The Open Group Base Specifications Issue 7
IEEE Std 1003.1, 2013 Edition
Copyright © 2001-2013 The IEEE and The Open Group

## 3. Definitions

For the purposes of POSIX.1-2008, the following terms and definitions apply. The Authoritative Dictionary of IEEE Standards Terms, Seventh Edition should be referenced for terms not defined in this section.

Note:
  No shading to denote extensions or options occurs in this chapter. Where the terms and definitions given in this chapter are used elsewhere in text related to extensions and options, they are shaded as appropriate.

### 3.1 Abortive Release

An abrupt termination of a network connection that may result in the loss of data.

### 3.2 Absolute Pathname

A pathname beginning with a single or more than two <slash> characters; see also Pathname.

Note:
  Pathname Resolution is defined in detail in Pathname Resolution .

### 3.3 Access Mode

A particular form of access permitted to a file.

### 3.4 Additional File Access Control Mechanism

An implementation-defined mechanism that is layered upon the access control mechanisms defined here, but which do not grant permissions beyond those defined herein, although they may further restrict them.

Note:
  File Access Permissions are defined in detail in File Access Permissions.

### 3.5 Address Space

The memory locations that can be referenced by a process or the threads of a process.

### 3.6 Advisory Information

An interface that advises the implementation on (portable) application behavior so that it can optimize the system.

### 3.7 Affirmative Response

An input string that matches one of the responses acceptable to the LC_MESSAGES category keyword yesexpr, matching an extended regular expression in the current locale.

Note:
  The LC_MESSAGES category is defined in detail in LC_MESSAGES.

### 3.8 Alert

To cause the user's terminal to give some audible or visual indication that an error or some other event has occurred. When the standard output is directed to a terminal device, the method for alerting the terminal user is unspecified. When the standard output is not directed to a terminal device, the alert is accomplished by writing the alert to standard output (unless the utility description indicates that the use of standard output produces undefined results in this case).

### 3.9 Alert Character (<alert>)

A character that in the output stream should cause a terminal to alert its user via a visual or audible notification. It is the character designated by `'\a'` in the C language. It is unspecified whether this character is the exact sequence transmitted to an output device by the system to accomplish the alert function.

### 3.10 Alias Name

In the shell command language, a word consisting solely of underscores, digits, and alphabetics from the portable character set and any of the following characters: `'!'`, `'%'`, `','`, `'@'`.

Implementations may allow other characters within alias names as an extension.

Note:
  The Portable Character Set is defined in detail in Portable Character Set.

### 3.11 Alignment

A requirement that objects of a particular type be located on storage boundaries with addresses that are particular multiples of a byte address.

Note:
  See also the ISO C standard, Section B3.

### 3.12 Alternate File Access Control Mechanism

An implementation-defined mechanism that is independent of the access control mechanisms defined herein, and which if enabled on a file may either restrict or extend the permissions of a given user. POSIX.1-2008 defines when such mechanisms can be enabled and when they are disabled.

Note:
  File Access Permissions are defined in detail in File Access Permissions.

### 3.13 Alternate Signal Stack

Memory associated with a thread, established upon request by the implementation for a thread, separate from the thread signal stack, in which signal handlers responding to signals sent to that thread may be executed.

### 3.14 Ancillary Data

Protocol-specific, local system-specific, or optional information. The information can be both local or end-to-end significant, header information, part of a data portion, protocol-specific, and implementation or system-specific.

### 3.15 Angle Brackets

The characters `'<'` (left-angle-bracket) and `'>'` (right-angle-bracket). When used in the phrase "enclosed in angle brackets", the symbol `'<'` immediately precedes the object to be enclosed, and `'>'` immediately follows it. When describing these characters in the portable character set, the names <less-than-sign> and <greater-than-sign> are used.

### 3.16 Apostrophe Character (<apostrophe>)

The character designated by `'\'` in the C language, also known as the single-quote character.

### 3.17 Application

A computer program that performs some desired function.

### 3.18 Application Address

Endpoint address of a specific application.

### 3.19 Application Program Interface (API)

The definition of syntax and semantics for providing computer system services.

### 3.20 Appropriate Privileges

An implementation-defined means of associating privileges with a process with regard to the function calls, function call options, and the commands that need special privileges. There may be zero or more such means. These means (or lack thereof) are described in the conformance document.

Note:
  Function calls are defined in the System Interfaces volume of POSIX.1-2008, and commands are defined in the Shell and Utilities volume of POSIX.1-2008.

### 3.21 Argument

In the shell command language, a parameter passed to a utility as the equivalent of a single string in the argv array created by one of the exec functions. An argument is one of the options, option-arguments, or operands following the command name.

Note:
  The Utility Argument Syntax is defined in detail in Utility Argument Syntax and XCU Command Search and Execution.

In the C language, an expression in a function call expression or a sequence of preprocessing tokens in a function-like macro invocation.

### 3.22 Arm (a Timer)

To start a timer measuring the passage of time, enabling notifying a process when the specified time or time interval has passed.

### 3.23 Asterisk Character (<asterisk>)

The character `'*'`.

### 3.24 Async-Cancel-Safe Function

A function that may be safely invoked by an application while the asynchronous form of cancellation is enabled. No function is async-cancel-safe unless explicitly described as such.

### 3.25 Asynchronous Events

Events that occur independently of the execution of the application.

### 3.26 Asynchronous Input and Output

A functionality enhancement to allow an application process to queue data input and output commands with asynchronous notification of completion.

### 3.27 Async-Signal-Safe Function

A function that may be invoked, without restriction, from signal-catching functions. No function is async-signal-safe unless explicitly described as such.

### 3.28 Asynchronously-Generated Signal

A signal that is not attributable to a specific thread. Examples are signals sent via kill(), signals sent from the keyboard, and signals delivered to process groups. Being asynchronous is a property of how the signal was generated and not a property of the signal number. All signals may be generated asynchronously.

Note:
  The kill() function is defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.29 Asynchronous I/O Completion

For an asynchronous read or write operation, when a corresponding synchronous read or write would have completed and when any associated status fields have been updated.

### 3.30 Asynchronous I/O Operation

An I/O operation that does not of itself cause the thread requesting the I/O to be blocked from further use of the processor.

This implies that the process and the I/O operation may be running concurrently.

### 3.31 Authentication

The process of validating a user or process to verify that the user or process is not a counterfeit.

### 3.32 Authorization

The process of verifying that a user or process has permission to use a resource in the manner requested.

To ensure security, the user or process would also need to be authenticated before granting access.

### 3.33 Background Job

See Background Process Group in Background Process Group (or Background Job).

### 3.34 Background Process

A process that is a member of a background process group.

### 3.35 Background Process Group (or Background Job)

Any process group, other than a foreground process group, that is a member of a session that has established a connection with a controlling terminal.

### 3.36 Backquote Character

The character `'`'`, also known as <grave-accent>.

### 3.37 Backslash Character (<backslash>)

The character designated by `'\\'` in the C language, also known as reverse solidus.

### 3.38 Backspace Character (<backspace>)

A character that, in the output stream, should cause printing (or displaying) to occur one column position previous to the position about to be printed. If the position about to be printed is at the beginning of the current line, the behavior is unspecified. It is the character designated by `'\b'` in the C language. It is unspecified whether this character is the exact sequence transmitted to an output device by the system to accomplish the backspace function. The backspace defined here is not necessarily the ERASE special character.

Note:
  Special Characters are defined in detail in Special Characters .

### 3.39 Barrier

A synchronization object that allows multiple threads to synchronize at a particular point in their execution.

### 3.40 Basename

The final, or only, filename in a pathname.

### 3.41 Basic Regular Expression (BRE)

A regular expression (see Regular Expression) used by the majority of utilities that select strings from a set of character strings.

Note:
  Basic Regular Expressions are described in detail in Basic Regular Expressions.

### 3.42 Batch Access List

A list of user IDs and group IDs of those users and groups authorized to place batch jobs in a batch queue.

A batch access list is associated with a batch queue. A batch server uses the batch access list of a batch queue as one of the criteria in deciding to put a batch job in a batch queue.

### 3.43 Batch Administrator

A user that is authorized to modify all the attributes of queues and jobs and to change the status of a batch server.

### 3.44 Batch Client

A computational entity that utilizes batch services by making requests of batch servers.

Batch clients often provide the means by which users access batch services, although a batch server may act as a batch client by virtue of making requests of another batch server.

### 3.45 Batch Destination

The batch server in a batch system to which a batch job should be sent for processing.

Acceptance of a batch job at a batch destination is the responsibility of a receiving batch server. A batch destination may consist of a batch server-specific portion, a network-wide portion, or both. The batch server-specific portion is referred to as the "batch queue". The network-wide portion is referred to as a "batch server name".

### 3.46 Batch Destination Identifier

A string that identifies a specific batch destination.

A string of characters in the portable character set used to specify a particular batch destination.

Note:
  The Portable Character Set is defined in detail in Portable Character Set.

### 3.47 Batch Directive

A line from a file that is interpreted by the batch server. The line is usually in the form of a comment and is an additional means of passing options to the qsub utility.

Note:
  The qsub utility is defined in detail in the Shell and Utilities volume of POSIX.1-2008.

### 3.48 Batch Job

A set of computational tasks for a computing system.

Batch jobs are managed by batch servers.

Once created, a batch job may be executing or pending execution. A batch job that is executing has an associated session leader (a process) that initiates and monitors the computational tasks of the batch job.

### 3.49 Batch Job Attribute

A named data type whose value affects the processing of a batch job.

The values of the attributes of a batch job affect the processing of that job by the batch server that manages the batch job.

### 3.50 Batch Job Identifier

A unique name for a batch job. A name that is unique among all other batch job identifiers in a batch system and that identifies the batch server to which the batch job was originally submitted.

### 3.51 Batch Job Name

A label that is an attribute of a batch job. The batch job name is not necessarily unique.

### 3.52 Batch Job Owner

The username@ hostname of the user submitting the batch job, where username is a user name (see also User Name) and hostname is a network host name.

### 3.53 Batch Job Priority

A value specified by the user that may be used by an implementation to determine the order in which batch jobs are selected to be executed. Job priority has a numeric value in the range -1024 to 1023.

Note:
  The batch job priority is not the execution priority (nice value) of the batch job.

### 3.54 Batch Job State

An attribute of a batch job which determines the types of requests that the batch server that manages the batch job can accept for the batch job. Valid states include QUEUED, RUNNING, HELD, WAITING, EXITING, and TRANSITING.

