Ubuntu Manpage: sd_notify, sd_notifyf - Notify service manager about start-up completion and other daemon

Provided by: systemd_204-5ubuntu20.31_amd64

NAME

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

SYNOPSIS

       #include <systemd/sd-daemon.h>

       int sd_notify(int unset_environment, const char *state);

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

DESCRIPTION

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 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_USEC= environment variable has been set for the service process, in every
half the time interval that is specified in the variable.

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.

RETURN VALUE

       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 don't, it is generally recommended to
       ignore the return value of this call.

NOTES

These functions are provided by the reference implementation of APIs for new-style daemons
and distributed with the systemd package. The algorithms they implement are simple, and
can easily be reimplemented in daemons if it is important to support this interface
without using the reference implementation.

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.

For details about the algorithms check the liberally licensed reference implementation
sources:
http://cgit.freedesktop.org/systemd/systemd/plain/src/libsystemd-daemon/sd-daemon.c and
http://cgit.freedesktop.org/systemd/systemd/plain/src/systemd/sd-daemon.h

sd_notify() and sd_notifyf() are implemented in the reference implementation's sd-daemon.c
and sd-daemon.h files. These interfaces are available as shared library, which can be
compiled and linked to with the libsystemd-daemonpkg-config(1) file. Alternatively,
applications consuming these APIs may copy the implementation into their source tree. For
more details about the reference implementation see sd-daemon(3).

If the reference implementation is used as drop-in files and -DDISABLE_SYSTEMD is set
during compilation these functions will always return 0 and otherwise become a NOP.

ENVIRONMENT

       $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.

EXAMPLES

       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);