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Process ID
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Each active process in the system is uniquely identified by a non-negative integer called a process ID. The range of this ID is from 0 to 30000.
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Parent process ID
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A new process is created by a currently active process; (see fork(2)). The parent process ID of a process is initially the process ID of its creator. If the creating process exits, the parent process ID of each child is set to the ID of a system process, init(8).
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Process Group
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Each active process is a member of a process group that is identified by a non-negative integer called the process group ID. This is the process ID of the group leader. This grouping permits the signaling of related processes (see termios(4)) and the job control mechanisms of csh(1).
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Session
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A session is a set of one or more process groups. A session is created by a successful call to setsid(2), which causes the caller to become the only member of the only process group in the new session.
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Session leader
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A process that has created a new session by a successful call to setsid(2), is known as a session leader. Only a session leader may acquire a terminal as its controlling terminal (see termios(4)).
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Controlling process
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A session leader with a controlling terminal is a controlling process.
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Controlling terminal
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A terminal that is associated with a session is known as the controlling terminal for that session and its members.
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Terminal Process Group ID
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A terminal may be acquired by a session leader as its controlling terminal. Once a terminal is associated with a session, any of the process groups within the session may be placed into the foreground by setting the terminal process group ID to the ID of the process group. This facility is used to arbitrate between multiple jobs contending for the same terminal. (See csh(1) and tty(4) for more information on job control.)
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Orphaned Process Group
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A process group is considered to be orphaned if it is not under the control of a job control shell. More precisely, a process group is orphaned when none of its members has a parent process that is in the same session as the group, but is in a different process group. Note that when a process exits, the parent process for its children is changed to be init(8), which is in a separate session. Not all members of an orphaned process group are necessarily orphaned processes (those whose creating process has exited). The process group of a session leader is orphaned by definition.
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Real User ID and Real Group ID
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Each user on the system is identified by a positive integer termed the real user ID.
Each user is also a member of one or more groups. One of these groups is distinguished from others and used in implementing accounting facilities. The positive integer corresponding to this distinguished group is termed the real group ID.
All processes have a real user ID and real group ID. These are initialized from the equivalent attributes of the process that created it.
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Effective User Id, Effective Group Id, and Group Access List
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Access to system resources is governed by two values: the effective user ID, and the group access list. The first member of the group access list is also known as the effective group ID. (In POSIX.1, the group access list is known as the set of supplementary group IDs, and it is unspecified whether the effective group ID is a member of the list.)
The effective user ID and effective group ID are initially the process's real user ID and real group ID respectively. Either may be modified through execution of a set-user-ID or set-group-ID file (possibly by one of its ancestors) (see execve(2)). By convention, the effective group ID (the first member of the group access list) is duplicated, so that the execution of a set-group-ID program does not result in the loss of the original (real) group ID.
The group access list is a set of group IDs used only in determining resource accessibility. Access checks are performed as described below in “File Access Permissions”.
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Saved Set User ID and Saved Set Group ID
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When a process executes a new file, the effective user ID is set to the owner of the file if the file is set-user-ID, and the effective group ID (first element of the group access list) is set to the group of the file if the file is set-group-ID. The effective user ID of the process is then recorded as the saved set-user-ID, and the effective group ID of the process is recorded as the saved set-group-ID. These values may be used to regain those values as the effective user or group ID after reverting to the real ID (see setuid(2)). (In POSIX.1, the saved set-user-ID and saved set-group-ID are optional, and are used in setuid and setgid, but this does not work as desired for the super-user.)
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Super-user
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A process is recognized as a super-user process and is granted special privileges if its effective user ID is 0.
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Special Processes
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The processes with process IDs of 0 and 1 are special. Process 0 is the scheduler. Process 1 is the initialization process init(8), and is the ancestor (parent) of every other process in the system. It is used to control the process structure. The kernel will allocate other kernel threads to handle certain periodic tasks or device related tasks, such as:
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acctwatch
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System accounting disk watcher, see acct(2), acct(5).
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aiodoned
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Asynchronous I/O done handler, see uvm(9).
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atabusX
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ATA bus handler, see ata(4).
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cardslotX
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CardBus slot watcher thread, see cardslot(4).
