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If you are developing daemons to run on OS X, it is highly recommended that you design your daemons to be launchd compliant. Using launchd provides better performance and flexibility for daemons. It also improves the ability of administrators to manage the daemons running on a given system.

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If you are running per-user background processes for OS X, launchd is also the preferred way to start these processes. These per-user processes are referred to as user agents. A user agent is essentially identical to a daemon, but is specific to a given logged-in user and executes only while that user is logged in.

Unless otherwise noted, for the purposes of this chapter, the terms 'daemon' and 'agent' can be used interchangeably. Thus, the term 'daemon' is used generically in this section to encompass both system-level daemons and user agents except where otherwise noted.

There are four ways to launch daemons using launchd. The preferred method is on-demand launching, but launchd can launch daemons that run continuously, and can replace inetdfor launching inetd-style daemons. In addition, launchd can start jobs at timed intervals.

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Although launchd supports non-launch-on-demand daemons, this use is not recommended. The launchd daemon was designed to remove the need for dependency ordering among daemons. If you do not make your daemon be launched on demand, you will have to handle these dependencies in another way, such as by using the legacy startup item mechanism.

Launching Custom Daemons Using launchd

With the introduction of launchd in OS X v10.4, an effort was made to improve the steps needed to launch and maintain daemons. What launchd provides is a harness for launching your daemon as needed. To client programs, the port representing your daemon's service is always available and ready to handle requests. In reality, the daemon may or may not be running. When a client sends a request to the port, launchd may have to launch the daemon so that it can handle the request. Once launched, the daemon can continue running or shut itself down to free up the memory and resources it holds. If a daemon shuts itself down, launchd once again relaunches it as needed to process requests.

In addition to the launch-on-demand feature, launchd provides the following benefits to daemon developers:

• Simplifies the process of making a daemon by handling many of the standard housekeeping chores normally associated with launching a daemon.

• Provides system administrators with a central place to manage daemons on the system.

• Supports inetd-style daemons.

• Eliminates the primary reason for running daemons as root. Because launchd runs as root, it can create low-numbered TCP/IP listen sockets and hand them off to the daemon.

• Simplifies error handling and dependency management for inter-daemon communication. Old minecraft folder. Because daemons launch on demand, communication requests do not fail if the daemon is not launched. They are simply delayed until the daemon can launch and process them.

The launchd Startup Process

After the system is booted and the kernel is running, launchd is run to finish the system initialization. As part of that initialization, it goes through the following steps:

1. It loads the parameters for each launch-on-demand system-level daemon from the property list files found in /System/Library/LaunchDaemons/ and /Library/LaunchDaemons/.

2. It registers the sockets and file descriptors requested by those daemons.

3. It launches any daemons that requested to be running all the time.

4. As requests for a particular service arrive, it launches the corresponding daemon and passes the request to it.

5. When the system shuts down, it sends a SIGTERM signal to all of the daemons that it started.

The process for per-user agents is similar. When a user logs in, a per-user launchd is started. It does the following:

1. It loads the parameters for each launch-on-demand user agent from the property list files found in /System/Library/LaunchAgents, /Library/LaunchAgents, and the user's individual Library/LaunchAgents directory.

2. It registers the sockets and file descriptors requested by those user agents.

3. It launches any user agents that requested to be running all the time.

4. As requests for a particular service arrive, it launches the corresponding user agent and passes the request to it.

5. When the user logs out, it sends a SIGTERM signal to all of the user agents that it started.

Because launchd registers the sockets and file descriptors used by all daemons before it launches any of them, daemons can be launched in any order. If a request comes in for a daemon that is not yet running, the requesting process is suspended until the target daemon finishes launching and responds.

If a daemon does not receive any requests over a specific period of time, it can choose to shut itself down and release the resources it holds. When this happens, launchd monitors the shutdown and makes a note to launch the daemon again when future requests arrive.

Important: If your daemon shuts down too quickly after being launched, launchd may think it has crashed. Daemons that continue this behavior may be suspended and not launched again when future requests arrive. To avoid this behavior, do not shut down for at least 10 seconds after launch.

