Glossary

 absolute pathname

See pathname resolution .

 access mode

A particular form of access permitted to a file.

 additional file access control mechanism

See file access permissions .

 address space

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

 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; see LC_MESSAGES .

 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 character to standard output (unless the utility description indicates that the use of standard output produces undefined results in this case).

 alert character

A character that in the output stream should cause a terminal to alert its user via a visual or audible notification. The alert character 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.

 alias name

A word consisting solely of underscores, digits and alphabetics from the portable character set (see Portable Character Set ) and any of the following characters:

 

!    %    ,    @

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

 alternate file access control mechanism

See file access permissions .

 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.

 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.

 appropriate privileges

An implementation-dependent means of associating privileges with a process with regard to the function calls and function call options defined in the XSH specification, and the commands in the XCU specification, that need special privileges. There may be zero or more such means.

 argument

In the shell, a parameter passed to a utility as the equivalent of a single string in the argv array created by one of the exec functions. See Utility Argument Syntax and the XCU specification, Command Search and Execution . An argument is one of the options, option-arguments or operands following the command name.

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

 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.

 assignment

See variable assignment .

 asterisk

The character "*".

 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.

 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.

 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.

 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.

 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.

 background job

See background process group .

 background process

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

 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.

 backquote

The character `, also known as a grave accent.

 backslash

The character "\", also known as a reverse solidus.

 backspace character

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 behaviour is unspecified. The backspace 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 character defined here is not necessarily the ERASE special character defined in Special Characters .

 base character

One of the set of characters defined in the Latin alphabet. In Western European languages other than English, these characters are commonly used with diacritical marks (accents, cedilla, and so on) to extend the range of characters in an alphabet.

 basename

The final, or only, filename in a pathname.

 basic regular expression

A pattern constructed according to the rules defined in Basic Regular Expressions .

 blank character

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 character.

 blank line

A line consisting solely of zero or more blank characters terminated by a newline character. See also empty line .

 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.

 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. See Output Devices and Terminal Types .

 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.

 braces

The characters "{" (left brace) and "}" (right brace), also known as curly braces. 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-brace> and <right-brace> are used.

 brackets

The characters "[" (left-bracket) and "]" (right-bracket), also known as square brackets. 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.

 break value

The address at which dynamic memory allocation starts.

 built-in utility

(Or built-in.) A utility implemented within a shell. The utilities referred to as special built-ins have special qualities, described in the XCU specification, Special Built-in Utilities . Unless qualified, the term built-in includes the special built-in utilities. The utilities referred to as regular built-ins are those named in the XCU specification, Command Search and Execution . There is no requirement that these utilities be actually built into the shell on the implementation, but they do have special command-search qualities.

 byte

An individually addressable unit of data storage that is equal to or larger than an octet, used to store a character or a portion of a character; see character . A byte is composed of a contiguous sequence of bits, the number of which is implementation-dependent. The least significant bit is called the low-order bit; the most significant is called the high-order bit. Note that this definition of byte deviates intentionally from the usage of byte in some international standards, where it is used as a synonym for octet (always eight bits). On a system based on the ISO/IEC 9945-2:1993 standard, a byte may be larger than eight bits so that it can be an integral portion of larger data objects that are not evenly divisible by eight bits (such as a 36-bit word that contains four 9-bit bytes).

 carriage-return character

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 character occurred. The carriage-return 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.

 character

A sequence of one or more bytes representing a single graphic symbol or control code. This term corresponds to the ISO C standard term multibyte character (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 Portable Character Set for a further explanation of the graphical representations of characters, or glyphs, as opposed to character encodings.

 character array

An array of type char.

 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; see LC_CTYPE .

 character set

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

 character special file

A file that refers to a device. One specific type of character special file is a terminal device file, whose access is defined in General Terminal Interface .

 character string

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

 child process

See process .

 circumflex

The character "^".

 clock

An object that measures the 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.

 clock tick

An interval of time; an implementation-dependent number of these occur each second.

 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.

 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 to elements of a character set may be further controlled by state information; see Character Encoding . The character set may contain fewer elements than the total number of possible numeric code values; that is, some code values may be unassigned.

 collating element

The smallest entity used to determine the logical ordering of character or wide-character strings. See 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.

