }
 
+        fflush(stdout);
+
         return 0;
 }