Creating a launchd Property List File

To run under launchd, you must provide a configuration property list file for your daemon. This file contains information about your daemon, including the list of sockets or file descriptors it uses to process requests. Specifying this information in a property list file lets launchd register the corresponding file descriptors and launch your daemon only after a request arrives for your daemon's services. Table 5-1 lists the required and recommended keys for all daemons.

The property list file is structured the same for both daemons and agents. You indicate whether it describes a daemon or agent by the directory you place it in. Property list files describing daemons are installed in /Library/LaunchDaemons, and those describing agents are installed in /Library/LaunchAgents or in the LaunchAgents subdirectory of an individual user's Library directory. (The appropriate location for executables that you launch from your job is /usr/local/libexec.)

For more information: For a complete listing of the keys, see the launchd.plist manual page.

For sample configuration property lists, look at the files in /System/Library/LaunchDaemons/. These files are used to configure many daemons that run on OS X.

Writing a 'Hello World!' launchd Job

The following simple example launches a daemon named hello, passing world as a single argument, and instructs launchd to keep the job running:

In this example, there are three keys in the top level dictionary. The first is Label, which uniquely identifies the job. when. The second is ProgramArguments which has a value of an array of strings which represent the tokenized arguments and the program to run. The third and final key is KeepAlive which indicates that this job needs to be running at all times, rather than the default launch-on-demand behavior, so launchd should always try to keep this job running.

Listening on Sockets

You can also include other keys in your configuration property list file. For example, if your daemon monitors a well-known port (one of the ports listed in /etc/services), add a Sockets entry as follows:

The string for SockServiceName typically comes from the leftmost column in /etc/services. The SockType is one of dgram (UDP) or stream (TCP/IP). If you need to pass a port number that is not listed in the well-known ports list, the format is the same, except the string contains a number instead of a name. For example:

Debugging launchd Jobs

There are some options that are useful for debugging your launchd job.

The following example enables core dumps, sets standard out and error to go to a log file, and instructs launchd to temporarily increase the debug level of its logging while acting on behalf of your job (remember to adjust your syslog.conf accordingly):

Running a Job Periodically

The following example creates a job that is run every five minutes (300 seconds):

Alternately, you can specify a calendar-based interval. The following example starts the job on the 7th day of every month at 13:45 (1:45 pm). Like the Unix cron subsystem, any missing key of the StartCalendarInterval dictionary is treated as a wildcard—in this case, the month is omitted, so the job is run every month.

Monitoring a Directory

The following example starts the job whenever any of the paths being watched have changed:

An additional file system trigger is the notion of a queue directory. The launchd daemon starts your job whenever the given directories are non-empty, and it keeps your job running as long as those directories are not empty:

Emulating inetd

The launchd daemon emulates the older inetd-style daemon semantics if you provide the inetdCompatibility key:

Behavior for Processes Managed by launchd

Processes that are managed by launchd must follow certain requirements so that they interact properly with launchd. This includes launch daemons and launch agents.

Required Behaviors

To support launchd, you must obey the following guidelines when writing your daemon code:

• You must provide a property list with some basic launch-on-demand criteria for your daemon. See Creating a launchd Property List File.

• You must not daemonize your process. This includes calling the daemon function, calling fork followed by exec, or calling fork followed by exit. If you do, launchd thinks your process has died. Depending on your property list key settings, launchd will either keep trying to relaunch your process until it gives up (with a 'respawning too fast' error message) or will be unable to restart it if it really does die.

• Daemons and agents that are installed globally must be owned by the root user. Agents installed for the current user must be owned by that user. All daemons and agents must not be group writable or world writable. (That is, they must have file mode set to 600 or 400.)

Recommended Behaviors

To support launchd, it is recommended that you obey the following guidelines when writing your daemon code:

• Wait until your daemon is fully initialized before attempting to process requests. Your daemon should always provide a reasonable response (rather than an error) when processing requests.

• Register the sockets and file descriptors used by your daemon in your launchd configuration property list file.

• If your daemon advertises a socket, check in with launchd as part of your daemon initialization. For an example implementation of the check-in process, see SampleD.

• During check-in, get the launch dictionary from launchd, extract and store its contents, and then discard the dictionary. Accessing the data structure used for the dictionary is very slow, so storing the whole dictionary locally and accessing it frequently could hurt performance.

• Provide a handler to catch the SIGTERM signal.

