print barcode in asp.net c# 2: Services, Daemons, and Processes in Objective-C

Creator ECC200 in Objective-C 2: Services, Daemons, and Processes

CHAPTER 2: Services, Daemons, and Processes
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When you manually start a program or script (or the computer starts the program, because it s been configured to do so), you re launching a process, which is any running application that takes up resources on the system. That process may consist of a single entity or a collection of entities, all working together in what are called threads. A thread is a stream of instructions that the computer can perform in sequence. A process can also have other processes that it invokes, known as child processes. Because you cannot have children without parents, the originating process is called the parent process. Most modern operating systems, such as Mac OS X, can run multiple processes simultaneously, because they are multi-threaded operating systems. If the process is running in the background and isn t directly under your control, it s often referred to as a daemon. Common daemons running on an OS X system include cupsd (for printing), syslogd (for logging), and blued (for interfacing with Bluetooth). Although the daemons running on your system often may have names ending in the letter d, they don't always. Daemons interact more often with other programs than they do with the user, and they sometimes have additional controller programs (in Mac OS X, many of them rely on scripts) that are used to provide various instructions to the daemon. NOTE: Whether an item is a daemon or a process can be a bit of a complicated topic. Many daemons can be run in non-daemon modes (interactively). You can take most programs or scripts and convert them to daemons to have them run in the background. The definitions we ve provided here for processes and daemons work with most of the Unix family of operating systems (Linux, OS X, Solaris, and so on). A service, however, is specific to Mac OS X. The Mac Developer Connection s Mac OS X Reference Library s Services Implementation Guide states that services in Mac OS X are features exported by your application for the benefit of other applications. This means that services allow applications to communicate with one another by passing information through a shared pasteboard (not shared memory). You may have seen Services in your menu options in each application's main menu. Each application that offers a service advertizes what operations its services can apply to data. It s these advertisements that make up the contents of your Services menus. Putting it all together, it s easier to think of each of the three this way: a process can be a daemon if it s running in the background; a daemon can be a service if its purpose is to act as a helper tool to interchange data between other programs. If you do not directly invoke a process, then it s typically considered a daemon. Apple recommends that daemons not use AppKit, so they re unlikely to be services. But not all developers are in tune with Apple s best practices, and this is not a recommendation that is always respected. The final aspect to review of these core underpinnings of the operating system is the application bundle. Applications appear to be single files, but in fact, they re more than that. An application bundle is a collection of programs and supporting files for those
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CHAPTER 2: Services, Daemons, and Processes
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programs that are displayed as a single application to the end user. A good example of an application bundle native to the Mac operating systems with Tiger and above is Automator. If you were to navigate to your Applications folder, control-click on the Automator icon, and click on Show Package Contents (or cd into it from the command line), you would discover that it s actually a directory of files. Inside this directory are a number of files and binaries. To actually run Automator from within the directory structure, you would need to navigate to the Contents folder and then into the MacOS folder to run the Automator binary. You will typically find binaries in the MacOS folder, and within the Resources you will find the supporting graphics, icon files, and compiled nib files that make up the application.
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