 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.

 collation sequence

The relative order of collating elements as determined by the setting of the LC_COLLATE category in the current locale. The character order, as defined for the LC_COLLATE category in the current locale, defines the relative order of all collating elements, such that each element occupies a unique position in the order. This is the order used in ranges of characters and collating elements in regular expressions and pattern matching. In addition, the definition of the collating weights of characters and collating elements uses collating elements to represent their respective positions within the collation sequence.

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

 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 this specification set, 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.

 command

A directive to the shell to perform a particular task; see the XCU specification, Shell Commands .

 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.

 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 basic letter.

 condition variable

A synchronization object which allows a thread to suspend execution, repeatedly, until some associated predicate becomes true.

 control character

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

 control operator

In the shell, 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. See the XCU specification, Token Recognition .

On some systems, the symbol (( is a control operator; its use produces unspecified results. Applications that wish to have nested subshells, such as:

((echo Hello);(echo World))

must separate the (( characters into two tokens by including white space between them. Some systems may treat these as invalid arithmetic expressions instead of subshells.

The (( and )) symbols are control operators in the KornShell, used for an alternative syntax of an arithmetic expression command. A portable application cannot use (( as a single token (with the exception of the $(( form for shell arithmetic).

 controlling process

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

 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 (see General Terminal Interface ) cause signals to be sent to all processes in the process group associated with the controlling terminal.

 conversion descriptor

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

 core file

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

 current working directory

See working directory .

 cursor position

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

 data segment

Memory associated with a process, that may be used to contain dynamically allocated data.

 device

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

 device ID

A non-negative integer used to identify a device.

 direct I/O

An operation that attempts to circumvent a system performance optimization for the optimization of the individual I/O operation.

 directory

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

 directory entry

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

 directory stream

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

 disarm (a timer)

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

 display

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

The XCU specification assigns precise requirements for the terms display and write. Some historical systems have chosen to implement certain utilities without using the traditional UNIX system file descriptor model. For example, the vi editor might employ direct screen memory updates on a personal computer, rather than a write() system call. An instance of user prompting might appear in a dialogue box, rather than with standard error. When the XCU specification uses the term display, the method of outputting to the terminal is unspecified; many historical implementations use termcap or terminfo, but this is not a requirement. The term write is used when the XCU specification mandates that a file descriptor be used and that the output can be redirected. However, it is assumed that when the writing is directly to the terminal (it has not been redirected elsewhere), there is no practical way for a user or test suite to determine whether a file descriptor is being used or not. Therefore, the use of a file descriptor is mandated only for the redirection case and the implementation is free to use any method when the output is not redirected. The verb write is used almost exclusively, with the very few exceptions of those utilities where output redirection need not be supported: tabs, talk, tput and vi.

 dollar sign

The character "$".

 dot

The filename consisting of a single dot character (.). See pathname resolution . In the context of shell special built-in utilities, see dot in the XCU specification, Special Built-in Utilities .

 dot-dot

The filename consisting solely of two dot characters (..). See pathname resolution .

 double-quote

The character """, also known as quotation-mark.

 downshifting

The conversion of an upper-case character to its lower-case representation.

 (clock) drift rate

The rate at which the time measured by a clock deviates from the actual passage of real time.

A positive drift rate causes a clock to gain time with respect to real time; a negative drift rate causes a clock to lose time with respect to real time.

 driver

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

 effective group ID

An attribute of a process that is used in determining various permissions, including file access permissions, described in file access permissions . See group ID . This value is subject to change during the process lifetime, as described in the exec family of functions and setgid().

 effective user ID

An attribute of a process that is used in determining various permissions, including file access permissions. See user ID . This value is subject to change during the process lifetime, as described in exec and setuid().

 eight-bit transparency

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

 empty directory

A directory that contains, at most, directory entries for dot and dot-dot.

 empty line

A line consisting of only a newline character. See also blank line .

 empty string

(Or null string.) A string whose first byte is a null byte.

 empty wide-character string

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

 epoch

The time zero hours, zero minutes, zero seconds, on January 1, 1970 Coordinated Universal Time. See seconds since the epoch .

 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.

 era

An alternative method for counting and displaying years. See LC_TIME .