In addition to the preceding list, the following is a list of things it is recommended you avoid in your code:

• Do not set the user or group ID for your daemon. Include the UserName, UID, GroupName, or GID keys in your daemon's configuration property list instead.

• Do not set the working directory. Include the WorkingDirectory key in your daemon's configuration property list instead.

• Do not call chroot to change the root directory. Include the RootDirectory key in your daemon's configuration property list instead.

• Do not call setsid to create a new session.

• Do not close any stray file descriptors.

• Do not change stdio to point to /dev/null. Include the StandardOutPath or StandardErrorPath keys in your daemon's configuration property list file instead.

• Do not set up resource limits with setrusage.

• Do not set the daemon priority with setpriority

Although many of the preceding behaviors may be standard tasks for daemons to perform, they are not recommended when running under launchd. The reason is that launchd configures the operating environment for the daemons that it manages. Changing this environment could interfere with the normal operation of your daemon.

Deciding When to Shut Down

If you do not expect your daemon to handle many requests, you might want to shut it down after a predetermined amount of idle time, rather than continue running. Although a well-written daemon does not consume any CPU resources when idle, it still consumes memory and could be paged out during periods of intense memory use.

The timing of when to shut down is different for each daemon and depends on several factors, including:

• The number and frequency of requests it receives

• The time it takes to launch the daemon

• The time it takes to shut down the daemon

• The need to retain state information

If your daemon does not receive frequent requests and can be launched and shut down quickly, you might prefer to shut it down rather than wait for it to be paged out to disk. Paging memory to disk, and subsequently reading it back, incurs two disk operations. If you do not need the data stored in memory, your daemon can shut down and avoid the step of writing memory to disk.

Special Dependencies

While launchd takes care of dependencies between daemons, in some cases, your daemon may depend on other system functionality that cannot be addressed in this manner. This section describes many of these special cases and how to handle them.

Network Availability

If your daemon depends on the network being available, this cannot be handled with dependencies because network interfaces can come and go at any time in OS X. To solve this problem, you should use the network reachability functionality or the dynamic store functionality in the System Configuration framework. This is documented in System Configuration Programming Guidelines and System Configuration Framework Reference. For more information about network reachability, see Determining Reachability and Getting Connected in System Configuration Programming Guidelines.

Disk or Server Availability

If your daemon depends on the availability of a mounted volume (whether local or remote), you can determine the status of that volume using the Disk Arbitration framework. This is documented in Disk Arbitration Framework Reference.

Non-launchd Daemons

If your daemon has a dependency on a non-launchd daemon, you must take additional care to ensure that your daemon works correctly if that non-launchd daemon has not started when your daemon is started. The best way to do this is to include a loop at start time that checks to see if the non-launchd daemon is running, and if not, sleeps for several seconds before checking again.

Be sure to set up handlers for SIGTERM prior to this loop to ensure that you are able to properly shut down if the daemon you rely on never becomes available.

User Logins

In general, a daemon should not care whether a user is logged in, and user agents should be used to provide per-user functionality. However, in some cases, this may be useful.

To determine what user is logged in at the console, you can use the System Configuration framework, as described in Technical Q&A QA1133.

Kernel Extensions

If your daemon requires that a certain kernel extension be loaded prior to executing, you have two options: load it yourself, or wait for it to be loaded.

The daemon may manually request that an extension be loaded. To do this, run kextload with the appropriate arguments using exec or variants thereof. I/O Kit kernel extensions should not be loaded with kextload; the I/O Kit will load them automatically when they are needed.

Note: The kextload executable must be run as root in order to load extensions into the kernel. For security reasons, it is not a setuid executable. This means that your daemon must either be running as the root user or must include a helper binary that is setuid root in order to use kextload to load a kernel extension.

Alternatively, our daemon may wait for a kernel service to be available. To do this, you should first register for service change notification. This is further documented in I/O Kit Framework Reference.

After registering for these notifications, you should check to see if the service is already available. By doing this after registering for notifications, you avoid waiting forever if the service becomes available between checking for availability and registering for the notification.

Note: In order for your kernel extension to be detected in a useful way, it must publish a node in the I/O registry to advertise the availability of its service. For I/O Kit drivers, this is usually handled by the I/O Kit family.

