sd_notify, sd_notifyf − Notify init system about start−up completion and other daemon status changes
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 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 an 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:
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 signalling 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: "BUSERROR=org.freedesktop.DBus.Error.TimedOut"
The main pid of the daemon, in case the init system did not fork off the process itself. Example: "MAINPID=4711"
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_notifyf() 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 don't, it is generally recommended to ignore the return value of this call.
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/tree/src/sd-daemon.c resp. http://cgit.freedesktop.org/systemd/tree/src/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−daemon pkg-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(7).
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.
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:
Example 2. Extended Start-up Notification
A daemon could send the following after completing initialization:
(unsigned long) getpid());
Example 3. Error Cause Notification
A daemon could send the following shortly before exiting, on failure
"STATUS=Failed to start up: %s\n"
Lennart Poettering <email@example.com>
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.