For the purposes of POSIX.1-2024, 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.
An abrupt termination of a network connection that may result in the loss of data.
A pathname beginning with a single or more than two <slash> characters; see also 3.254 Pathname
.
A particular form of access permitted to a file.
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.
The memory locations that can be referenced by a process or the threads of a process.
An interface that advises the implementation on (portable) application behavior so that it can optimize the system.
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.
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).
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.
In the shell command language, a word consisting solely of alphabetics and digits from the portable character set and any of the following characters: '!', '%', ',', '-', '@', '_'.
Implementations may allow other characters within alias names as an extension.
A requirement that objects of a particular type be located on storage boundaries with addresses that are particular multiples of
a byte address.
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.1-2024 defines when such mechanisms can be enabled and
when they are disabled.
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.
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.
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.
The character designated by '\'' in the C language, also known as the single-quote character.
A computer program that performs some desired function.
When the User Portability Utilities option is supported, requirements placed on applications relating to the use of standard utilities shall also apply to the actions of a user who is entering shell command language statements into an interactive shell.
Endpoint address of a specific application.
The definition of syntax and semantics for providing computer system services.
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.
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.
In the C language, an expression in a function call expression or a sequence of preprocessing tokens in a function-like macro invocation.
To start a timer measuring the passage of time, enabling notifying a process when the specified time or time interval has passed.
The character '*'.
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.
Events that occur independently of the execution of the application.
A functionality enhancement to allow an application process to queue data input and output commands with asynchronous notification of completion.
A function that can be called, without restriction, from signal-catching functions. Note that, although there is no restriction
on the calls themselves, for certain functions there are restrictions on subsequent behavior after the function is called from a
signal-catching function. No function is async-signal-safe unless explicitly described as such.
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.
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.
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.
An operation that cannot be broken up into smaller parts that could be performed separately. An atomic operation is guaranteed to complete either fully or not at all. In the context of the functionality provided by the <stdatomic.h> header, there are different types of atomic operation that are defined in detail in 4.15.1 Memory Ordering .
The process of validating a user or process to verify that the user or process is not a counterfeit.
The process of verifying that a user or process has permission to use a resource in the manner requested.
To assure security, the user or process would also need to be authenticated before granting access.
In the context of the System Interfaces volume of POSIX.1-2024, a background process group (see 3.37 Background Process Group ).
In the context of the shell, a job that the shell is not waiting for before it executes further commands or, if interactive,
prompts for further commands. A background job can be a job-control background job or a non-job-control background job. A
job-control background job is a job that started execution (either in the background or the foreground) while job control was
enabled and is currently in the background. A non-job-control background job is an asynchronous AND-OR list that started execution
while job control was disabled and was assigned a job number. An implementation need not support non-job-control background jobs;
that is, the shell may, but need not, assign job numbers to asynchronous AND-OR lists that start execution while job control is
disabled.
A process that is a member of a background process group.
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.
The character '`', also known as <grave-accent>.
The character designated by '\\' in the C language, also known as reverse solidus.
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.
A synchronization object that allows multiple threads to synchronize at a particular point in their execution.
For pathnames containing at least one filename: the final, or only, filename in the pathname. For pathnames consisting only of <slash> characters: either '/' or "//" if the pathname consists of exactly two <slash> characters, and '/' otherwise.
A regular expression (see 3.308 Regular Expression ) used by the majority of utilities that select
strings from a set of character strings.
The process of assigning a network address to an endpoint.
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>.
A line consisting solely of zero or more <blank> characters terminated by a <newline>; see also 3.120 Empty Line .
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.
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.
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.
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.
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 '}'.
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.
The transfer of data from one endpoint to several endpoints, as described in RFC 919 and RFC 922.
A utility implemented within a shell. There are two main types of built-in utilities: special built-ins and regular built-ins.
Unless qualified, the term "built-in" includes both types. The utilities referred to as special built-ins have special qualities.