### 3.55 Batch Name Service

A service that assigns batch names that are unique within the batch name space, and that can translate a unique batch name into the location of the named batch entity.

### 3.56 Batch Name Space

The environment within which a batch name is known to be unique.

### 3.57 Batch Node

A host containing part or all of a batch system.

A batch node is a host meeting at least one of the following conditions:

- Capable of executing a batch client

- Contains a routing batch queue

- Contains an execution batch queue

### 3.58 Batch Operator

A user that is authorized to modify some, but not all, of the attributes of jobs and queues, and may change the status of the batch server.

### 3.59 Batch Queue

A manageable object that represents a set of batch jobs and is managed by a single batch server.

Note:
  A set of batch jobs is called a batch queue largely for historical reasons. Jobs are selected from the batch queue for execution based on attributes such as priority, resource requirements, and hold conditions.

  See also XCU Batch Queues.

### 3.60 Batch Queue Attribute

A named data type whose value affects the processing of all batch jobs that are members of the batch queue.

A batch queue has attributes that affect the processing of batch jobs that are members of the batch queue.

### 3.61 Batch Queue Position

The place, relative to other jobs in the batch queue, occupied by a particular job in a batch queue. This is defined in part by submission time and priority; see also Batch Queue Priority.

### 3.62 Batch Queue Priority

The maximum job priority allowed for any batch job in a given batch queue.

The batch queue priority is set and may be changed by users with appropriate privileges. The priority is bounded in an implementation-defined manner.

### 3.63 Batch Rerunability

An attribute of a batch job indicating that it may be rerun after an abnormal termination from the beginning without affecting the validity of the results.

### 3.64 Batch Restart

The action of resuming the processing of a batch job from the point of the last checkpoint. Typically, this is done if the batch job has been interrupted because of a system failure.

### 3.65 Batch Server

A computational entity that provides batch services.

### 3.66 Batch Server Name

A string of characters in the portable character set used to specify a particular server in a network.

Note:
  The Portable Character Set is defined in detail in Portable Character Set.

### 3.67 Batch Service

Computational and organizational services performed by a batch system on behalf of batch jobs.

Batch services are of two types: requested and deferred.

Note:
  Batch Services are listed in XCU Batch Services Summary.

### 3.68 Batch Service Request

A solicitation of services from a batch client to a batch server.

A batch service request may entail the exchange of any number of messages between the batch client and the batch server.

When naming specific types of service requests, the term "request" is qualified by the type of request, as in Queue Batch Job Request and Delete Batch Job Request.

### 3.69 Batch Submission

The process by which a batch client requests that a batch server create a batch job via a Queue Job Request to perform a specified computational task.

### 3.70 Batch System

A collection of one or more batch servers.

### 3.71 Batch Target User

The name of a user on the batch destination batch server.

The target user is the user name under whose account the batch job is to execute on the destination batch server.

### 3.72 Batch User

A user who is authorized to make use of batch services.

### 3.73 Bind

The process of assigning a network address to an endpoint.

### 3.74 Blank Character (<blank>)

One of the characters that belong to the blank character class as defined via the LC_CTYPE category in the current locale. In the POSIX locale, a <blank> character is either a <tab> or a <space>.

### 3.75 Blank Line

A line consisting solely of zero or more <blank> characters terminated by a <newline>; see also Empty Line.

### 3.76 Blocked Process (or Thread)

A process (or thread) that is waiting for some condition (other than the availability of a processor) to be satisfied before it can continue execution.

### 3.77 Blocking

A property of an open file description that causes function calls associated with it to wait for the requested action to be performed before returning.

### 3.78 Block-Mode Terminal

A terminal device operating in a mode incapable of the character-at-a-time input and output operations described by some of the standard utilities.

Note:
  Output Devices and Terminal Types are defined in detail in Output Devices and Terminal Types.

### 3.79 Block Special File

A file that refers to a device. A block special file is normally distinguished from a character special file by providing access to the device in a manner such that the hardware characteristics of the device are not visible.

### 3.80 Braces

The characters `'{'` (left-curly-bracket) and `'}'` (right-curly-bracket). When used in the phrase "enclosed in (curly) braces" the symbol `'{'` immediately precedes the object to be enclosed, and `'}'` immediately follows it. When describing these characters in the portable character set, the names <left-curly-bracket> and <left-brace> are used for `'{'`, and <right-curly-bracket> and <right-brace> are used for `'}'`.

### 3.81 Brackets

The characters `'['` (left-square-bracket) and `']'` (right-square-bracket). When used in the phrase "enclosed in (square) brackets" the symbol `'['` immediately precedes the object to be enclosed, and `']'` immediately follows it. When describing these characters in the portable character set, the names <left-square-bracket> and <right-square-bracket> are used.

### 3.82 Broadcast

The transfer of data from one endpoint to several endpoints, as described in RFC 919 and RFC 922.

### 3.83 Built-In Utility (or Built-In)

A utility implemented within a shell. The utilities referred to as special built-ins have special qualities. Unless qualified, the term "built-in" includes the special built-in utilities. Regular built-ins are not required to be actually built into the shell on the implementation, but they do have special command-search qualities.

Note:
  Special Built-In Utilities are defined in detail in XCU Special Built-In Utilities.

  Regular Built-In Utilities are defined in detail in XCU Command Search and Execution.

### 3.84 Byte

An individually addressable unit of data storage that is exactly an octet, used to store a character or a portion of a character; see also Character. A byte is composed of a contiguous sequence of 8 bits. The least significant bit is called the "low-order" bit; the most significant is called the "high-order" bit.

Note:
  The definition of byte from the ISO C standard is broader than the above and might accommodate hardware architectures with different sized addressable units than octets.

### 3.85 Byte Input/Output Functions

The functions that perform byte-oriented input from streams or byte-oriented output to streams: fgetc(), fgets(), fprintf(), fputc(), fputs(), fread(), fscanf(), fwrite(), getc(), getchar(), getdelim(), getline(), gets(), printf(), putc(), putchar(), puts(), scanf(), ungetc(), vfprintf(), and vprintf().

Note:
  Functions are defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.86 Carriage-Return Character (<carriage-return>)

A character that in the output stream indicates that printing should start at the beginning of the same physical line in which the carriage-return occurred. It is the character designated by `'\r'` in the C language. It is unspecified whether this character is the exact sequence transmitted to an output device by the system to accomplish the movement to the beginning of the line.

### 3.87 Character

A sequence of one or more bytes representing a single graphic symbol or control code.

Note:
  This term corresponds to the ISO C standard term multi-byte character, where a single-byte character is a special case of a multi-byte character. Unlike the usage in the ISO C standard, character here has no necessary relationship with storage space, and byte is used when storage space is discussed.

  See the definition of the portable character set in Portable Character Set for a further explanation of the graphical representations of (abstract) characters, as opposed to character encodings.

### 3.88 Character Array

An array of elements of type char.

### 3.89 Character Class

A named set of characters sharing an attribute associated with the name of the class. The classes and the characters that they contain are dependent on the value of the LC_CTYPE category in the current locale.

Note:
  The LC_CTYPE category is defined in detail in LC_CTYPE .

### 3.90 Character Set

A finite set of different characters used for the representation, organization, or control of data.

### 3.91 Character Special File

A file that refers to a device (such as a terminal device file) or that has special properties (such as /dev/null).

Note:
  The General Terminal Interface is defined in detail in General Terminal Interface.

### 3.92 Character String

A contiguous sequence of characters terminated by and including the first null byte.

### 3.93 Child Process

A new process created (by fork(), posix_spawn(), or posix_spawnp()) by a given process. A child process remains the child of the creating process as long as both processes continue to exist.

Note:
  The fork(), posix_spawn(), and posix_spawnp() functions are defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.94 Circumflex Character (<circumflex>)

The character `'^'`.

### 3.95 Clock

A software or hardware object that can be used to measure the apparent or actual passage of time.

The current value of the time measured by a clock can be queried and, possibly, set to a value within the legal range of the clock.

### 3.96 Clock Jump

The difference between two successive distinct values of a clock, as observed from the application via one of the "get time'' operations.

### 3.97 Clock Tick

An interval of time; an implementation-defined number of these occur each second. Clock ticks are one of the units that may be used to express a value found in type clock_t.

### 3.98 Coded Character Set

A set of unambiguous rules that establishes a character set and the one-to-one relationship between each character of the set and its bit representation.

### 3.99 Codeset

The result of applying rules that map a numeric code value to each element of a character set. An element of a character set may be related to more than one numeric code value but the reverse is not true. However, for state-dependent encodings the relationship between numeric code values and elements of a character set may be further controlled by state information. The character set may contain fewer elements than the total number of possible numeric code values; that is, some code values may be unassigned.

Note:
  Character Encoding is defined in detail in Character Encoding.

### 3.100 Collating Element

The smallest entity used to determine the logical ordering of character or wide-character strings; see also Collation Sequence. A collating element consists of either a single character, or two or more characters collating as a single entity. The value of the LC_COLLATE category in the current locale determines the current set of collating elements.

### 3.101 Collation

The logical ordering of character or wide-character strings according to defined precedence rules. These rules identify a collation sequence between the collating elements, and such additional rules that can be used to order strings consisting of multiple collating elements.

### 3.102 Collation Sequence

The relative order of collating elements as determined by the setting of the LC_COLLATE category in the current locale. The collation sequence is used for sorting and is determined from the collating weights assigned to each collating element. In the absence of weights, the collation sequence is the order in which collating elements are specified between order_start and order_end keywords in the LC_COLLATE category.

Multi-level sorting is accomplished by assigning elements one or more collation weights, up to the limit {COLL_WEIGHTS_MAX}. On each level, elements may be given the same weight (at the primary level, called an equivalence class; see also Equivalence Class) or be omitted from the sequence. Strings that collate equally using the first assigned weight (primary ordering) are then compared using the next assigned weight (secondary ordering), and so on.

Note:
  {COLL_WEIGHTS_MAX} is defined in detail in <limits.h>.

### 3.103 Column Position

A unit of horizontal measure related to characters in a line.

It is assumed that each character in a character set has an intrinsic column width independent of any output device. Each printable character in the portable character set has a column width of one. The standard utilities, when used as described in POSIX.1-2008, assume that all characters have integral column widths. The column width of a character is not necessarily related to the internal representation of the character (numbers of bits or bytes).

The column position of a character in a line is defined as one plus the sum of the column widths of the preceding characters in the line. Column positions are numbered starting from 1.

### 3.104 Command

A directive to the shell to perform a particular task.

Note:
  Shell Commands are defined in detail in XCU Shell Commands.