 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.

 execute

To perform the actions described in the XCU specification, Command Search and Execution . See also invoke .

 expand

In the shell, when not qualified, the act of applying all the expansions described in the XCU specification, Word Expansions .

 extended regular expression

A pattern constructed according to the rules defined in Extended Regular Expressions .

 extended signed integral type

A signed integral type or an implementation-dependent type with similar properties.

 extended security controls

The access control (see file access permissions ) and privilege (see appropriate privileges ) mechanisms have been defined to allow implementation-dependent extended security controls. These permit an implementation to provide security mechanisms to support different security policies from those described in this specification set. These mechanisms do not alter or override the defined semantics of any of the functions or utilities in this specification set.

 extended unsigned integral type

An unsigned integral type or an implementation-dependent type with similar properties.

 feature test macro

A macro used to determine whether a particular set of features will be included from a header. See the XSH specification, The Compilation Environment .

 field

In the shell, a unit of text that is the result of parameter expansion (see the XCU specification, Parameter Expansion ), arithmetic expansion (see the XCU specification, Arithmetic Expansion ), command substitution (see the XCU specification, Command Substitution ), or field splitting (see the XCU specification, Field Splitting ). During command processing (see the XCU specification, Simple Commands ), the resulting fields are used as the command name and its arguments.

 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. Other characteristics of FIFOs are described in open(), read(), write() and lseek().

 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 and directory. Other types of files may be supported by the implementation.

 file access permissions

The standard file access control mechanism uses the file permission bits, as described below. These bits are set at the time of file creation by functions such as open(), creat(), mkdir() and mkfifo() and are changed by chmod(). These bits are read by stat() or fstat().

Implementations may provide additional or alternate file access control mechanisms, or both. An additional access control mechanism will only further restrict the access permissions defined by the file permission bits. An alternate file access control mechanism will:

• specify file permission bits for the file owner class, file group class, and file other class of that file, corresponding to the access permissions, to be returned by stat() or fstat()

• be enabled only by explicit user action, on a per-file basis by the file owner or a user with the appropriate privilege

• be disabled for a file after the file permission bits are changed for that file with chmod(). The disabling of the alternate mechanism need not disable any additional mechanisms supported by an implementation.

Whenever a process requests file access permission for read, write or execute/search, if no additional mechanism denies access, access is determined as follows:

• If a process has the appropriate privilege:

  • If read, write or directory search permission is requested, access is granted.

  • If execute permission is requested, access is granted if execute permission is granted to at least one user by the file permission bits or by an alternate access control mechanism; otherwise, access is denied.

• Otherwise:

  • The file permission bits of a file contain read, write and execute/search permissions for the file owner class, file group class and file other class.

  • Access is granted if an alternate access control mechanism is not enabled and the requested access permission bit is set for the class (file owner class, file group class, or file other class) to which the process belongs, or if an alternate access control mechanism is enabled and it allows the requested access; otherwise, access is denied.

 file description

See open file description .

 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 file descriptor is from zero to {OPEN_MAX}. A process can have no more than {OPEN_MAX} file descriptors open simultaneously. File descriptors may also be used to implement message catalogue descriptors and directory streams. See open file description and {OPEN_MAX} in <limits.h>.

 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-dependent.

 file hierarchy

Files in the system are organised in a hierarchical structure in which all of the non-terminal nodes are directories and all of the terminal nodes are any other type of file. Because multiple directory entries may refer to the same file, the hierarchy is properly described as a directed graph.

 file mode

An object containing the file mode bits and file type of a file, as described in <sys/stat.h>.

 file mode bits

A file's file permission bits, set-user-ID-on-execution bit (S_ISUID) and set-group-ID-on-execution bit (S_ISGID); see <sys/stat.h>.

 filename

A name consisting of 1 to {NAME_MAX} bytes used to name a file. The characters composing the name may be selected from the set of all character values excluding the slash character and the null byte. The filenames dot and dot-dot have special meaning; see pathname resolution . A filename is sometimes referred to as a pathname component.

Filenames should be constructed from the portable filename character set because the use of other characters can be confusing or ambiguous in certain contexts. (For instance, the use of a colon (:) in a pathname could cause ambiguity if that pathname were included in a PATH definition.)

 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.

 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.

 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.

 file permission bits

Information about a file that is used, along with other information, to determine if 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, as described in <sys/stat.h>. The detailed usage of the file permission bits in access decisions is described in file access permissions .

 file serial number

A per-file-system unique identifier for a file.

 file system

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

 file times update

Each file has three associated time values that are updated when file data has been accessed, file data has been modified, or file status has been changed, respectively. These values are returned in the file characteristics structure, as described in <sys/stat.h>.

