sd_notify, sd_notifyf, sd_pid_notify, sd_pid_notifyf, sd_pid_notify_with_fds, sd_notify_barrier - Notify service manager about start-up completion and other service status changes
int sd_notify(int unset_environment, const char *state);
int sd_notifyf(int unset_environment, const char *format, ...);
int sd_pid_notify(pid_t pid, int unset_environment, const char *state);
int sd_pid_notifyf(pid_t pid, int unset_environment, const char *format, ...);
int sd_pid_notify_with_fds(pid_t pid, int unset_environment, const char *state, const int *fds, unsigned n_fds);
int sd_notify_barrier(int unset_environment, uint64_t timeout);
sd_notify() may be called by a service to notify the service manager about state 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" (i.e. "READY=0" is not defined).
sd_watchdog_enabled(3) for the details of how the service can check whether the watchdog is enabled.
It is recommended to prefix variable names that are not listed above with X_ to avoid namespace clashes.
Hence, to eliminate all race conditions involving lookup of the client's unit and attribution of notifications to units correctly, sd_notify_barrier() may be used. This call acts as a synchronization point and ensures all notifications sent before this call have been picked up by the service manager when it returns successfully. Use of sd_notify_barrier() is needed for clients which are not invoked by the service manager, otherwise this synchronization mechanism is unnecessary for attribution of notifications to the unit.
sd_notifyf() is similar to sd_notify() but takes a printf()-like format string plus arguments.
sd_pid_notify() and sd_pid_notifyf() are similar to sd_notify() and sd_notifyf() but take a process ID (PID) to use as originating PID for the message as first argument. This is useful to send notification messages on behalf of other processes, provided the appropriate privileges are available. If the PID argument is specified as 0, the process ID of the calling process is used, in which case the calls are fully equivalent to sd_notify() and sd_notifyf().
sd_pid_notify_with_fds() is similar to sd_pid_notify() but takes an additional array of file descriptors. These file descriptors are sent along the notification message to the service manager. This is particularly useful for sending "FDSTORE=1" messages, as described above. The additional arguments are a pointer to the file descriptor array plus the number of file descriptors in the array. If the number of file descriptors is passed as 0, the call is fully equivalent to sd_pid_notify(), i.e. no file descriptors are passed. Note that sending file descriptors to the service manager on messages that do not expect them (i.e. without "FDSTORE=1") they are immediately closed on reception.
sd_notify_barrier() allows the caller to synchronize against reception of previously sent notification messages and uses the "BARRIER=1" command. It takes a relative timeout value in microseconds which is passed to ppoll(2). A value of UINT64_MAX is interpreted as infinite timeout.
On failure, these calls return a negative errno-style error code. If $NOTIFY_SOCKET was not set and hence no status message could be sent, 0 is returned. If the status was sent, these functions return a positive value. In order to support both service managers that implement this scheme and those which do not, it is generally recommended to ignore the return value of this call. Note that the return value simply indicates whether the notification message was enqueued properly, it does not reflect whether the message could be processed successfully. Specifically, no error is returned when a file descriptor is attempted to be stored using FDSTORE=1 but the service is not actually configured to permit storing of file descriptors (see above).
Please note that this is a description of the notification technique as used by elogind. If the service manager you use does not set the needed environment variables, none of the functions described here do anything but return 0.
These APIs are implemented as a shared library, which can be compiled and linked to with the libelogind pkg-config(1) file.
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 service, using SCM_CREDENTIALS.
Example 1. Start-up Notification
When a service finished starting up, it might issue the following call to notify the service manager:
Example 2. Extended Start-up Notification
A service could send the following after completing initialization:
(unsigned long) getpid());
Example 3. Error Cause Notification
A service could send the following shortly before exiting, on failure:
sd_notifyf(0, "STATUS=Failed to start up: %s\n"
Example 4. Store a File Descriptor in the Service Manager
To store an open file descriptor in the service manager, in order to continue operation after a service restart without losing state, use "FDSTORE=1":
sd_pid_notify_with_fds(0, 0, "FDSTORE=1\nFDNAME=foobar", &fd, 1);
Example 5. Eliminating race conditions
When the client sending the notifications is not spawned by the service manager, it may exit too quickly and the service manager may fail to attribute them correctly to the unit. To prevent such races, use sd_notify_barrier() to synchronize against reception of all notifications sent before this call is made.
/* set timeout to 5 seconds */
sd_notify_barrier(0, 5 * 1000000);