### 3.105 Command Language Interpreter

An interface that interprets sequences of text input as commands. It may operate on an input stream or it may interactively prompt and read commands from a terminal. It is possible for applications to invoke utilities through a number of interfaces, which are collectively considered to act as command interpreters. The most obvious of these are the sh utility and the system() function, although popen() and the various forms of exec may also be considered to behave as interpreters.

Note:
  The sh utility is defined in detail in the Shell and Utilities volume of POSIX.1-2008.

  The system(), popen(), and exec functions are defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.106 Composite Graphic Symbol

A graphic symbol consisting of a combination of two or more other graphic symbols in a single character position, such as a diacritical mark and a base character.

### 3.107 Condition Variable

A synchronization object which allows a thread to suspend execution, repeatedly, until some associated predicate becomes true. A thread whose execution is suspended on a condition variable is said to be blocked on the condition variable.

### 3.108 Connected Socket

A connection-mode socket for which a connection has been established, or a connectionless-mode socket for which a peer address has been set. See also Connection, Connection Mode, Connectionless Mode, and Socket.

### 3.109 Connection

An association established between two or more endpoints for the transfer of data

### 3.110 Connection Mode

The transfer of data in the context of a connection; see also Connectionless Mode.

### 3.111 Connectionless Mode

The transfer of data other than in the context of a connection; see also Connection Mode and Datagram.

### 3.112 Control Character

A character, other than a graphic character, that affects the recording, processing, transmission, or interpretation of text.

### 3.113 Control Operator

In the shell command language, a token that performs a control function. It is one of the following symbols:

    &   &&   (   )   ;   ;;   newline   |   ||

The end-of-input indicator used internally by the shell is also considered a control operator.

Note:
  Token Recognition is defined in detail in XCU Token Recognition .

### 3.114 Controlling Process

The session leader that established the connection to the controlling terminal. If the terminal subsequently ceases to be a controlling terminal for this session, the session leader ceases to be the controlling process.

### 3.115 Controlling Terminal

A terminal that is associated with a session. Each session may have at most one controlling terminal associated with it, and a controlling terminal is associated with exactly one session. Certain input sequences from the controlling terminal cause signals to be sent to all processes in the foreground process group associated with the controlling terminal.

Note:
  The General Terminal Interface is defined in detail in General Terminal Interface.

### 3.116 Conversion Descriptor

A per-process unique value used to identify an open codeset conversion.

### 3.117 Core File

A file of unspecified format that may be generated when a process terminates abnormally.

### 3.118 CPU Time (Execution Time)

The time spent executing a process or thread, including the time spent executing system services on behalf of that process or thread. If the Threads option is supported, then the value of the CPU-time clock for a process is implementation-defined. With this definition the sum of all the execution times of all the threads in a process might not equal the process execution time, even in a single-threaded process, because implementations may differ in how they account for time during context switches or for other reasons.

### 3.119 CPU-Time Clock

A clock that measures the execution time of a particular process or thread.

### 3.120 CPU-Time Timer

A timer attached to a CPU-time clock.

### 3.121 Current Job

In the context of job control, the job that will be used as the default for the fg or bg utilities. There is at most one current job; see also Job Control Job ID.

### 3.122 Current Working Directory

See Working Directory in Working Directory (or Current Working Directory).

### 3.123 Cursor Position

The line and column position on the screen denoted by the terminal's cursor.

### 3.124 Datagram

A unit of data transferred from one endpoint to another in connectionless mode service.

### 3.125 Data Segment

Memory associated with a process, that can contain dynamically allocated data.

### 3.126 Deferred Batch Service

A service that is performed as a result of events that are asynchronous with respect to requests.

Note:
  Once a batch job has been created, it is subject to deferred services.

### 3.127 Device

A computer peripheral or an object that appears to the application as such.

### 3.128 Device ID

A non-negative integer used to identify a device.

### 3.129 Directory

A file that contains directory entries. No two directory entries in the same directory have the same name.

### 3.130 Directory Entry (or Link)

An object that associates a filename with a file. Several directory entries can associate names with the same file.

### 3.131 Directory Stream

A sequence of all the directory entries in a particular directory. An open directory stream may be implemented using a file descriptor.

### 3.132 Disarm (a Timer)

To stop a timer from measuring the passage of time, disabling any future process notifications (until the timer is armed again).

### 3.133 Display

To output to the user's terminal. If the output is not directed to a terminal, the results are undefined.

### 3.134 Display Line

A line of text on a physical device or an emulation thereof. Such a line will have a maximum number of characters which can be presented.

Note:
  This may also be written as "line on the display".

### 3.135 Dollar-Sign Character (<dollar-sign>)

The character `'$'`.

### 3.136 Dot

In the context of naming files, the filename consisting of a single <period> character ( `'.'` ).

Note:
  In the context of shell special built-in utilities, see dot in XCU Special Built-In Utilities.

  Pathname Resolution is defined in detail in Pathname Resolution.

### 3.137 Dot-Dot

The filename consisting solely of two <period> characters ( `".."` ).

Note:
  Pathname Resolution is defined in detail in Pathname Resolution .

### 3.138 Double-Quote Character

The character `'"'` also known as <quotation-mark>.

Note:
  The "double" adjective in this term refers to the two strokes in the character glyph. POSIX.1-2008 never uses the term "double-quote" to refer to two apostrophes or quotation-marks.

### 3.139 Downshifting

The conversion of an uppercase character that has a single-character lowercase representation into this lowercase representation.

### 3.140 Driver

A module that controls data transferred to and received from devices.

Note:
  Drivers are traditionally written to be a part of the system implementation, although they are frequently written separately from the writing of the implementation. A driver may contain processor-specific code, and therefore be non-portable.

### 3.141 Effective Group ID

An attribute of a process that is used in determining various permissions, including file access permissions; see also Group ID.

### 3.142 Effective User ID

An attribute of a process that is used in determining various permissions, including file access permissions; see also User ID.

### 3.143 Eight-Bit Transparency

The ability of a software component to process 8-bit characters without modifying or utilizing any part of the character in a way that is inconsistent with the rules of the current coded character set.

### 3.144 Empty Directory

A directory that contains, at most, directory entries for dot and dot-dot, and has exactly one link to it (other than its own dot entry, if one exists), in dot-dot. No other links to the directory may exist. It is unspecified whether an implementation can ever consider the root directory to be empty.

### 3.145 Empty Line

A line consisting of only a <newline>; see also Blank Line.

### 3.146 Empty String (or Null String)

A string whose first byte is a null byte.

### 3.147 Empty Wide-Character String

A wide-character string whose first element is a null wide-character code.

### 3.148 Encoding Rule

The rules used to convert between wide-character codes and multi-byte character codes.

Note:
  Stream Orientation and Encoding Rules are defined in detail in XSH Stream Orientation and Encoding Rules.

### 3.149 Entire Regular Expression

The concatenated set of one or more basic regular expressions or extended regular expressions that make up the pattern specified for string selection.

Note:
  Regular Expressions are defined in detail in Regular Expressions.

### 3.150 Epoch

The time zero hours, zero minutes, zero seconds, on January 1, 1970 Coordinated Universal Time (UTC).

Note:
  See also Seconds Since the Epoch defined in Seconds Since the Epoch.

### 3.151 Equivalence Class

A set of collating elements with the same primary collation weight.

Elements in an equivalence class are typically elements that naturally group together, such as all accented letters based on the same base letter.

The collation order of elements within an equivalence class is determined by the weights assigned on any subsequent levels after the primary weight.

### 3.152 Era

A locale-specific method for counting and displaying years.

Note:
  The LC_TIME category is defined in detail in LC_TIME.

### 3.153 Event Management

The mechanism that enables applications to register for and be made aware of external events such as data becoming available for reading.

### 3.154 Executable File

A regular file acceptable as a new process image file by the equivalent of the exec family of functions, and thus usable as one form of a utility. The standard utilities described as compilers can produce executable files, but other unspecified methods of producing executable files may also be provided. The internal format of an executable file is unspecified, but a conforming application cannot assume an executable file is a text file.

### 3.155 Execute

To perform command search and execution actions, as defined in the Shell and Utilities volume of POSIX.1-2008; see also Invoke.

Note:
  Command Search and Execution is defined in detail in XCU Command Search and Execution.

### 3.156 Execution Time

See CPU Time in CPU Time (Execution Time).

### 3.157 Execution Time Monitoring

A set of execution time monitoring primitives that allow online measuring of thread and process execution times.

### 3.158 Expand

In the shell command language, when not qualified, the act of applying word expansions.

Note:
  Word Expansions are defined in detail in XCU wordexp.

### 3.159 Extended Regular Expression (ERE)

A regular expression (see also Regular Expression) that is an alternative to the Basic Regular Expression using a more extensive syntax, occasionally used by some utilities.

Note:
  Extended Regular Expressions are described in detail in Extended Regular Expressions.

### 3.160 Extended Security Controls

Implementation-defined security controls allowed by the file access permission and appropriate privileges (see also Appropriate Privileges) mechanisms, through which an implementation can support different security policies from those described in POSIX.1-2008.

Note:
  See also Extended Security Controls defined in Extended Security Controls.

  File Access Permissions are defined in detail in File Access Permissions.

### 3.161 Feature Test Macro

A macro used to determine whether a particular set of features is included from a header.

Note:
  See also XSH The Compilation Environment.

### 3.162 Field

In the shell command language, a unit of text that is the result of parameter expansion, arithmetic expansion, command substitution, or field splitting. During command processing, the resulting fields are used as the command name and its arguments.

Note:
  Parameter Expansion is defined in detail in XCU Parameter Expansion.

  Arithmetic Expansion is defined in detail in XCU Arithmetic Expansion.

  Command Substitution is defined in detail in XCU Command Substitution.

  Field Splitting is defined in detail in XCU Field Splitting.

  For further information on command processing, see XCU Simple Commands.

### 3.163 FIFO Special File (or FIFO)

A type of file with the property that data written to such a file is read on a first-in-first-out basis.

Note:
  Other characteristics of FIFOs are described in the System Interfaces volume of POSIX.1-2008, lseek(), open(), read(), and write().

### 3.164 File

An object that can be written to, or read from, or both. A file has certain attributes, including access permissions and type. File types include regular file, character special file, block special file, FIFO special file, symbolic link, socket, and directory. Other types of files may be supported by the implementation.

### 3.165 File Description

See Open File Description in Open File Description.