For each function or utility in this specification set that reads or writes file data or changes the file status, the appropriate time-related fields are noted as "marked for update". At an update point in time, any marked fields are set to the current time and the update marks cleared. Two such update points are when the file is no longer open by any process and when stat() or fstat() is performed on the file. Additional update points are unspecified. Marks for update, and updates themselves, are not done for files on read-only file systems.

 file type

See file .

 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.

 first open (of a file)

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

 foreground job

See foreground process group .

 foreground process

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

 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 exactly one process group of the session as the foreground process group of that controlling terminal. See General Terminal Interface .

 foreground process group ID

The process group ID of the foreground process group.

 form-feed character

A character that in the output stream indicates that printing should start on the next page of an output device. The form-feed 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.

 graphic character

A character, other than a control character, that has a visual representation when handwritten, printed or displayed.

 group database

A system database of implementation-dependent format 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.

 group ID

A non-negative integer 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.

 group name

A string that is used to identify a group, as described in group database . To be portable across XSI-conformant systems, the value must be composed of characters from the portable filename character set. The hyphen should not be used as the first character of a portable group name.

 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.

 hard link

The relationship between two directory entries that represent the same file; see directory entry . This term is contrasted against symbolic link; see symbolic link .

 home directory

The current directory associated with a user at the time of login.

 incomplete line

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

 Inf

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

 interactive shell

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

 internationalisation

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.

 invoke

To perform the actions described in the XCU specification, Command Search and Execution , except that searching for shell functions and special built-in utilities is suppressed. See also execute .

 ISO/IEC 646:1983

ISO 7-bit coded character set for information interchange. The reference version of the standard contains 95 graphic characters, which are identical to the graphic characters defined in the ASCII coded character set.

 ISO 6937:1983

ISO 7-bit or 8-bit coded character set for text communication using public communication networks, private communication networks, or interchange media such as magnetic tapes and discs.

 ISO 8859-1:1987

ISO 8-bit single-byte coded character set Part 1, Latin Alphabet No 1. This standard character set comprises 191 graphic characters covering the requirements of most of Western Europe.

 job

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

 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.

 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:

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


 Table: Job Control Job ID Formats

 last close (of a file)

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

 line

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

 link

See directory entry .

 link count

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

 local customs

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

 locale

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

 localisation

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

 login

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

 login name

A user name that is associated with a login.

 map

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

 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.

 memory object

Either a file or shared memory object.

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

 message

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.

 message catalogue

A file or storage area containing program messages, command prompts and responses to prompts for a particular native language, territory and codeset.

 message catalogue descriptor

A per-process unique value used to identify an open message catalogue. A message catalogue descriptor may be implemented using a file descriptor.

 message queue

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.

 mode

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

 mount point

Either the system root directory or a directory for which the st_dev field of structure stat (see <sys/stat.h>) differs from that of its parent directory.

 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.

 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.

 name

In the shell, a word consisting solely of underscores, digits and alphabetics from the portable character set (see Portable Character Set ). The first character of a name must not be a digit.

There are no explicit limits in this specification set on the sizes of names, words (see word ), lines or other objects. However, other implicit limits do apply: shell script lines produced by many of the standard utilities cannot exceed {LINE_MAX} and the sum of exported variables comes under the {ARG_MAX} limit. Historical shells dynamically allocate memory for names and words and parse incoming lines a byte at a time. Lines cannot have an arbitrary {LINE_MAX} limit because of historical practice such as makefiles, where make removes the newline characters associated with the commands for a target and presents the shell with one very long line. The text on INPUT FILES in the XCU specification, Section 1.9, Utility Description Defaults does allow a shell to run out of memory, but it cannot have arbitrary programming limits.

 named STREAM

A STREAMS-based file descriptor that is attached to a name in the file-system namespace. 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.

 NaN (not a number)

A value that can be stored in a floating type but that is not a valid floating point number. Not all systems support the NaN value.

 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.

 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. See LC_MESSAGES .

 newline character

A character that in the output stream indicates that printing should start at the beginning of the next line. The newline 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.

 non-spacing characters

A character, such as a character representing a diacritical mark in the ISO 6937:1983 standard coded character set, which is used in combination with other characters to form composite graphic symbols.

 NUL

A character with all bits set to zero.

 null byte

A byte with all bits set to zero.

 null pointer

The value that is obtained by converting the number 0 into a pointer; for example, (void *) 0. The C language guarantees that this value will not match that of any legitimate pointer, so it is used by many functions that return pointers to indicate an error.

 null string

See empty string .

 null wide-character code

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

 number sign

The character #, also known as hash sign.

