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Processes in UNIX

This excerpt from Modern Operating Systems digs into the UNIX kernel and looks at the basic concepts UNIX supports, namely processes, memory, the file system, and input/output. This article will emphasize the features common to all UNIX versions rather than focusing on any one specific version.
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It is time to dig deeply into the UNIX kernel and look more closely at the basic concepts UNIX supports, namely, processes, memory, the file system, and input/output. These notions are important because the system calls—the interface to the operating system itself—manipulate them. For example, system calls exist to create processes, allocate memory, open files, and do I/O.

Unfortunately, with so many versions of UNIX in existence, there are some differences between them. In this chapter, we will emphasize the features common to all of them rather than focus on any one specific version. Thus in certain sections (especially implementation sections), the discussion may not apply equally to every version.

10.3.1 Fundamental Concepts

The only active entities in a UNIX system are the processes. UNIX processes are very similar to the classical sequential processes that we studied in Chap 2. Each process runs a single program and initially has a single thread of control. In other words, it has one program counter, which keeps track of the next instruction to be executed. Most versions of UNIX allow a process to create additional threads once it starts executing.

UNIX is a multiprogramming system, so multiple, independent processes may be running at the same time. Each user may have several active processes at once, so on a large system, there may be hundreds or even thousands of processes running. In fact, on most single-user workstations, even when the user is absent, dozens of background processes, called daemons, are running. These are started automatically when the system is booted. (''Daemon'' is a variant spelling of ''demon,'' which is a self-employed evil spirit.) A typical daemon is the cron daemon. It wakes up once a minute to check if there is any work for it to do. If so, it does the work. Then it goes back to sleep until it is time for the next check.

This daemon is needed because it is possible in UNIX to schedule activities minutes, hours, days, or even months in the future. For example, suppose a user has a dentist appointment at 3 o'clock next Tuesday. He can make an entry in the cron daemon's database telling the daemon to beep at him at, say, 2:30. When the appointed day and time arrives, the cron daemon sees that it has work to do, and starts up the beeping program as a new process.

The cron daemon is also used to start up periodic activities, such as making daily disk backups at 4 A.M., or reminding forgetful users every year on October 31 to stock up on trick-or-treat goodies for Halloween. Other daemons handle incoming and outgoing electronic mail, manage the line printer queue, check if there are enough free pages in memory, and so forth. Daemons are straightforward to implement in UNIX because each one is a separate process, independent of all other processes.

Processes are created in UNIX in an especially simple manner. The fork system call creates an exact copy of the original process. The forking process is called the parent process. The new process is called the child process. The parent and child each have their own, private memory images. If the parent subsequently changes any of its variables, the changes are not visible to the child, and vice versa.

Open files are shared between parent and child. That is, if a certain file was open in the parent before the fork, it will continue to be open in both the parent and the child afterward. Changes made to the file by either one will be visible to the other. This behavior is only reasonable, because these changes are also visible to any unrelated process that opens the file as well.

The fact that the memory images, variables, registers, and everything else are identical in the parent and child leads to a small difficulty: How do the processes know which one should run the parent code and which one should run the child code? The secret is that the fork system call returns a 0 to the child and a nonzero value, the child's PID (Process IDentifier) to the parent. Both processes normally check the return value, and act accordingly, as shown in Fig. 10-1.

Figure 10-1. Process creation in UNIX.

Processes are named by their PIDs. When a process is created, the parent is given the child's PID, as mentioned above. If the child wants to know its own PID, there is a system call, getpid, that provides it. PIDs are used in a variety of ways. For example, when a child terminates, the parent is given the PID of the child that just finished. This can be important because a parent may have many children. Since children may also have children, an original process can build up an entire tree of children, grandchildren, and further descendants.

Processes in UNIX can communicate with each other using a form of message passing. It is possible to create a channel between two processes into which one process can write a stream of bytes for the other to read. These channels are called pipes. Synchronization is possible because when a process tries to read from an empty pipe it is blocked until data are available.

Shell pipelines are implemented with pipes. When the shell sees a line like

sort <f | head 

it creates two processes, sort and head, and sets up a pipe between them in such a way that sort's standard output is connected to head's standard input. In this way, all the data that sort writes go directly to head, instead of going to a file. If the pipe fills up, the system stops running sort until head has removed some data from the pipe.

Processes can also communicate in another way: software interrupts. A process can send what is called a signal to another process. Processes can tell the system what they want to happen when a signal arrives. The choices are to ignore it, to catch it, or to let the signal kill the process (the default for most signals). If a process elects to catch signals sent to it, it must specify a signal handling procedure. When a signal arrives, control will abruptly switch to the handler. When the handler is finished and returns, control goes back to where it came from, analogous to hardware I/O interrupts. A process can only send signals to members of its process group, which consists of its parent (and further ancestors), siblings, and children (and further descendants). A process may also send a signal to all members of its process group with a single system call.

Signals are also used for other purposes. For example, if a process is doing floating-point arithmetic, and inadvertently divides by 0, it gets a a SIGFPE (floating-point exception) signal. The signals that are required by POSIX are listed in Fig. 10-2. Many UNIX systems have additional signals as well, but programs using them may not be portable to other versions of UNIX.

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