### 3.166 File Descriptor

A per-process unique, non-negative integer used to identify an open file for the purpose of file access. The value of a newly-created file descriptor is from zero to {OPEN_MAX}-1. A file descriptor can have a value greater than or equal to {OPEN_MAX} if the value of {OPEN_MAX} has decreased (see sysconf) since the file descriptor was opened. File descriptors may also be used to implement message catalog descriptors and directory streams; see also Open File Description.

Note:
  {OPEN_MAX} is defined in detail in <limits.h>.

### 3.167 File Group Class

The property of a file indicating access permissions for a process related to the group identification of a process. A process is in the file group class of a file if the process is not in the file owner class and if the effective group ID or one of the supplementary group IDs of the process matches the group ID associated with the file. Other members of the class may be implementation-defined.

### 3.168 File Mode

An object containing the file mode bits and file type of a file.

Note:
  File mode bits and file types are defined in detail in <sys/stat.h> .

### 3.169 File Mode Bits

A file's file permission bits, set-user-ID-on-execution bit (S_ISUID), set-group-ID-on-execution bit (S_ISGID), and, on directories, the restricted deletion flag bit (S_ISVTX).

Note:
  File Mode Bits are defined in detail in <sys/stat.h>.

### 3.170 Filename

A sequence of bytes consisting of 1 to {NAME_MAX} bytes used to name a file. The bytes composing the name shall not contain the <NUL> or <slash> characters. In the context of a pathname, each filename shall be followed by a <slash> or a <NUL> character; elsewhere, a filename followed by a <NUL> character forms a string (but not necessarily a character string). The filenames dot and dot-dot have special meaning. A filename is sometimes referred to as a "pathname component". See also Pathname.

Note:
  Pathname Resolution is defined in detail in Pathname Resolution .

### 3.171 Filename String

A string consisting of a filename followed by a <NUL> character.

### 3.172 File Offset

The byte position in the file where the next I/O operation begins. Each open file description associated with a regular file, block special file, or directory has a file offset. A character special file that does not refer to a terminal device may have a file offset. There is no file offset specified for a pipe or FIFO.

### 3.173 File Other Class

The property of a file indicating access permissions for a process related to the user and group identification of a process. A process is in the file other class of a file if the process is not in the file owner class or file group class.

### 3.174 File Owner Class

The property of a file indicating access permissions for a process related to the user identification of a process. A process is in the file owner class of a file if the effective user ID of the process matches the user ID of the file.

### 3.175 File Permission Bits

Information about a file that is used, along with other information, to determine whether a process has read, write, or execute/search permission to a file. The bits are divided into three parts: owner, group, and other. Each part is used with the corresponding file class of processes. These bits are contained in the file mode.

Note:
  File modes are defined in detail in <sys/stat.h>.

  File Access Permissions are defined in detail in File Access Permissions.

### 3.176 File Serial Number

A per-file system unique identifier for a file.

### 3.177 File System

A collection of files and certain of their attributes. It provides a name space for file serial numbers referring to those files.

### 3.178 File Type

See File in File.

### 3.179 Filter

A command whose operation consists of reading data from standard input or a list of input files and writing data to standard output. Typically, its function is to perform some transformation on the data stream.

### 3.180 First Open (of a File)

When a process opens a file that is not currently an open file within any process.

### 3.181 Flow Control

The mechanism employed by a communications provider that constrains a sending entity to wait until the receiving entities can safely receive additional data without loss.

### 3.182 Foreground Job

See Foreground Process Group in Foreground Process Group (or Foreground Job).

### 3.183 Foreground Process

A process that is a member of a foreground process group.

### 3.184 Foreground Process Group (or Foreground Job)

A process group whose member processes have certain privileges, denied to processes in background process groups, when accessing their controlling terminal. Each session that has established a connection with a controlling terminal has at most one process group of the session as the foreground process group of that controlling terminal.

Note:
  The General Terminal Interface is defined in detail in General Terminal Interface.

### 3.185 Foreground Process Group ID

The process group ID of the foreground process group.

### 3.186 Form-Feed Character (<form-feed>)

A character that in the output stream indicates that printing should start on the next page of an output device. It is the character designated by `'\f'` in the C language. If the form-feed is not the first character of an output line, the result is unspecified. It is unspecified whether this character is the exact sequence transmitted to an output device by the system to accomplish the movement to the next page.

### 3.187 Graphic Character

A member of the graph character class of the current locale.

Note:
  The graph character class is defined in detail in LC_CTYPE .

### 3.188 Group Database

A system database that contains at least the following information for each group ID:

- Group name

- Numerical group ID

- List of users allowed in the group

The list of users allowed in the group is used by the newgrp utility.

Note:
  The newgrp utility is defined in detail in the Shell and Utilities volume of POSIX.1-2008.

### 3.189 Group ID

A non-negative integer, which can be contained in an object of type gid_t, that is used to identify a group of system users. Each system user is a member of at least one group. When the identity of a group is associated with a process, a group ID value is referred to as a real group ID, an effective group ID, one of the supplementary group IDs, or a saved set-group-ID.

### 3.190 Group Name

A string that is used to identify a group; see also Group Database. To be portable across conforming systems, the value is composed of characters from the portable filename character set. The <hyphen> should not be used as the first character of a portable group name.

### 3.191 Hard Limit

A system resource limitation that may be reset to a lesser or greater limit by a privileged process. A non-privileged process is restricted to only lowering its hard limit.

### 3.192 Hard Link

The relationship between two directory entries that represent the same file; see also Directory Entry (or Link). The result of an execution of the ln utility (without the -s option) or the link() function. This term is contrasted against symbolic link; see also Symbolic Link.

### 3.193 Home Directory

The directory specified by the HOME environment variable.

### 3.194 Host Byte Order

The arrangement of bytes in any integer type when using a specific machine architecture.

Note:
  Two common methods of byte ordering are big-endian and little-endian. Big-endian is a format for storage of binary data in which the most significant byte is placed first, with the rest in descending order. Little-endian is a format for storage or transmission of binary data in which the least significant byte is placed first, with the rest in ascending order. See also Host and Network Byte Orders.

### 3.195 Incomplete Line

A sequence of one or more non- <newline> characters at the end of the file.

### 3.196 Inf

A value representing +infinity or a value representing -infinity that can be stored in a floating type. Not all systems support the Inf values.

### 3.197 Instrumented Application

An application that contains at least one call to the trace point function posix_trace_event(). Each process of an instrumented application has a mapping of trace event names to trace event type identifiers. This mapping is used by the trace stream that is created for that process.

### 3.198 Interactive Shell

A processing mode of the shell that is suitable for direct user interaction.

### 3.199 Internationalization

The provision within a computer program of the capability of making itself adaptable to the requirements of different native languages, local customs, and coded character sets.

### 3.200 Interprocess Communication

A functionality enhancement to add a high-performance, deterministic interprocess communication facility for local communication.

### 3.201 Invoke

To perform command search and execution actions, except that searching for shell functions and special built-in utilities is suppressed; see also Execute.

Note:
  Command Search and Execution is defined in detail in XCU Command Search and Execution.

### 3.202 Job

A set of processes, comprising a shell pipeline, and any processes descended from it, that are all in the same process group.

Note:
  See also XCU Pipelines.

### 3.203 Job Control

A facility that allows users selectively to stop (suspend) the execution of processes and continue (resume) their execution at a later point. The user typically employs this facility via the interactive interface jointly supplied by the terminal I/O driver and a command interpreter.

### 3.204 Job Control Job ID

A handle that is used to refer to a job. The job control job ID can be any of the forms shown in the following table:

Table: Job Control Job ID Formats

Job Control Job ID | Meaning
--- | ---
%% | Current job.
%+ | Current job.
%- | Previous job.
%n | Job number n.
%string | Job whose command begins with string.
%?string | Job whose command contains string.

### 3.205 Last Close (of a File)

When a process closes a file, resulting in the file not being an open file within any process.

### 3.206 Line

A sequence of zero or more non- <newline> characters plus a terminating <newline> character.

### 3.207 Linger

The period of time before terminating a connection, to allow outstanding data to be transferred.

### 3.208 Link

See Directory Entry in Directory Entry (or Link).

### 3.209 Link Count

The number of directory entries that refer to a particular file.

### 3.210 Local Customs

The conventions of a geographical area or territory for such things as date, time, and currency formats.

### 3.211 Local Interprocess Communication (Local IPC)

The transfer of data between processes in the same system.

### 3.212 Locale

The definition of the subset of a user's environment that depends on language and cultural conventions.

Note:
  Locales are defined in detail in Locale.

### 3.213 Localization

The process of establishing information within a computer system specific to the operation of particular native languages, local customs, and coded character sets.

### 3.214 Login

The unspecified activity by which a user gains access to the system. Each login is associated with exactly one login name.

### 3.215 Login Name

A user name that is associated with a login.

### 3.216 Map

To create an association between a page-aligned range of the address space of a process and some memory object, such that a reference to an address in that range of the address space results in a reference to the associated memory object. The mapped memory object is not necessarily memory-resident.

### 3.217 Marked Message

A STREAMs message on which a certain flag is set. Marking a message gives the application protocol-specific information. An application can use ioctl() to determine whether a given message is marked.

Note:
  The ioctl() function is defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.218 Matched

A state applying to a sequence of zero or more characters when the characters in the sequence correspond to a sequence of characters defined by a basic regular expression or extended regular expression pattern.

Note:
  Regular Expressions are defined in detail in Regular Expressions.

### 3.219 Memory Mapped Files

A facility to allow applications to access files as part of the address space.

### 3.220 Memory Object

One of:

- A file (see File)

- A shared memory object (see Shared Memory Object)

- A typed memory object (see Typed Memory Object)

When used in conjunction with mmap(), a memory object appears in the address space of the calling process.

Note:
  The mmap() function is defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.221 Memory-Resident

The process of managing the implementation in such a way as to provide an upper bound on memory access times.

### 3.222 Message

In the context of programmatic message passing, information that can be transferred between processes or threads by being added to and removed from a message queue. A message consists of a fixed-size message buffer.

### 3.223 Message Catalog

In the context of providing natural language messages to the user, a file or storage area containing program messages, command prompts, and responses to prompts for a particular native language, territory, and codeset.

### 3.224 Message Catalog Descriptor

In the context of providing natural language messages to the user, a per-process unique value used to identify an open message catalog. A message catalog descriptor may be implemented using a file descriptor.

### 3.225 Message Queue

In the context of programmatic message passing, an object to which messages can be added and removed. Messages may be removed in the order in which they were added or in priority order.

### 3.226 Mode

A collection of attributes that specifies a file's type and its access permissions.