 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 run time. The internal format of an object file is unspecified, but a conforming application cannot assume an object file is a text file.

 offset maximum

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

 open file

A file that is currently associated with a file descriptor.

 open file description

A record of how a process or group of processes are 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. A file offset, file status and file access modes are attributes of an open file description.

 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. See Utility Argument Syntax .

 operator

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

 option

An argument to a command that is generally used to specify changes in the utility's default behaviour; see Utility Argument Syntax .

 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. See Utility Argument Syntax .

 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.

 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.

 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.

 parameter

In the shell, 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. See the XCU specification, 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.

 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.

 parent process

See process .

 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-dependent system process.

 pathname

A character string that is used to identify a file. A pathname consists of, at most, {PATH_MAX} bytes, including the terminating null byte. It has an optional beginning slash, followed by zero or more filenames separated by slashes. If the pathname refers to a directory, it may also have one or more trailing slashes. Multiple successive slashes are considered to be the same as one slash. A pathname that begins with two successive slashes may be interpreted in an implementation-dependent manner, although more than two leading slashes are treated as a single slash. The interpretation of the pathname is described in pathname resolution .

 pathname component

See filename .

 pathname resolution

Pathname resolution is performed for a process to resolve a pathname to a particular file in a file hierarchy. There may be multiple pathnames that resolve to the same file.

Each filename in the pathname is located in the directory specified by its predecessor (for example, in the pathname fragment a/b, file b is located in directory a). Pathname resolution fails if this cannot be accomplished. If the pathname begins with a slash, the predecessor of the first filename in the pathname is taken to be the root directory of the process (such pathnames are referred to as absolute pathnames). If the pathname does not begin with a slash, the predecessor of the first filename of the pathname is taken to be the current working directory of the process (such pathnames are referred to as relative pathnames).

The interpretation of a pathname component is dependent on the values of {NAME_MAX} and {_POSIX_NO_TRUNC} associated with the path prefix of that component. If any pathname component is longer than {NAME_MAX}, because {_POSIX_NO_TRUNC} is in effect on all XSI-conformant systems for the path prefix of that component (see pathconf()), the implementation will consider this an error condition.

If a symbolic link (see symbolic link ) is encountered during pathname resolution, then pathname resolution is complete if all of the following are true:

• This is the last component of the pathname.

• The pathname has no trailing slash.

• The function is required to act on the symbolic link itself, or certain arguments direct that the function act on the symbolic link itself.

In all other cases, the system prefixes the remaining pathname, if any, with the contents of the symbolic link. The function may fail, setting errno to [ENAMETOOLONG], if the combined length exceeds {PATH_MAX}. Otherwise, the resolved pathname is the resolution of the pathname just created. The result is either an absolute pathname that is resolved from the root directory of the process or a relative pathname that is resolved from the directory containing the symbolic link.

The special filename dot refers to the directory specified by its predecessor. The special filename dot-dot refers to the parent directory of its predecessor directory. As a special case, in the root directory, dot-dot may refer to the root directory itself.

A pathname consisting of a single slash resolves to the root directory of the process. A null pathname is invalid.

 path prefix

A pathname, with an optional ending slash, that refers to a directory.

 pattern

A sequence of characters used either with regular expression notation (see Regular Expressions ) or for pathname expansion (see the XCU specification, Pathname Expansion ), as a means of selecting various character strings or pathnames, respectively.

The syntaxes of the two patterns are similar, but not identical; this specification set always indicates the type of pattern being referred to in the immediate context of the use of the term.

 period

The character (.). The term period is contrasted against dot, which is used to describe a specific directory entry.

 permissions

See file access permissions .

 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.

 pipe

An object accessed by one of the pair of file descriptors created by the pipe() function. Once created, the file descriptors can be used to manipulate it, and it behaves identically to a FIFO special file when accessed in this way. It has no name in the file hierarchy.

 positional parameter

In the shell, a parameter denoted by a single digit or one or more digits in curly braces. See the XCU specification, Positional Parameters .

 portable character set

The collection of characters that are required to be present in all locales supported by XSI-conformant systems:

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 ! # % ^ & * ( ) _ + - = { } [ ]
: " ~ ; ' ` < > ? , . | \ / @ $

Also included are the alert, backspace, tab, newline, vertical-tab, form-feed, carriage-return and space characters and the null character, NUL.