For other kernel extensions, you must explicitly register the service by publishing a nub, which must be an instance of IOService.

For more information about I/O Kit services and matching, see IOKit Fundamentals, I/O Kit Framework Reference (user space reference), and Kernel Framework Reference (kernel space reference).

For More Information

The manual pages for launchd and launchd.plist are the two best sources for information about launchd.

In addition, you can find a source daemon accompanying the launchd source code (available from http://www.macosforge.org/). This daemon is also provided from the Mac Developer Library as the SampleD sample code project.

The Daemons and Agents technical note provides additional information about how launchd daemons and agents work under the hood.

Finally, many Apple-provided daemons support launchd. Their property list files can be found in /System/Library/LaunchDaemons. Some of these daemons are also available as open source from http://www.opensource.apple.com/ or http://www.macosforge.org/.

Copyright © 2003, 2016 Apple Inc. All Rights Reserved. Terms of Use | Privacy Policy | Updated: 2016-09-13

In multitasking computer operating systems, a daemon (/ˈdiːmən/ or /ˈdeɪmən/)^[1] is a computer program that runs as a background process, rather than being under the direct control of an interactive user. Traditionally, the process names of a daemon end with the letter d, for clarification that the process is in fact a daemon, and for differentiation between a daemon and a normal computer program. For example, syslogd is a daemon that implements system logging facility, and sshd is a daemon that serves incoming SSH connections.

In a Unix environment, the parent process of a daemon is often, but not always, the init process. A daemon is usually created either by a process forking a child process and then immediately exiting, thus causing init to adopt the child process, or by the init process directly launching the daemon. In addition, a daemon launched by forking and exiting typically must perform other operations, such as dissociating the process from any controlling terminal (tty). Such procedures are often implemented in various convenience routines such as daemon(3) in Unix.

Systems often start daemons at boot time that will respond to network requests, hardware activity, or other programs by performing some task. Daemons such as cron may also perform defined tasks at scheduled times.

Terminology[edit]

The term was coined by the programmers at MIT's Project MAC. They took the name from Maxwell's demon, an imaginary being from a thought experiment that constantly works in the background, sorting molecules.^[2]Unix systems inherited this terminology. Maxwell's demon is consistent with Greek mythology's interpretation of a daemon as a supernatural being working in the background, with no particular bias towards good or evil. However, BSD and some of its derivatives have adopted a Christian demon as their mascot rather than a Greek daemon.^{[citation needed]}

The word daemon is an alternative spelling of demon,^[3] and is pronounced /ˈdiːmən/DEE-mən. In the context of computer software, the original pronunciation /ˈdiːmən/ has drifted to /ˈdeɪmən/DAY-mən for some speakers.^[1]

Alternative terms for daemon are service (used in Windows, from Windows NT onwards — and later also in Linux), started task (IBM z/OS),^[4] and ghost job (XDS UTS).

After the term was adopted for computer use, it was rationalized as a 'backronym' for Disk And Execution MONitor.^[5]

Daemons that connect to a computer network are examples of network services.

Implementations[edit]

Unix-like systems[edit]

In a strictly technical sense, a Unix-like system process is a daemon when its parent process terminates and the daemon is assigned the init process (process number 1) as its parent process and has no controlling terminal. However, more generally, a daemon may be any background process, whether a child of the init process or not.

On a Unix-like system, the common method for a process to become a daemon, when the process is started from the command line or from a startup script such as an init script or a SystemStarter script, involves:

• Optionally removing unnecessary variables from environment.
• Executing as a background task by forking and exiting (in the parent 'half' of the fork). This allows daemon's parent (shell or startup process) to receive exit notification and continue its normal execution.
• Detaching from the invoking session, usually accomplished by a single operation, setsid():
  • Dissociating from the controlling tty.
  • Creating a new session and becoming the session leader of that session.
  • Becoming a process group leader.
• If the daemon wants to ensure that it will not acquire a new controlling tty even by accident (which happens when a session leader without a controlling tty opens a free tty), it may fork and exit again. This means that it is no longer a session leader in the new session, and cannot acquire a controlling tty.
• Setting the root directory (/) as the current working directory so that the process does not keep any directory in use that may be on a mounted file system (allowing it to be unmounted).
• Changing the umask to 0 to allow open(), creat(), and other operating system calls to provide their own permission masks and not to depend on the umask of the caller.
• Redirecting file descriptors 0, 1 and 2 for the standard streams (stdin, stdout and stderr) to /dev/null or a logfile, and closing all the other file descriptors inherited from the parent process.