Note:
  File Access Permissions are defined in detail in File Access Permissions.

### 3.227 Monotonic Clock

A clock measuring real time, whose value cannot be set via clock_settime() and which cannot have negative clock jumps.

### 3.228 Mount Point

Either the system root directory or a directory for which the st_dev field of structure stat differs from that of its parent directory.

Note:
  The stat structure is defined in detail in <sys/stat.h>.

### 3.229 Multi-Character Collating Element

A sequence of two or more characters that collate as an entity. For example, in some coded character sets, an accented character is represented by a non-spacing accent, followed by the letter. Other examples are the Spanish elements ch and ll.

### 3.230 Mutex

A synchronization object used to allow multiple threads to serialize their access to shared data. The name derives from the capability it provides; namely, mutual-exclusion. The thread that has locked a mutex becomes its owner and remains the owner until that same thread unlocks the mutex.

### 3.231 Name

In the shell command language, a word consisting solely of underscores, digits, and alphabetics from the portable character set. The first character of a name is not a digit.

Note:
  The Portable Character Set is defined in detail in Portable Character Set.

### 3.232 Named STREAM

A STREAMS-based file descriptor that is attached to a name in the file system name space. All subsequent operations on the named STREAM act on the STREAM that was associated with the file descriptor until the name is disassociated from the STREAM.

### 3.233 NaN (Not a Number)

A set of values that may be stored in a floating type but that are neither Inf nor valid floating-point numbers. Not all systems support NaN values.

### 3.234 Native Language

A computer user's spoken or written language, such as American English, British English, Danish, Dutch, French, German, Italian, Japanese, Norwegian, or Swedish.

### 3.235 Negative Response

An input string that matches one of the responses acceptable to the LC_MESSAGES category keyword noexpr, matching an extended regular expression in the current locale.

Note:
  The LC_MESSAGES category is defined in detail in LC_MESSAGES.

### 3.236 Network

A collection of interconnected hosts.

Note:
  The term "network" in POSIX.1-2008 is used to refer to the network of hosts. The term "batch system" is used to refer to the network of batch servers.

### 3.237 Network Address

A network-visible identifier used to designate specific endpoints in a network. Specific endpoints on host systems have addresses, and host systems may also have addresses.

### 3.238 Network Byte Order

The way of representing any integer type such that, when transmitted over a network via a network endpoint, the int type is transmitted as an appropriate number of octets with the most significant octet first, followed by any other octets in descending order of significance.

Note:
  This order is more commonly known as big-endian ordering. See also Host and Network Byte Orders.

### 3.239 Newline Character (<newline>)

A character that in the output stream indicates that printing should start at the beginning of the next line. It is the character designated by `'\n'` in the C language. It is unspecified whether this character is the exact sequence transmitted to an output device by the system to accomplish the movement to the next line.

### 3.240 Nice Value

A number used as advice to the system to alter process scheduling. Numerically smaller values give a process additional preference when scheduling a process to run. Numerically larger values reduce the preference and make a process less likely to run. Typically, a process with a smaller nice value runs to completion more quickly than an equivalent process with a higher nice value. The symbol {NZERO} specifies the default nice value of the system.

### 3.241 Non-Blocking

A property of an open file description that causes function calls involving it to return without delay when it is detected that the requested action associated with the function call cannot be completed without unknown delay.

Note:
  The exact semantics are dependent on the type of file associated with the open file description. For data reads from devices such as ttys and FIFOs, this property causes the read to return immediately when no data was available. Similarly, for writes, it causes the call to return immediately when the thread would otherwise be delayed in the write operation; for example, because no space was available. For networking, it causes functions not to await protocol events (for example, acknowledgements) to occur. See also XSH Socket I/O Mode.

### 3.242 Non-Spacing Characters

A character, such as a character representing a diacritical mark in the ISO/IEC 6937:1994 standard coded graphic character set, which is used in combination with other characters to form composite graphic symbols.

### 3.243 NUL

A character with all bits set to zero.

### 3.244 Null Byte

A byte with all bits set to zero.

### 3.245 Null Pointer

A pointer obtained by converting an integer constant expression with the value 0, or such an expression cast to type void *, to a pointer type; for example, (char *)0. The C language guarantees that a null pointer compares unequal to a pointer to any object or function, so it is used by many functions that return pointers to indicate an error.

### 3.246 Null String

See Empty String in Empty String (or Null String).

### 3.247 Null Wide-Character Code

A wide-character code with all bits set to zero.

### 3.248 Number-Sign Character (<number-sign>)

The character `'#'`, also known as hash sign.

### 3.249 Object File

A regular file containing the output of a compiler, formatted as input to a linkage editor for linking with other object files into an executable form. The methods of linking are unspecified and may involve the dynamic linking of objects at runtime. The internal format of an object file is unspecified, but a conforming application cannot assume an object file is a text file.

### 3.250 Octet

Unit of data representation that consists of eight contiguous bits.

### 3.251 Offset Maximum

An attribute of an open file description representing the largest value that can be used as a file offset.

### 3.252 Opaque Address

An address such that the entity making use of it requires no details about its contents or format.

### 3.253 Open File

A file that is currently associated with a file descriptor.

### 3.254 Open File Description

A record of how a process or group of processes is accessing a file. Each file descriptor refers to exactly one open file description, but an open file description can be referred to by more than one file descriptor. The file offset, file status, and file access modes are attributes of an open file description.

### 3.255 Operand

An argument to a command that is generally used as an object supplying information to a utility necessary to complete its processing. Operands generally follow the options in a command line.

Note:
  Utility Argument Syntax is defined in detail in Utility Argument Syntax.

### 3.256 Operator

In the shell command language, either a control operator or a redirection operator.

### 3.257 Option

An argument to a command that is generally used to specify changes in the utility's default behavior.

Note:
  Utility Argument Syntax is defined in detail in Utility Argument Syntax.

### 3.258 Option-Argument

A parameter that follows certain options. In some cases an option-argument is included within the same argument string as the option-in most cases it is the next argument.

Note:
  Utility Argument Syntax is defined in detail in Utility Argument Syntax.

### 3.259 Orientation

A stream has one of three orientations: unoriented, byte-oriented, or wide-oriented.

Note:
  For further information, see XSH Stream Orientation and Encoding Rules.

### 3.260 Orphaned Process Group

A process group in which the parent of every member is either itself a member of the group or is not a member of the group's session.

### 3.261 Page

The granularity of process memory mapping or locking.

Physical memory and memory objects can be mapped into the address space of a process on page boundaries and in integral multiples of pages. Process address space can be locked into memory (made memory-resident) on page boundaries and in integral multiples of pages.

### 3.262 Page Size

The size, in bytes, of the system unit of memory allocation, protection, and mapping. On systems that have segment rather than page-based memory architectures, the term "page" means a segment.

### 3.263 Parameter

In the shell command language, an entity that stores values. There are three types of parameters: variables (named parameters), positional parameters, and special parameters. Parameter expansion is accomplished by introducing a parameter with the `'$'` character.

Note:
  See also XCU Parameters and Variables.

In the C language, an object declared as part of a function declaration or definition that acquires a value on entry to the function, or an identifier following the macro name in a function-like macro definition.

### 3.264 Parent Directory

When discussing a given directory, the directory that both contains a directory entry for the given directory and is represented by the pathname dot-dot in the given directory.

When discussing other types of files, a directory containing a directory entry for the file under discussion.

This concept does not apply to dot and dot-dot.

### 3.265 Parent Process

The process which created (or inherited) the process under discussion.

### 3.266 Parent Process ID

An attribute of a new process identifying the parent of the process. The parent process ID of a process is the process ID of its creator, for the lifetime of the creator. After the creator's lifetime has ended, the parent process ID is the process ID of an implementation-defined system process.

### 3.267 Pathname

A string that is used to identify a file. In the context of POSIX.1-2008, a pathname may be limited to {PATH_MAX} bytes, including the terminating null byte. It has optional beginning <slash> characters, followed by zero or more filenames separated by <slash> characters. A pathname can optionally contain one or more trailing <slash> characters. Multiple successive <slash> characters are considered to be the same as one <slash>, except for the case of exactly two leading <slash> characters.

Note:
  If a pathname consists of only bytes corresponding to characters from the portable filename character set (see Portable Filename Character Set), <slash> characters, and a single terminating <NUL> character, the pathname will be usable as a character string in all supported locales; otherwise, the pathname might only be a string (rather than a character string). Additionally, since the single-byte encoding of the <slash> character is required to be the same across all locales and to not occur within a multi-byte character, references to a <slash> character within a pathname are well-defined even when the pathname is not a character string. However, this property does not necessarily hold for the remaining characters within the portable filename character set.

  Pathname Resolution is defined in detail in Pathname Resolution.

### 3.268 Pathname Component

See Filename in Filename.

### 3.269 Path Prefix

The part of a pathname up to, but not including, the last component and any trailing <slash> characters, unless the pathname consists entirely of <slash> characters, in which case the path prefix is `'/'` for a pathname containing either a single <slash> or three or more <slash> characters, and `'//'` for the pathname //. The path prefix of a pathname containing no <slash> characters is empty, but is treated as referring to the current working directory.

Note:
  The term is used both in the sense of identifying part of a pathname that forms the prefix and of joining a non-empty path prefix to a filename to form a pathname. In the latter case, the path prefix need not have a trailing <slash> (in which case the joining is done with a <slash> character).

### 3.270 Pattern

A sequence of characters used either with regular expression notation or for pathname expansion, as a means of selecting various character strings or pathnames, respectively.

Note:
  Regular Expressions are defined in detail in Regular Expressions.

  See also XCU Pathname Expansion.

The syntaxes of the two types of patterns are similar, but not identical; POSIX.1-2008 always indicates the type of pattern being referred to in the immediate context of the use of the term.

### 3.271 Period Character (<period>)

The character `'.'`. The term "period" is contrasted with dot (see also Dot), which is used to describe a specific directory entry.

### 3.272 Permissions

Attributes of an object that determine the privilege necessary to access or manipulate the object.

Note:
  File Access Permissions are defined in detail in File Access Permissions.

### 3.273 Persistence

A mode for semaphores, shared memory, and message queues requiring that the object and its state (including data, if any) are preserved after the object is no longer referenced by any process.

Persistence of an object does not imply that the state of the object is maintained across a system crash or a system reboot.

### 3.274 Pipe

An object identical to a FIFO which has no links in the file hierarchy.

Note:
  The pipe() function is defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.275 Polling

A scheduling scheme whereby the local process periodically checks until the pre-specified events (for example, read, write) have occurred.