This term is contrasted against the smaller portable filename character set. See Portable Character Set .

 portable filename character set

The set of characters from which portable filenames are constructed. For a filename to be portable across implementations conforming to this specification set and the ISO POSIX-1 standard, it must consist only of the following characters:

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. The hyphen must not be used as the first character of a portable filename. Upper- and lower-case letters retain their unique identities between conforming implementations. In the case of a portable pathname, the slash character may also be used.

 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.

 preempted process (or thread)

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

 printable character

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

 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 character, all in the current locale; see LC_CTYPE .

 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.

 priority band

The queueing 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.

 priority-based scheduling

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

 privilege

See appropriate privileges .

 process

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

Many of the system resources defined by this specification 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.

A process is created by another process issuing the fork() function. The process that issues fork() is known as the parent process, and the new process created by the fork() is known as the child process.

 process group

A collection of processes that permits the signalling 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.

 process group ID

The unique identifier representing a process group during its lifetime. A process group ID is a positive integer. A process group ID will not be reused by the system until the process group lifetime ends.

 process group leader

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

 process group lifetime

A 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 last process' lifetime or to the last remaining process calling the setsid() or setpgid() functions.

 process ID

The unique identifier representing a process. A process ID is a positive integer. A process ID will not be reused by the system until the process lifetime ends. In addition, if there exists a process group whose process group ID is equal to that process ID, the process ID will not be reused by the system until the process group lifetime ends. A process that is not a system process will not have a process ID of 1.

 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 with a fork() function, 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(), wait3(), 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.

 process list

See thread list .

 process virtual time

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

 program

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

 pseudo-terminal

A pseudo-terminal provides the process with an interface that is identical to the terminal subsystem. A pseudo-terminal is composed of 2 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.

This specification requires a STREAMS-based implementation of pseudo-terminals to be available, but does not preclude others also being available.

 radix character

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

 read-only file system

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

 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 synchronise 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 initialised for this behaviour.

 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 group ID . This value is subject to change during the process lifetime, as described in setgid().

 real time

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

 real user ID

The attribute of a process that, at the time of process creation, identifies the user who created the process. See user ID . This value is subject to change during the process lifetime, as described in setuid().

 redirection

In the shell, a method of associating files with the input or output of commands. See the XCU specification, Redirection .

 redirection operator

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

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

 reentrant function

A function whose effect, when called by two or more threads, is guaranteed to be as if the threads each executed the function one after another in an undefined order, even if the actual execution is interleaved.

 referenced shared memory object

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

 refresh

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

 regular expression

A pattern constructed according to the rules defined in Regular Expressions .

 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.

 regular file

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

 relative pathname

See pathname resolution .

 (time) resolution

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

 root directory

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

 runnable process (or thread)

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

 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.

 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().

 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().

 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 exec and setuid().

 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.

 scheduling allocation domain

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

 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 this specification, a thread has scheduling contention scope of either PTHREAD_SCOPE_SYSTEM or PTHREAD_SCOPE_PROCESS.

 scheduling policy

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

In the context of XSI, a scheduling policy affects thread ordering:

• when a thread is a running thread and it becomes a blocked thread

• when a thread is a running thread and it becomes a preempted thread

• when a thread is a blocked thread and it becomes a runnable thread

• when a running thread calls a function that can change the priority or scheduling policy of a thread

• in other scheduling policy-defined circumstances.

Conforming implementations are required to define the manner in which each of the scheduling policies may modify the priorities or otherwise affect the ordering of threads at each of the occurrences listed above. Additionally, conforming implementations will define at what other circumstances and in what manner each scheduling policy may modify the priorities or affect the ordering of threads.

 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.

 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.

 seconds since the epoch

A value to be interpreted as the number of seconds between a specified time and the epoch. A Coordinated Universal Time name (specified in terms of seconds (tm_sec), minutes (tm_min), hours (tm_hour), days since January 1 of the year (tm_yday), and calendar year minus 1900 (tm_year)) is related to a time represented as seconds since the Epoch, according to the expression below.