If the process is started by a super-server daemon, such as inetd, launchd, or systemd, the super-server daemon will perform those functions for the process,^[6][7][8] except for old-style daemons not converted to run under systemd and specified as Type=forking^[8] and 'multi-threaded' datagram servers under inetd.^[6]

MS-DOS[edit]

In the Microsoft DOS environment, daemon-like programs were implemented as terminate and stay resident (TSR) software.

Windows NT[edit]

On Microsoft Windows NT systems, programs called Windows services perform the functions of daemons. They run as processes, usually do not interact with the monitor, keyboard, and mouse, and may be launched by the operating system at boot time. In Windows 2000 and later versions, Windows services are configured and manually started and stopped using the Control Panel, a dedicated control/configuration program, the Service Controller component of the Service Control Manager (sc command), the net start and net stop commands or the PowerShell scripting system.

However, any Windows application can perform the role of a daemon, not just a service, and some Windows daemons have the option of running as a normal process.

Classic Mac OS and macOS[edit]

On the classic Mac OS, optional features and services were provided by files loaded at startup time that patched the operating system; these were known as system extensions and control panels. Later versions of classic Mac OS augmented these with fully fledged faceless background applications: regular applications that ran in the background. To the user, these were still described as regular system extensions.

macOS, which is a Unix system, uses daemons. Note that macOS uses the term 'services' to designate software that performs functions selected from the Services menu, rather than using that term for daemons as Windows does.

Etymology[edit]

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According to Fernando J. Corbató, who worked on Project MAC in 1963, his team was the first to use the term daemon, inspired by Maxwell's demon, an imaginary agent in physics and thermodynamics that helped to sort molecules, stating, 'We fancifully began to use the word daemon to describe background processes that worked tirelessly to perform system chores'.^[9]

In the general sense, daemon is an older form of the word 'demon', from the Greek δαίμων. In the Unix System Administration HandbookEvi Nemeth states the following about daemons:^[10]

Many people equate the word 'daemon' with the word 'demon', implying some kind of satanic connection between UNIX and the underworld. This is an egregious misunderstanding. 'Daemon' is actually a much older form of 'demon'; daemons have no particular bias towards good or evil, but rather serve to help define a person's character or personality. The ancient Greeks' concept of a 'personal daemon' was similar to the modern concept of a 'guardian angel'—eudaemonia is the state of being helped or protected by a kindly spirit. As a rule, UNIX systems seem to be infested with both daemons and demons.

Jacobsen lawn prince lawnmower service manual. A further characterization of the mythological symbolism is that a daemon is something that is not visible yet is always present and working its will. In the Theages, attributed to Plato, Socrates describes his own personal daemon to be something like the modern concept of a moral conscience: 'The favour of the gods has given me a marvelous gift, which has never left me since my childhood. It is a voice that, when it makes itself heard, deters me from what I am about to do and never urges me on'.^{[citation needed]}

See also[edit]

• Bounce message (also known as mailer daemon)

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References[edit]

1. ^ ^abEric S. Raymond. 'daemon'. The Jargon File. Retrieved 2008-10-22.
2. ^Fernando J. Corbató (2002-01-23). 'Take Our Word for It'. Retrieved 2006-08-20.
3. ^'Merriam-Webster definition of daemon'. Merriam-Webster Online. Retrieved 2009-08-05.
4. ^'IBM Knowledge Center - Glossary of z/OS terms and abbreviations'. IBM.
5. ^'Daemon Definition'. www.linfo.org.
6. ^ ^abinetd(8) – FreeBSD System Manager's Manual
7. ^launchd.plist(5) – Darwin and macOS File Formats Manual
8. ^ ^ab'systemd.service'. freedesktop.org. Retrieved August 25, 2012.
9. ^'The Origin of the word Daemon'.
10. ^'The BSD Daemon'. Freebsd.org. Retrieved 2008-11-15.

External links[edit]

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