### 3.276 Portable Character Set

The collection of characters that are required to be present in all locales supported by conforming systems.

Note:
  The Portable Character Set is defined in detail in Portable Character Set.

This term is contrasted against the smaller portable filename character set; see also Portable Filename Character Set.

### 3.277 Portable Filename

A filename consisting only of characters from the portable filename character set.

Note:
  Applications should avoid using filenames that have the <hyphen> character as the first character since this may cause problems when filenames are passed as command line arguments.

### 3.278 Portable Filename Character Set

The set of characters from which portable filenames are constructed.

    A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
    a b c d e f g h i j k l m n o p q r s t u v w x y z
    0 1 2 3 4 5 6 7 8 9 . _ -

The last three characters are the <period>, <underscore>, and <hyphen> characters, respectively. See also Pathname.

### 3.279 Positional Parameter

In the shell command language, a parameter denoted by a single digit or one or more digits in curly braces.

Note:
  For further information, see XCU Positional Parameters.

### 3.280 Preallocation

The reservation of resources in a system for a particular use.

Preallocation does not imply that the resources are immediately allocated to that use, but merely indicates that they are guaranteed to be available in bounded time when needed.

### 3.281 Preempted Process (or Thread)

A running thread whose execution is suspended due to another thread becoming runnable at a higher priority.

### 3.282 Previous Job

In the context of job control, the job that will be used as the default for the fg or bg utilities if the current job exits. There is at most one previous job; see also Job Control Job ID.

### 3.283 Printable Character

One of the characters included in the print character classification of the LC_CTYPE category in the current locale.

Note:
  The LC_CTYPE category is defined in detail in LC_CTYPE .

### 3.284 Printable File

A text file consisting only of the characters included in the print and space character classifications of the LC_CTYPE category and the <backspace>, all in the current locale.

Note:
  The LC_CTYPE category is defined in detail in LC_CTYPE .

### 3.285 Priority

A non-negative integer associated with processes or threads whose value is constrained to a range defined by the applicable scheduling policy. Numerically higher values represent higher priorities.

### 3.286 Priority Band

The queuing order applied to normal priority STREAMS messages. High priority STREAMS messages are not grouped by priority bands. The only differentiation made by the STREAMS mechanism is between zero and non-zero bands, but specific protocol modules may differentiate between priority bands.

### 3.287 Priority Inversion

A condition in which a thread that is not voluntarily suspended (waiting for an event or time delay) is not running while a lower priority thread is running. Such blocking of the higher priority thread is often caused by contention for a shared resource.

### 3.288 Priority Scheduling

A performance and determinism improvement facility to allow applications to determine the order in which threads that are ready to run are granted access to processor resources.

### 3.289 Priority-Based Scheduling

Scheduling in which the selection of a running thread is determined by the priorities of the runnable processes or threads.

### 3.290 Privilege

See Appropriate Privileges in Appropriate Privileges.

### 3.291 Process

An address space with one or more threads executing within that address space, and the required system resources for those threads.

Note:
  Many of the system resources defined by POSIX.1-2008 are shared among all of the threads within a process. These include the process ID, the parent process ID, process group ID, session membership, real, effective, and saved set-user-ID, real, effective, and saved set-group-ID, supplementary group IDs, current working directory, root directory, file mode creation mask, and file descriptors.

### 3.292 Process Group

A collection of processes that permits the signaling of related processes. Each process in the system is a member of a process group that is identified by a process group ID. A newly created process joins the process group of its creator.

### 3.293 Process Group ID

The unique positive integer identifier representing a process group during its lifetime.

Note:
  See also Process Group ID Reuse defined in Process ID Reuse .

### 3.294 Process Group Leader

A process whose process ID is the same as its process group ID.

### 3.295 Process Group Lifetime

The period of time that begins when a process group is created and ends when the last remaining process in the group leaves the group, due either to the end of the lifetime of the last process or to the last remaining process calling the setsid() or setpgid() functions.

Note:
  The setsid() and setpgid() functions are defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.296 Process ID

The unique positive integer identifier representing a process during its lifetime.

Note:
  See also Process ID Reuse defined in Process ID Reuse.

### 3.297 Process Lifetime

The period of time that begins when a process is created and ends when its process ID is returned to the system. After a process is created by fork(), posix_spawn(), or posix_spawnp(), it is considered active. At least one thread of control and address space exist until it terminates. It then enters an inactive state where certain resources may be returned to the system, although some resources, such as the process ID, are still in use. When another process executes a wait(), waitid(), or waitpid() function for an inactive process, the remaining resources are returned to the system. The last resource to be returned to the system is the process ID. At this time, the lifetime of the process ends.

Note:
  The fork(), posix_spawn(), posix_spawnp(), wait(), waitid(), and waitpid() functions are defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.298 Process Memory Locking

A performance improvement facility to bind application programs into the high-performance random access memory of a computer system. This avoids potential latencies introduced by the operating system in storing parts of a program that were not recently referenced on secondary memory devices.

### 3.299 Process Termination

There are two kinds of process termination:

1. Normal termination occurs by a return from main(), when requested with the exit(), _exit(), or _Exit() functions; or when the last thread in the process terminates by returning from its start function, by calling the pthread_exit() function, or through cancellation.

2. Abnormal termination occurs when requested by the abort() function or when some signals are received.

Note:
  The _exit(), _Exit(), abort(), and exit() functions are defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.300 Process-To-Process Communication

The transfer of data between processes.

### 3.301 Process Virtual Time

The measurement of time in units elapsed by the system clock while a process is executing.

### 3.302 Program

A prepared sequence of instructions to the system to accomplish a defined task. The term "program" in POSIX.1-2008 encompasses applications written in the Shell Command Language, complex utility input languages (for example, awk, lex, sed, and so on), and high-level languages.

### 3.303 Protocol

A set of semantic and syntactic rules for exchanging information.

### 3.304 Pseudo-Terminal

A facility that provides an interface that is identical to the terminal subsystem, except where noted otherwise in POSIX.1-2008. A pseudo-terminal is composed of two devices: the "master device" and a "slave device". The slave device provides processes with an interface that is identical to the terminal interface, although there need not be hardware behind that interface. Anything written on the master device is presented to the slave as an input and anything written on the slave device is presented as an input on the master side.

### 3.305 Radix Character

The character that separates the integer part of a number from the fractional part.

### 3.306 Read-Only File System

A file system that has implementation-defined characteristics restricting modifications.

Note:
  File Times Update is described in detail in File Times Update.

### 3.307 Read-Write Lock

Multiple readers, single writer (read-write) locks allow many threads to have simultaneous read-only access to data while allowing only one thread to have write access at any given time. They are typically used to protect data that is read-only more frequently than it is changed.

Read-write locks can be used to synchronize threads in the current process and other processes if they are allocated in memory that is writable and shared among the cooperating processes and have been initialized for this behavior.

### 3.308 Real Group ID

The attribute of a process that, at the time of process creation, identifies the group of the user who created the process; see also Group ID.

### 3.309 Real Time

Time measured as total units elapsed by the system clock without regard to which thread is executing.

### 3.310 Realtime Signal Extension

A determinism improvement facility to enable asynchronous signal notifications to an application to be queued without impacting compatibility with the existing signal functions.

### 3.311 Real User ID

The attribute of a process that, at the time of process creation, identifies the user who created the process; see also User ID.

### 3.312 Record

A collection of related data units or words which is treated as a unit.

### 3.313 Redirection

In the shell command language, a method of associating files with the input or output of commands.

Note:
  For further information, see XCU Redirection.

### 3.314 Redirection Operator

In the shell command language, a token that performs a redirection function. It is one of the following symbols:

    <     >     >|     <<     >>     <&     >&     <<-     <>

### 3.315 Referenced Shared Memory Object

A shared memory object that is open or has one or more mappings defined on it.

### 3.316 Refresh

To ensure that the information on the user's terminal screen is up-to-date.

### 3.317 Regular Expression

A pattern that selects specific strings from a set of character strings.

Note:
  Regular Expressions are described in detail in Regular Expressions .

### 3.318 Region

In the context of the address space of a process, a sequence of addresses.

In the context of a file, a sequence of offsets.

### 3.319 Regular File

A file that is a randomly accessible sequence of bytes, with no further structure imposed by the system.

### 3.320 Relative Pathname

A pathname not beginning with a <slash> character.

Note:
  Pathname Resolution is defined in detail in Pathname Resolution .

### 3.321 Relocatable File

A file holding code or data suitable for linking with other object files to create an executable or a shared object file.

### 3.322 Relocation

The process of connecting symbolic references with symbolic definitions. For example, when a program calls a function, the associated call instruction transfers control to the proper destination address at execution.

### 3.323 Requested Batch Service

A service that is either rejected or performed prior to a response from the service to the requester.

### 3.324 (Time) Resolution

The minimum time interval that a clock can measure or whose passage a timer can detect.

### 3.325 Robust Mutex

A mutex with the robust attribute set.

Note:
  The robust attribute is defined in detail by the pthread_mutexattr_getrobust() function.

### 3.326 Root Directory

A directory, associated with a process, that is used in pathname resolution for pathnames that begin with a <slash> character.

### 3.327 Runnable Process (or Thread)

A thread that is capable of being a running thread, but for which no processor is available.

### 3.328 Running Process (or Thread)

A thread currently executing on a processor. On multi-processor systems there may be more than one such thread in a system at a time.

### 3.329 Saved Resource Limits

An attribute of a process that provides some flexibility in the handling of unrepresentable resource limits, as described in the exec family of functions and setrlimit().

Note:
  The exec and setrlimit() functions are defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.330 Saved Set-Group-ID

An attribute of a process that allows some flexibility in the assignment of the effective group ID attribute, as described in the exec family of functions and setgid().

Note:
  The exec and setgid() functions are defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.331 Saved Set-User-ID

An attribute of a process that allows some flexibility in the assignment of the effective user ID attribute, as described in the exec family of functions and setuid().

Note:
  The exec and setuid() functions are defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.332 Scheduling

The application of a policy to select a runnable process or thread to become a running process or thread, or to alter one or more of the thread lists.

### 3.333 Scheduling Allocation Domain

The set of processors on which an individual thread can be scheduled at any given time.

### 3.334 Scheduling Contention Scope

A property of a thread that defines the set of threads against which that thread competes for resources.

For example, in a scheduling decision, threads sharing scheduling contention scope compete for processor resources. In POSIX.1-2008, a thread has scheduling contention scope of either PTHREAD_SCOPE_SYSTEM or PTHREAD_SCOPE_PROCESS.