Regular built-ins are not required to be actually built into the shell on the implementation, but they usually have special
command-search qualities, or affect the current execution environment.
Regular Built-In Utilities are defined in detail in XCU 1.6 Built-In Utilities .
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 3.58 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.
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(), printf(), putc(), putchar(), puts(), scanf(), ungetc(), vfprintf(), and vprintf().
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.
A sequence of one or more bytes representing a member of a character set.
See the definition of the portable character set in 6.1 Portable Character Set for a further explanation of the graphical representations of (abstract) characters, as opposed to character encodings.
An array of elements of type char.
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.
A finite set of different characters used for the representation, organization, or control of data.
A file that refers to a device (such as a terminal device file) or that has special properties (such as /dev/null).
A contiguous sequence of characters terminated by and including the first null byte.
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.
The character '^'.
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.
The difference between two successive distinct values of a clock, as observed from the application via one of the "get time" operations.
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.
In the context of the System Interfaces volume of POSIX.1-2024, a block as defined in the ISO C standard.
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.
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.
The smallest entity used to determine the logical ordering of character or wide-character strings; see also 3.74 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.
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.
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 3.126 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.
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.1-2024, 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.
A directive to the shell to perform a particular task.
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.
The system(), popen(), and exec functions are defined in detail in the System Interfaces volume of POSIX.1-2024.
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.
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.
There are two types of condition variable: those of type pthread_cond_t which are initialized using pthread_cond_init() and those of type cnd_t which are initialized using
cnd_init(). If an application attempts to use the two types interchangeably (that
is, pass a condition variable of type pthread_cond_t to a function that takes a cnd_t, or vice versa), the behavior
is undefined.
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 3.81 Connection , 3.82 Connection Mode , 3.83 Connectionless Mode , and 3.342 Socket .
An association established between two or more endpoints for the transfer of data
The transfer of data in the context of a connection; see also 3.83 Connectionless Mode .
The transfer of data other than in the context of a connection; see also 3.82 Connection Mode and 3.96 Datagram .
A character, other than a graphic character, that affects the recording, processing, transmission, or interpretation of text.
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.
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.
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.
A per-process unique value used to identify an open codeset conversion.
An unspecified object of unspecified format that may be generated when a process terminates abnormally.
The time spent executing a process or thread, including the time spent executing system services on behalf of that process or thread. 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.
A clock that measures the execution time of a particular process or thread.
A timer attached to a CPU-time clock.
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 3.182 Job ID .
See Working Directory in 3.421 Working Directory (or Current Working Directory) .
The line and column position on the screen denoted by the terminal's cursor.
A unit of data transferred from one endpoint to another in connectionless mode service.
A situation in which there are two conflicting actions in different threads, at least one of which is not atomic, and neither "happens before" the other, where the "happens before" relation is defined formally in 4.15.1 Memory Ordering .
Memory associated with a process, that can contain dynamically allocated data.
See Radix Character in 3.294 Radix Character (or Decimal-Point Character) .
The following standard utilities are declaration utilities: export, readonly, and, under certain conditions, command. An implementation may provide other declaration utilities.
A computer peripheral or an object that appears to the application as such.
A non-negative integer used to identify a device.
A file that contains directory entries. No two directory entries in the same directory have the same name.
An object that associates a filename with a file. Several directory entries can associate names with the same file.
A sequence of all the directory entries in a particular directory. An open directory stream may be implemented using a file descriptor.
To stop a timer from measuring the passage of time, disabling any future process notifications (until the timer is armed again).
To output to the user's terminal. If the output is not directed to a terminal, the results are undefined.
A line of text on a physical device or an emulation thereof. Such a line has a maximum number of characters which can be
presented.
The character '$'.
In the context of naming files, the filename consisting of a single <period> character ('.').
Pathname Resolution is defined in detail in 4.16 Pathname Resolution .
The filename consisting solely of two <period> characters ("..").
See Portable Messages Object Source File in 3.266 Portable Messages Object Source File (or Dot-Po File) .