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cryptoret
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The software crypto daemon.
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fssbsX
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File system snapshot thread, see fss(4).
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ioflush
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The in-kernel periodic flush the buffer cache to disk task, which replaces the old update program.
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nfsio, nfskqpoll
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NFS handing daemons.
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lfs_writer
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Log filesystem writer.
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pagedaemon
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The page daemon.
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raidX, raidioX, raid_parity, raid_recon, raid_reconip, raid_copyback
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Raid framework related threads, see raid(4).
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scsibusX
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SCSI bus handler, see scsi(4).
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smbiodX, smbkq
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SMBFS handling daemon, see netsmb(4).
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swdmover
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The software data mover I/O thread, see dmoverio(4).
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sysmon
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The systems monitoring framework daemon.
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usbX, usbtask
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USB bus handler, see usb(4).
There are more machine-dependent kernel threads allocated by different drivers. See the specific driver manual pages for more information.
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Descriptor
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An integer assigned by the system when a file is referenced by open(2) or dup(2), or when a socket is created by pipe(2), socket(2), or socketpair(2), which uniquely identifies an access path to that file or socket from a given process or any of its children.
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File Name
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Names consisting of up to 255 (MAXNAMELEN) characters may be used to name an ordinary file, special file, or directory.
These characters may be selected from the set of all ASCII character excluding 0 (NUL) and the ASCII code for ‘/' (slash). (The parity bit, bit 7, must be 0).
Note that it is generally unwise to use ‘*', ‘?', ‘[' or ‘]' as part of file names because of the special meaning attached to these characters by the shell.
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Pathname
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A path name is a NUL-terminated character string starting with an optional slash ‘/', followed by zero or more directory names separated by slashes, optionally followed by a file name. The total length of a path name must be less than 1024 (MAXPATHLEN) characters.
If a path name begins with a slash, the path search begins at the root directory. Otherwise, the search begins from the current working directory. A slash by itself names the root directory. An empty string is not a valid pathname.
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Directory
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A directory is a special type of file that contains entries that are references to other files. Directory entries are called links. By convention, a directory contains at least two links, ‘.' and ‘..', referred to as dot and dot-dot respectively. Dot refers to the directory itself and dot-dot refers to its parent directory.
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Root Directory and Current Working Directory
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Each process has associated with it a concept of a root directory and a current working directory for the purpose of resolving path name searches. A process's root directory need not be the root directory of the root file system.
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File Access Permissions
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Every file in the file system has a set of access permissions. These permissions are used in determining whether a process may perform a requested operation on the file (such as opening a file for writing). Access permissions are established at the time a file is created. They may be changed at some later time through the chmod(2) call.
File access is broken down according to whether a file may be: read, written, or executed. Directory files use the execute permission to control if the directory may be searched.
File access permissions are interpreted by the system as they apply to three different classes of users: the owner of the file, those users in the file's group, anyone else. Every file has an independent set of access permissions for each of these classes. When an access check is made, the system decides if permission should be granted by checking the access information applicable to the caller.
Read, write, and execute/search permissions on a file are granted to a process if:
The process's effective user ID is that of the super-user. (Note: even the super-user cannot execute a non-executable file).
The process's effective user ID matches the user ID of the owner of the file and the owner permissions allow the access.
The process's effective user ID does not match the user ID of the owner of the file, and either the process's effective group ID matches the group ID of the file, or the group ID of the file is in the process's group access list, and the group permissions allow the access.
Neither the effective user ID nor effective group ID and group access list of the process match the corresponding user ID and group ID of the file, but the permissions for ``other users'' allow access.
Otherwise, permission is denied.
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Sockets and Address Families
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A socket is an endpoint for communication between processes. Each socket has queues for sending and receiving data.
Sockets are typed according to their communications properties. These properties include whether messages sent and received at a socket require the name of the partner, whether communication is reliable, the format used in naming message recipients, etc.
Each instance of the system supports some collection of socket types; consult socket(2) for more information about the types available and their properties.
Each instance of the system supports some number of sets of communications protocols. Each protocol set supports addresses of a certain format. An Address Family is the set of addresses for a specific group of protocols. Each socket has an address chosen from the address family in which the socket was created.