### 3.335 Scheduling Policy

A set of rules that is used to determine the order of execution of processes or threads to achieve some goal.

Note:
  Scheduling Policy is defined in detail in Scheduling Policy.

### 3.336 Screen

A rectangular region of columns and lines on a terminal display. A screen may be a portion of a physical display device or may occupy the entire physical area of the display device.

### 3.337 Scroll

To move the representation of data vertically or horizontally relative to the terminal screen. There are two types of scrolling:

1. The cursor moves with the data.

2. The cursor remains stationary while the data moves.

### 3.338 Semaphore

A minimum synchronization primitive to serve as a basis for more complex synchronization mechanisms to be defined by the application program.

Note:
  Semaphores are defined in detail in Semaphore.

### 3.339 Session

A collection of process groups established for job control purposes. Each process group is a member of a session. A process is considered to be a member of the session of which its process group is a member. A newly created process joins the session of its creator. A process can alter its session membership; see setsid(). There can be multiple process groups in the same session.

Note:
  The setsid() function is defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.340 Session Leader

A process that has created a session.

Note:
  For further information, see the setsid() function defined in the System Interfaces volume of POSIX.1-2008.

### 3.341 Session Lifetime

The period between when a session is created and the end of the lifetime of all the process groups that remain as members of the session.

### 3.342 Shared Memory Object

An object that represents memory that can be mapped concurrently into the address space of more than one process.

### 3.343 Shell

A program that interprets sequences of text input as commands. It may operate on an input stream or it may interactively prompt and read commands from a terminal.

### 3.344 Shell, the

The Shell Command Language Interpreter; a specific instance of a shell.

Note:
  For further information, see the sh utility defined in the Shell and Utilities volume of POSIX.1-2008.

### 3.345 Shell Script

A file containing shell commands. If the file is made executable, it can be executed by specifying its name as a simple command. Execution of a shell script causes a shell to execute the commands within the script. Alternatively, a shell can be requested to execute the commands in a shell script by specifying the name of the shell script as the operand to the sh utility.

Note:
  Simple Commands are defined in detail in XCU Simple Commands.

  The sh utility is defined in detail in the Shell and Utilities volume of POSIX.1-2008.

### 3.346 Signal

A mechanism by which a process or thread may be notified of, or affected by, an event occurring in the system. Examples of such events include hardware exceptions and specific actions by processes. The term signal is also used to refer to the event itself.

### 3.347 Signal Stack

Memory established for a thread, in which signal handlers catching signals sent to that thread are executed.

### 3.348 Single-Quote Character

The character designated by `'\'` in the C language, also known as <apostrophe>.

### 3.349 Slash Character (<slash>)

The character `'/'`, also known as solidus.

### 3.350 Socket

A file of a particular type that is used as a communications endpoint for process-to-process communication as described in the System Interfaces volume of POSIX.1-2008.

### 3.351 Socket Address

An address associated with a socket or remote endpoint, including an address family identifier and addressing information specific to that address family. The address may include multiple parts, such as a network address associated with a host system and an identifier for a specific endpoint.

### 3.352 Soft Limit

A resource limitation established for each process that the process may set to any value less than or equal to the hard limit.

### 3.353 Source Code

When dealing with the Shell Command Language, input to the command language interpreter. The term "shell script" is synonymous with this meaning.

When dealing with an ISO/IEC-conforming programming language, source code is input to a compiler conforming to that ISO/IEC standard.

Source code also refers to the input statements prepared for the following standard utilities: awk, bc, ed, lex, localedef, make, sed, and yacc.

Source code can also refer to a collection of sources meeting any or all of these meanings.

Note:
  The awk, bc, ed, lex, localedef, make, sed, and yacc utilities are defined in detail in the Shell and Utilities volume of POSIX.1-2008.

### 3.354 Space Character (<space>)

The character defined in the portable character set as <space>. The <space> character is a member of the space character class of the current locale, but represents the single character, and not all of the possible members of the class; see also White Space.

### 3.355 Spawn

A process creation primitive useful for systems that have difficulty with fork() and as an efficient replacement for fork()/ exec.

### 3.356 Special Built-In

See Built-In Utility in Built-In Utility (or Built-In).

### 3.357 Special Parameter

In the shell command language, a parameter named by a single character from the following list:

    *   @   #   ?   !   -   $   0

Note:
  For further information, see XCU Special Parameters.

### 3.358 Spin Lock

A synchronization object used to allow multiple threads to serialize their access to shared data.

### 3.359 Sporadic Server

A scheduling policy for threads and processes that reserves a certain amount of execution capacity for processing aperiodic events at a given priority level.

### 3.360 Standard Error

An output stream usually intended to be used for diagnostic messages.

### 3.361 Standard Input

An input stream usually intended to be used for primary data input.

### 3.362 Standard Output

An output stream usually intended to be used for primary data output.

### 3.363 Standard Utilities

The utilities described in the Shell and Utilities volume of POSIX.1-2008.

### 3.364 Stream

Appearing in lowercase, a stream is a file access object that allows access to an ordered sequence of characters, as described by the ISO C standard. Such objects can be created by the fdopen(), fmemopen(), fopen(), open_memstream(), or popen() functions, and are associated with a file descriptor. A stream provides the additional services of user-selectable buffering and formatted input and output; see also STREAM.

Note:
  For further information, see XSH Standard I/O Streams.

  The fdopen(), fmemopen(), fopen(), open_memstream(), and popen() functions are defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.365 STREAM

Appearing in uppercase, STREAM refers to a full-duplex connection between a process and an open device or pseudo-device. It optionally includes one or more intermediate processing modules that are interposed between the process end of the STREAM and the device driver (or pseudo-device driver) end of the STREAM; see also Stream.

Note:
  For further information, see XSH STREAMS.

### 3.366 STREAM End

The STREAM end is the driver end of the STREAM and is also known as the downstream end of the STREAM.

### 3.367 STREAM Head

The STREAM head is the beginning of the STREAM and is at the boundary between the system and the application process. This is also known as the upstream end of the STREAM.

### 3.368 STREAMS Multiplexor

A driver with multiple STREAMS connected to it. Multiplexing with STREAMS connected above is referred to as N-to-1, or "upper multiplexing". Multiplexing with STREAMS connected below is referred to as 1-to-N or "lower multiplexing".

### 3.369 String

A contiguous sequence of bytes terminated by and including the first null byte.

### 3.370 Subshell

A shell execution environment, distinguished from the main or current shell execution environment.

Note:
  For further information, see XCU Shell Execution Environment.

### 3.371 Successfully Transferred

For a write operation to a regular file, when the system ensures that all data written is readable on any subsequent open of the file (even one that follows a system or power failure) in the absence of a failure of the physical storage medium.

For a read operation, when an image of the data on the physical storage medium is available to the requesting process.

### 3.372 Supplementary Group ID

An attribute of a process used in determining file access permissions. A process has up to {NGROUPS_MAX} supplementary group IDs in addition to the effective group ID. The supplementary group IDs of a process are set to the supplementary group IDs of the parent process when the process is created.

### 3.373 Suspended Job

A job that has received a SIGSTOP, SIGTSTP, SIGTTIN, or SIGTTOU signal that caused the process group to stop. A suspended job is a background job, but a background job is not necessarily a suspended job.

### 3.374 Symbolic Constant

An object-like macro defined with a constant value.

Unless stated otherwise, the following shall apply to every symbolic constant:

- It expands to a compile-time constant expression with an integer type.

- It may be defined as another type of constant-e.g., an enumeration constant-as well as being a macro.

- It need not be usable in #if preprocessing directives.

### 3.375 Symbolic Link

A type of file with the property that when the file is encountered during pathname resolution, a string stored by the file is used to modify the pathname resolution. The stored string has a length of {SYMLINK_MAX} bytes or fewer.

Note:
  Pathname Resolution is defined in detail in Pathname Resolution .

### 3.376 Synchronized Input and Output

A determinism and robustness improvement mechanism to enhance the data input and output mechanisms, so that an application can ensure that the data being manipulated is physically present on secondary mass storage devices.

### 3.377 Synchronized I/O Completion

The state of an I/O operation that has either been successfully transferred or diagnosed as unsuccessful.

### 3.378 Synchronized I/O Data Integrity Completion

For read, when the operation has been completed or diagnosed if unsuccessful. The read is complete only when an image of the data has been successfully transferred to the requesting process. If there were any pending write requests affecting the data to be read at the time that the synchronized read operation was requested, these write requests are successfully transferred prior to reading the data.

For write, when the operation has been completed or diagnosed if unsuccessful. The write is complete only when the data specified in the write request is successfully transferred and all file system information required to retrieve the data is successfully transferred.

File attributes that are not necessary for data retrieval (access time, modification time, status change time) need not be successfully transferred prior to returning to the calling process.

### 3.379 Synchronized I/O File Integrity Completion

Identical to a synchronized I/O data integrity completion with the addition that all file attributes relative to the I/O operation (including access time, modification time, status change time) are successfully transferred prior to returning to the calling process.

### 3.380 Synchronized I/O Operation

An I/O operation performed on a file that provides the application assurance of the integrity of its data and files.

### 3.381 Synchronous I/O Operation

An I/O operation that causes the thread requesting the I/O to be blocked from further use of the processor until that I/O operation completes.

Note:
  A synchronous I/O operation does not imply synchronized I/O data integrity completion or synchronized I/O file integrity completion.

### 3.382 Synchronously-Generated Signal

A signal that is attributable to a specific thread.

For example, a thread executing an illegal instruction or touching invalid memory causes a synchronously-generated signal. Being synchronous is a property of how the signal was generated and not a property of the signal number.

### 3.383 System

An implementation of POSIX.1-2008.

### 3.384 System Boot

An unspecified sequence of events that may result in the loss of transitory data; that is, data that is not saved in permanent storage. For example, message queues, shared memory, semaphores, and processes.

### 3.385 System Clock

A clock with at least one second resolution that contains seconds since the Epoch.

### 3.386 System Console

A device that receives messages sent by the syslog() function, and the fmtmsg() function when the MM_CONSOLE flag is set.

Note:
  The syslog() and fmtmsg() functions are defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.387 System Crash

An interval initiated by an unspecified circumstance that causes all processes (possibly other than special system processes) to be terminated in an undefined manner, after which any changes to the state and contents of files created or written to by an application prior to the interval are undefined, except as required elsewhere in POSIX.1-2008.

### 3.388 System Databases

An implementation provides two system databases: the "group database" (see also Group Database) and the "user database" (see also User Database).