The character '"' , also known as <quotation-mark>.
The conversion of an uppercase character that has a single-character lowercase representation into this lowercase representation.
A module that controls data transferred to and received from devices.
An attribute of a process that is used in determining various permissions, including file access permissions; see also 3.165 Group ID .
An attribute of a process that is used in determining various permissions, including file access permissions; see also 3.408 User ID .
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.
A directory that contains, at most, directory entries for dot and dot-dot, and has exactly one hard link to it other than its own dot entry (if one exists), in dot-dot. No other hard links to the directory can exist. It is unspecified whether an implementation can ever consider the root directory to be empty.
A line consisting of only a <newline>; see also 3.46 Blank Line .
A string whose first byte is a null byte.
A wide-character string whose first element is a null wide-character code.
The rules used to convert between wide-character codes and multi-byte character codes.
The concatenated set of one or more basic regular expressions or extended regular expressions that make up the pattern specified
for string selection.
The time zero hours, zero minutes, zero seconds, on January 1, 1970 Coordinated Universal Time (UTC).
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.
A locale-specific method for counting and displaying years.
The mechanism that enables applications to register for and be made aware of external events such as data becoming available for reading.
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.
To perform command search and execution actions, as defined in the Shell and Utilities volume of POSIX.1-2024; see also 3.179 Invoke .
See CPU Time in 3.90 CPU Time (Execution Time) .
A set of execution time monitoring primitives that allow online measuring of thread and process execution times.
In the shell command language, when not qualified, the act of applying word expansions.
A regular expression (see also 3.308 Regular Expression ) that is an alternative to the Basic Regular
Expression using a more extensive syntax, occasionally used by some utilities.
Implementation-defined security controls allowed by the file access permission and appropriate privileges (see also 3.21 Appropriate Privileges ) mechanisms, through which an implementation can support different security policies
from those described in POSIX.1-2024.
File Access Permissions are defined in detail in 4.7 File Access Permissions .
A macro used to determine whether a particular set of features is included from a header.
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.
Arithmetic Expansion is defined in detail in XCU 2.6.4 Arithmetic Expansion .
Command Substitution is defined in detail in XCU 2.6.3 Command Substitution .
Field Splitting is defined in detail in XCU 2.6.5 Field Splitting .
For further information on command processing, see XCU 2.9.1 Simple Commands .
A type of file with the property that data written to such a file is read on a first-in-first-out basis.
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.
See Open File Description in 3.241 Open File Description .
A per-process unique, non-negative integer used to identify an open file for the purpose of file access. The values 0, 1, and 2
have special meaning and conventional uses, and are referred to as standard input, standard output, and standard
error, respectively. Programs usually take their input from standard input, and write output on standard output. Diagnostic
messages are usually written on standard error. 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 3.241 Open File Description .
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.
Any advisory lock, including a record lock (see 3.302 Record Lock ), obtained on a file for the
purpose of coordinating transactions among cooperating processes accessing the same file with the same lock type. See also 3.237 OFD-Owned File Lock and 3.289 Process-Owned File Lock .
An object containing the file mode bits and some information about the file type of a file.
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).
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 3.254 Pathname .
A string consisting of a filename followed by a <NUL> character.
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.
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.
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.
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.
File Access Permissions are defined in detail in 4.7 File Access Permissions .
A per-file system unique identifier for a file.
A collection of files and certain of their attributes. It provides a name space for file serial numbers referring to those files.
See File in 3.139 File .
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.
When a process opens a file that is not currently an open file within any process.
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.
In the context of the System Interfaces volume of POSIX.1-2024, a foreground process group (see 3.160 Foreground Process Group ).
In the context of the shell, a job that the shell is waiting for before it executes further commands or, if interactive, prompts
for further commands.
A process that is a member of a foreground process group.
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.
The process group ID of the foreground process group.
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.
A member of the graph character class of the current locale.
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.