If the year < 1970 or the value is negative, the relationship is undefined. If the year 1970 and the value is non-negative, the value is related to a Coordinated Universal Time name according to the expression:

 

tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 +
    (tm_year-70)*31536000 + ((tm_year-69)/4)*86400

 semaphore

A shareable resource that has a non-negative integral value. When the value is zero, there is a (possibly empty) set of threads awaiting the availability of the semaphore.

 semaphore lock operation

An operation that is applied to a semaphore. If, prior to the operation, the value of the semaphore is zero, the semaphore lock operation causes the calling thread to be blocked and added to the set of threads awaiting the semaphore. Otherwise, the value is decremented.

 semaphore unlock operation

An operation that is applied to a semaphore. If, prior to the operation, there are any threads in the set of threads awaiting the semaphore, then some thread from that set will be removed from the set and become unblocked. Otherwise, the semaphore value is incremented.

 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.

 session leader

A process that has created a session; see setsid().

 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.

 shared memory object

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

 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.

 shell, the

The XSI Shell Command Language Interpreter (see sh), a specific instance of a shell.

 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 (see the XCU specification, Simple Commands ). 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.

 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.

 signal stack

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

 single-quote

The character "'", also known as apostrophe.

 slash

The character "/", also known as solidus.

 socket

A communications endpoint associated with a file descriptor that provides communications services using a specified communications protocol. See the Networking Services specification.

 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.

 source code

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

When dealing with the C language, input to a C compiler conforming to the ISO C standard.

When dealing with another XSI-compliant language, input to a compiler conforming to that language 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.

 special parameter

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

*   @   #   ?   !   -   $   0

See the XCU specification, Special Parameters .

 space character

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 white space .)

 standard error

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

 standard input

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

 standard output

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

 standard utilities

The utilities described in the XCU specification.

 stream

Appearing in lower case, 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(), fopen() 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 the XSH specification, Standard I/O Streams .

 STREAM

Appearing in upper case, 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 the XSH specification, STREAMS .

 STREAM end

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

 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.

 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.

 string

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

 subshell

A shell execution environment, distinguished from the main or current shell execution environment by the attributes described in the XCU specification, Shell Execution Environment .

 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.

 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. Whether a process' effective group ID is included in or omitted from its list of supplementary group IDs is unspecified.

 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.

 symbolic link

A type of file that contains a pathname. The pathname is interpolated into a pathname being resolved, during pathname resolution, to create a new pathname when it is encountered.

 synchronised I/O completion

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

 synchronised 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 synchronised read operation was requested, these write requests shall be 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.

 synchronised I/O file integrity completion

Identical to a synchronised 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) will be successfully transferred prior to returning to the calling process.

 synchronised I/O operation

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

 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 that a synchronous I/O operation does not imply synchronised I/O data integrity completion or synchronised I/O file integrity completion.

 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.

 system

An implementation of the XSI.

 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 this specification set.

 system console

An optional file that receives messages sent by fmtmsg() when the MM_CONSOLE flag is set.

 system documentation

All documentation provided with an XSI-conformant implementation except for the Conformance Statement Questionnaire (CSQ). Electronically distributed documents for an XSI-conformant implementation are considered part of the system documentation.

 system process

An implementation-dependent object, other than a process executing an application, that has a process ID.

 system scheduling priority

A number used as advice to the system to alter process scheduling priorities. Raising the value should give a process additional preference when scheduling a process to run. Lowering the value should reduce the preference and make a process less likely to run. Typically, a process with higher system scheduling priority will run to completion more quickly than an equivalent process with lower system scheduling priority. A scheduling priority of zero specifies the default policy of the system.

This definition is not intended to suggest that all processes in a system have priorities that are comparable. Scheduling policy extensions such as adding real-time priorities make the notion of a single underlying priority for all scheduling policies problematic. Some systems may implement the features related to nice to affect all processes on the system, others to affect just the general time-sharing activities implied by this specification set, and others may have no effect at all. Because of the use of "implementation-dependent" in nice and renice, a wide range of implementation strategies is possible.

 system reboot

An implementation-dependent 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.

 tab character

A character that in the output stream indicates that printing or displaying should start at the next horizontal tabulation position on the current line. The tab 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 behaviour is unspecified. It is unspecified whether this character is the exact sequence transmitted to an output device by the system to accomplish the tabulation.

 terminal

(Or terminal device.) A character special file that obeys the specifications of the general terminal interface as described in General Terminal Interface .

 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.

 text file

A file that contains characters organised into one or more lines. The lines must not contain NUL characters and none can exceed {LINE_MAX} bytes in length, including the newline character. Although the XSI 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.

