sd_notify man page on Archlinux

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SD_NOTIFY(3)			   sd_notify			  SD_NOTIFY(3)

       sd_notify, sd_notifyf - Notify service manager about start-up
       completion and other daemon status changes

       #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

	   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".

	   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..."

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

	   If a daemon fails, the D-Bus error-style error code. Example:

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

	   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

       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.

	   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"
			 (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"

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

systemd 212							  SD_NOTIFY(3)

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