sd_notify (3)

#include <systemd/sd-daemon.h>

int sd_notify(int unset_environment, const char *state); int sd_notifyf(int unset_environment, const char *format, ...);

sd_notify() shall be called by a daemon to notify the init system about status changes. It can be used to send arbitrary information, encoded in an environment-block-like string. Most importantly it can be used for start-up completion notification.

If the unset_environment parameter is non-zero, sd_notify() will unset the $NOTIFY_SOCKET environment variable before returning (regardless of whether the function call itself succeeded or not). Further calls to sd_notify() will then fail, but the variable is no longer inherited by child processes.

The state parameter should contain a newline-separated list of variable assignments, similar in style to an environment block. A trailing newline is implied if none is specified. The string may contain any kind of variable assignments, but the following shall be considered well-known:

READY=1

Tells the init system that daemon startup is finished. This is only used by systemd if the service definition file has Type=notify set. The passed argument is a boolean "1" or "0". Since there is little value in signaling non-readiness, the only value daemons should send is "READY=1".

STATUS=...

Passes a single-line status string back to the init system that describes the daemon state. This is free-form and can be used for various purposes: general state feedback, fsck-like programs could pass completion percentages and failing programs could pass a human readable error message. Example: "STATUS=Completed 66% of file system check..."

ERRNO=...

If a daemon fails, the errno-style error code, formatted as string. Example: "ERRNO=2" for ENOENT.

BUSERROR=...

If a daemon fails, the D-Bus error-style error code. Example: "BUSERROR=org.freedesktop.DBus.Error.TimedOut"

MAINPID=...

The main pid of the daemon, in case the init system did not fork off the process itself. Example: "MAINPID=4711"

WATCHDOG=1

Tells systemd to update the watchdog timestamp. This is the keep-alive ping that services need to issue in regular intervals if WatchdogSec= is enabled for it. See systemd.service(5) for details. It is recommended to send this message if the $WATCHDOG_PID environment variable has been set to the PID of the service process, in every half the time interval that is specified in the $WATCHDOG_USEC environment variable. See sd_watchdog_enabled(3) for details.

It is recommended to prefix variable names that are not shown in the list above with X_ to avoid namespace clashes.

Note that systemd will accept status data sent from a daemon only if the NotifyAccess= option is correctly set in the service definition file. See systemd.service(5) for details.

sd_notifyf() is similar to sd_notify() but takes a printf()-like format string plus arguments.

On failure, these calls return a negative errno-style error code. If $NOTIFY_SOCKET was not set and hence no status data could be sent, 0 is returned. If the status was sent, these functions return with a positive return value. In order to support both, init systems that implement this scheme and those which do not, it is generally recommended to ignore the return value of this call.

These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.

Internally, these functions send a single datagram with the state string as payload to the AF_UNIX socket referenced in the $NOTIFY_SOCKET environment variable. If the first character of $NOTIFY_SOCKET is "@", the string is understood as Linux abstract namespace socket. The datagram is accompanied by the process credentials of the sending daemon, using SCM_CREDENTIALS.

$NOTIFY_SOCKET

Set by the init system for supervised processes for status and start-up completion notification. This environment variable specifies the socket sd_notify() talks to. See above for details.

Example 1. Start-up Notification

When a daemon finished starting up, it might issue the following call to notify the init system:

sd_notify(0, "READY=1");

Example 2. Extended Start-up Notification

A daemon could send the following after completing initialization:

sd_notifyf(0, "READY=1\n"
              "STATUS=Processing requests...\n"
              "MAINPID=%lu",
              (unsigned long) getpid());

Example 3. Error Cause Notification

A daemon could send the following shortly before exiting, on failure

sd_notifyf(0, "STATUS=Failed to start up: %s\n"
              "ERRNO=%i",
              strerror(errno),
              errno);

systemd(1), sd-daemon(3), daemon(7), systemd.service(5), sd_watchdog_enabled(3)