The term text file does not prevent the inclusion of control or other non-printable characters (other than NUL). Therefore, standard utilities that list text files as inputs or outputs are either able to process the special characters gracefully or they explicitly describe their limitations within their individual sections. The only difference between text and binary files is that text files have lines of less than {LINE_MAX} bytes, with no NUL characters, each terminated by a newline character. The definition allows a file with a single newline character, but not a totally empty file, to be called a text file. If a file ends with an incomplete line it is not strictly a text file by this definition. The newline character referred to in this specification set is not some generic line separator, but a single character; files created on systems where they use multiple characters for ends of lines are not portable to all XSI-conformant systems without some translation process.

 thread

A single flow of control within a process. Each thread has its own thread ID, scheduling priority and policy, errno value, 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-supplied functions and automatic variables, are accessible to all threads in the same process.

 thread ID

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

 thread list

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

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

 thread-safe

A function that may be safely invoked concurrently by multiple threads. Examples are any "pure" function, a function which holds a mutex locked while it is accessing static storage, or objects shared among threads.

 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.

 tilde

The character "~".

 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.

 timer overrun

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

 token

A sequence of characters that the shell considers as a single unit when reading input, according to the rules in the XCU specification, Token Recognition . A token is either an operator or a word.

 upshifting

The conversion of a lower-case character to its upper-case representation.

 user database

A system database of implementation-dependent format 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-dependent.

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

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

 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.

 user name

A string that is used to identify a user, as described in user database . To be portable across XSI-conformant systems, the value must be composed of characters from the portable filename character set. The hyphen should not be used as the first character of a portable user name.

 utility

A program that can be called by name from a shell to perform a specific task, or related set of tasks. This program is either an executable file, such as might be produced by a compiler or linker system from computer source code, or a file of shell source code, directly interpreted by the shell. The program may have been produced by the user, provided by the system implementor, or acquired from an independent distributor. The term utility does not apply to the special built-in utilities provided as part of the XSI Shell Command Language; see the XCU specification, Special Built-in Utilities . The system may implement certain utilities as shell functions (see the XCU specification, Function Definition Command ) or built-in utilities, but only an application that is aware of the command search order described in the XCU specification, Command Search and Execution  or of performance characteristics can discern differences between the behaviour of such a function or built-in utility and that of a true executable file.

 variable

In the shell, a named parameter. See the XCU specification, Parameters and Variables .

 variable assignment

In the shell, a word consisting of the following parts:

varname=value

When used in a context where assignment is defined to occur (see the XCU specification, Simple Commands ) and at no other time, the value (representing a word or field) will be assigned as the value of the variable denoted by varname. The varname and value parts meet the requirements for a name and a word, respectively, except that they are delimited by the embedded unquoted equals-sign in addition to the delimiting described in the XCU specification, Token Recognition . In all cases, the variable will be created if it did not already exist. If value is not specified, the variable will be given a null value.

An alternative form of variable assignment:

symbol=value

(where symbol is a valid word delimited by an equals-sign, but not a valid name) produces unspecified results. This form is used by the KornShell name[expression]=value syntax.

 vertical-tab character

A character that in the output stream indicates that printing should start at the next vertical tabulation position. The vertical-tab 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 behaviour is unspecified. It is unspecified whether this character is the exact sequence transmitted to an output device by the system to accomplish the tabulation.

 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 characters (space and tab characters), newline characters, carriage-return characters, form-feed characters and vertical-tab characters.

 wide-character code (C language)

An integer value corresponding to a single graphic symbol or control code. See C Language Wide-character Codes .

 wide-character string

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

 word

In the shell, 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 (see the XCU specification, 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; see the XCU specification, Word Expansions .

 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.

 world-wide portability interface

Functions for handling characters in a codeset-independent manner.

 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 .

 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 this specification set.

 zombie process

An inactive process that will be deleted at some later time when its parent process executes wait() or waitpid().

 [n, m] and [n, m)

Notations denoting mathematical ranges. The square brackets [ and ] include the limit; the parentheses ( and ) exclude the limit; that is, if x is in [0, 1], it can be from 0 to 1 inclusive, but if x is in [0, 1), it can be from 0 up to but not including 1.

 ±0

The algebraic sign provides additional information about any variable that has the value zero. Although all precisions have distinct representations for +0, -0, +Inf and -Inf, the signs are significant in some circumstances, such as division by zero, and not in others.