### 3.389 System Documentation

All documentation provided with an implementation except for the conformance document. Electronically distributed documents for an implementation are considered part of the system documentation.

### 3.390 System Process

An object other than a process executing an application, that is provided by the system and has a process ID.

### 3.391 System Reboot

See System Boot defined in System Boot.

### 3.392 System Trace Event

A trace event that is generated by the implementation, in response either to a system-initiated action or to an application-requested action, except for a call to posix_trace_event(). When supported by the implementation, a system-initiated action generates a process-independent system trace event and an application-requested action generates a process-dependent system trace event. For a system trace event not defined by POSIX.1-2008, the associated trace event type identifier is derived from the implementation-defined name for this trace event, and the associated data is of implementation-defined content and length.

### 3.393 System-Wide

Pertaining to events occurring in all processes existing in an implementation at a given point in time.

### 3.394 Tab Character (<tab>)

A character that in the output stream indicates that printing or displaying should start at the next horizontal tabulation position on the current line. It is the character designated by `'\t'` in the C language. If the current position is at or past the last defined horizontal tabulation position, the behavior is unspecified. It is unspecified whether this character is the exact sequence transmitted to an output device by the system to accomplish the tabulation.

### 3.395 Terminal (or Terminal Device)

A character special file that obeys the specifications of the general terminal interface.

Note:
  The General Terminal Interface is defined in detail in General Terminal Interface.

### 3.396 Text Column

A roughly rectangular block of characters capable of being laid out side-by-side next to other text columns on an output page or terminal screen. The widths of text columns are measured in column positions.

### 3.397 Text File

A file that contains characters organized into zero or more lines. The lines do not contain NUL characters and none can exceed {LINE_MAX} bytes in length, including the <newline> character. Although POSIX.1-2008 does not distinguish between text files and binary files (see the ISO C standard), many utilities only produce predictable or meaningful output when operating on text files. The standard utilities that have such restrictions always specify "text files" in their STDIN or INPUT FILES sections.

### 3.398 Thread

A single flow of control within a process. Each thread has its own thread ID, scheduling priority and policy, errno value, floating point environment, thread-specific key/value bindings, and the required system resources to support a flow of control. Anything whose address may be determined by a thread, including but not limited to static variables, storage obtained via malloc(), directly addressable storage obtained through implementation-defined functions, and automatic variables, are accessible to all threads in the same process.

Note:
  The malloc() function is defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.399 Thread ID

Each thread in a process is uniquely identified during its lifetime by a value of type pthread_t called a thread ID.

### 3.400 Thread List

An ordered set of runnable threads that all have the same ordinal value for their priority.

The ordering of threads on the list is determined by a scheduling policy or policies. The set of thread lists includes all runnable threads in the system.

### 3.401 Thread-Safe

A thread-safe function can be safely invoked concurrently with other calls to the same function, or with calls to any other thread-safe functions, by multiple threads. Each function defined in the System Interfaces volume of POSIX.1-2008 is thread-safe unless explicitly stated otherwise. Examples are any "pure" function, a function which holds a mutex locked while it is accessing static storage, or objects shared among threads.

### 3.402 Thread-Specific Data Key

A process global handle of type pthread_key_t which is used for naming thread-specific data.

Although the same key value may be used by different threads, the values bound to the key by pthread_setspecific() and accessed by pthread_getspecific() are maintained on a per-thread basis and persist for the life of the calling thread.

Note:
  The pthread_getspecific() and pthread_setspecific() functions are defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.403 Tilde Character (<tilde>)

The character `'˜'`.

### 3.404 Timeouts

A method of limiting the length of time an interface will block; see also Blocked Process (or Thread) .

### 3.405 Timer

A mechanism that can notify a thread when the time as measured by a particular clock has reached or passed a specified value, or when a specified amount of time has passed.

### 3.406 Timer Overrun

A condition that occurs each time a timer, for which there is already an expiration signal queued to the process, expires.

### 3.407 Token

In the shell command language, a sequence of characters that the shell considers as a single unit when reading input. A token is either an operator or a word.

Note:
  The rules for reading input are defined in detail in XCU Token Recognition.

### 3.408 Trace Analyzer Process

A process that extracts trace events from a trace stream to retrieve information about the behavior of an application.

### 3.409 Trace Controller Process

A process that creates a trace stream for tracing a process.

### 3.410 Trace Event

A data object that represents an action executed by the system, and that is recorded in a trace stream.

### 3.411 Trace Event Type

A data object type that defines a class of trace event.

### 3.412 Trace Event Type Mapping

A one-to-one mapping between trace event types and trace event names.

### 3.413 Trace Filter

A filter that allows the trace controller process to specify those trace event types that are to be ignored; that is, not generated.

### 3.414 Trace Generation Version

A data object that is an implementation-defined character string, generated by the trace system and describing the origin and version of the trace system.

### 3.415 Trace Log

The flushed image of a trace stream, if the trace stream is created with a trace log.

### 3.416 Trace Point

An action that may cause a trace event to be generated.

### 3.417 Trace Stream

An opaque object that contains trace events plus internal data needed to interpret those trace events.

### 3.418 Trace Stream Identifier

A handle to manage tracing operations in a trace stream.

### 3.419 Trace System

A system that allows both system and user trace events to be generated into a trace stream. These trace events can be retrieved later.

### 3.420 Traced Process

A process for which at least one trace stream has been created. A traced process is also called a target process.

### 3.421 Tracing Status of a Trace Stream

A status that describes the state of an active trace stream. The tracing status of a trace stream can be retrieved from the trace stream attributes. An active trace stream can be in one of two states: running or suspended.

### 3.422 Typed Memory Name Space

A system-wide name space that contains the names of the typed memory objects present in the system. It is configurable for a given implementation.

### 3.423 Typed Memory Object

A combination of a typed memory pool and a typed memory port. The entire contents of the pool are accessible from the port. The typed memory object is identified through a name that belongs to the typed memory name space.

### 3.424 Typed Memory Pool

An extent of memory with the same operational characteristics. Typed memory pools may be contained within each other.

### 3.425 Typed Memory Port

A hardware access path to one or more typed memory pools.

### 3.426 Unbind

Remove the association between a network address and an endpoint.

### 3.427 Unit Data

See Datagram in Datagram.

### 3.428 Upshifting

The conversion of a lowercase character that has a single-character uppercase representation into this uppercase representation.

### 3.429 User Database

A system database that contains at least the following information for each user ID:

- User name

- Numerical user ID

- Initial numerical group ID

- Initial working directory

- Initial user program

The initial numerical group ID is used by the newgrp utility. Any other circumstances under which the initial values are operative are implementation-defined.

If the initial user program field is null, an implementation-defined program is used.

If the initial working directory field is null, the interpretation of that field is implementation-defined.

Note:
  The newgrp utility is defined in detail in the Shell and Utilities volume of POSIX.1-2008.

### 3.430 User ID

A non-negative integer that is used to identify a system user. When the identity of a user is associated with a process, a user ID value is referred to as a real user ID, an effective user ID, or a saved set-user-ID.

### 3.431 User Name

A string that is used to identify a user; see also User Database. To be portable across systems conforming to POSIX.1-2008, the value is composed of characters from the portable filename character set. The <hyphen> character should not be used as the first character of a portable user name.

### 3.432 User Trace Event

A trace event that is generated explicitly by the application as a result of a call to posix_trace_event().

### 3.433 Utility

A program, excluding special built-in utilities provided as part of the Shell Command Language, that can be called by name from a shell to perform a specific task, or related set of tasks.

Note:
  For further information on special built-in utilities, see XCU Special Built-In Utilities.

### 3.434 Variable

In the shell command language, a named parameter.

Note:
  For further information, see XCU Parameters and Variables.

### 3.435 Vertical-Tab Character (<vertical-tab>)

A character that in the output stream indicates that printing should start at the next vertical tabulation position. It is the character designated by `'\v'` in the C language. If the current position is at or past the last defined vertical tabulation position, the behavior is unspecified. It is unspecified whether this character is the exact sequence transmitted to an output device by the system to accomplish the tabulation.

### 3.436 White Space

A sequence of one or more characters that belong to the space character class as defined via the LC_CTYPE category in the current locale.

In the POSIX locale, white space consists of one or more <blank> ( <space> and <tab> characters), <newline>, <carriage-return>, <form-feed>, and <vertical-tab> characters.

### 3.437 Wide-Character Code (C Language)

An integer value corresponding to a single graphic symbol or control code.

Note:
  C Language Wide-Character Codes are defined in detail in C Language Wide-Character Codes.

### 3.438 Wide-Character Input/Output Functions

The functions that perform wide-oriented input from streams or wide-oriented output to streams: fgetwc(), fgetws(), fputwc(), fputws(), fwprintf(), fwscanf(), getwc(), getwchar(), putwc(), putwchar(), ungetwc(), vfwprintf(), vfwscanf(), vwprintf(), vwscanf(), wprintf(), and wscanf().

Note:
  These functions are defined in detail in the System Interfaces volume of POSIX.1-2008.

### 3.439 Wide-Character String

A contiguous sequence of wide-character codes terminated by and including the first null wide-character code.

### 3.440 Word

In the shell command language, a token other than an operator. In some cases a word is also a portion of a word token: in the various forms of parameter expansion, such as ${name-word}, and variable assignment, such as name=word, the word is the portion of the token depicted by word. The concept of a word is no longer applicable following word expansions-only fields remain.

Note:
  For further information, see XCU Parameter Expansion and wordexp.

### 3.441 Working Directory (or Current Working Directory)

A directory, associated with a process, that is used in pathname resolution for pathnames that do not begin with a <slash> character.

### 3.442 Worldwide Portability Interface

Functions for handling characters in a codeset-independent manner.

### 3.443 Write

To output characters to a file, such as standard output or standard error. Unless otherwise stated, standard output is the default output destination for all uses of the term "write''; see the distinction between display and write in Display.

### 3.444 XSI

The X/Open System Interfaces (XSI) option is the core application programming interface for C and sh programming for systems conforming to the Single UNIX Specification. This is a superset of the mandatory requirements for conformance to POSIX.1-2008.

### 3.445 XSI-Conformant

A system which allows an application to be built using a set of services that are consistent across all systems that conform to POSIX.1-2008 and that support the XSI option.

Note:
  See also Conformance.

### 3.446 Zombie Process

A process that has terminated and that is deleted when its exit status has been reported to another process which is waiting for that process to terminate.

### 3.447 ±0

The algebraic sign provides additional information about any variable that has the value zero when the representation allows the sign to be determined.

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