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. The value (gid_t)-1 shall not be a valid group ID, but does have a defined use in some interfaces defined in this standard.
A string that is used to identify a group; see also 3.164 Group Database . To be portable across conforming systems, the value is composed of characters from the portable filename character set. The <hyphen-minus> should not be used as the first character of a portable group name.
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.
See Directory Entry in 3.104 Directory Entry (or Hard Link) . A file can have multiple hard links as a 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 3.364 Symbolic Link .
A contiguous region of bytes within a file, all having the value of zero. Not all bytes with the value zero need belong to a hole; however, all seekable files shall have a virtual hole starting at the current size of the file. A hole is typically created via truncate(), or if an lseek() call has been made to position beyond the end of a file and data subsequently written at that point, although it is up to the implementation to define when sparse files can be created and with what granularity for the size of holes.
The directory specified by the HOME environment variable.
The arrangement of bytes in any integer type when using a specific machine architecture.
A sequence of one or more non-<newline> characters at the end of the file.
A value representing +infinity or a value representing -infinity that can be stored in a floating type. Not all systems support the Inf values.
A terminal device.
A processing mode of the shell that is suitable for direct user interaction.
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.
A functionality enhancement to add a high-performance, deterministic interprocess communication facility for local communication.
To perform command search and execution actions, except that searching for shell functions and special built-in utilities is
suppressed; see also 3.130 Execute .
A background job, a foreground job, or a suspended job.
In the context of the shell, jobs are created when a list (see XCU 2.9.3
Lists ) is executed while job control is enabled, and may be created when an asynchronous AND-OR list is executed while job
control is disabled.
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.
The term is also used in connection with system interfaces that can be used by a command interpreter to implement job control (see for example setpgid ).
A handle that is used to refer to a job. The job ID can be any of the forms shown in the following table:
|
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. |
A thread that was created either using pthread_create() with
the detachstate attribute not set to PTHREAD_CREATE_DETACHED or using thrd_create(), and for which neither pthread_detach() nor pthread_join() has been called and returned zero, and neither thrd_detach() nor thrd_join() has been
called and returned thrd_success.
When a process closes a file, resulting in the file not being an open file within any process.
A sequence of zero or more non-<newline> characters plus a terminating <newline> character.
The period of time before terminating a connection, to allow outstanding data to be transferred.
In the context of the file hierarchy, either a hard link or a symbolic link.
In the context of the c17 utility, the action performed by the link
editor (or linker).
The number of directory entries that refer to a particular file.
An address space with one or more threads executing within that address space, and the required system resources
for those threads.
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 can 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 live threads in the same process.
The conventions of a geographical area or territory for such things as date, time, and currency formats.
The transfer of data between processes in the same system.
The definition of the subset of a user's environment that depends on language and cultural conventions.
The process of establishing information within a computer system specific to the operation of particular native languages, local customs, and coded character sets.
An operation that does not require the use of a lock such as a mutex in order to avoid data races.
The unspecified activity by which a user gains access to the system. Each login is associated with exactly one login name.
A user name that is associated with a login.
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.
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.
A facility to allow applications to access files as part of the address space.
One of:
When used in conjunction with mmap(), a memory object appears in the
address space of the calling process.
The process of managing the implementation in such a way as to provide an upper bound on memory access times.
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.
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.
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.
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.
A file containing message identifiers and translations in an unspecified format. Used by the gettext family of functions and the gettext and ngettext utilities for internationalization and localization of programs and scripts. Messages objects have the filename suffix .mo, and can be created by the msgfmt utility.
See also 3.386 Text Domain .
A collection of attributes that specifies a file's type and its access permissions.
A clock measuring real time, whose value cannot be set via clock_settime() and which cannot have negative clock jumps.
Either the system root directory or a directory for which the st_dev field of structure stat differs
from that of its parent directory.
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.
A library containing object files that were produced by compiling with c17 using the flags output by getconf POSIX_V8_THREADS_CFLAGS, or by compiling using a non-standard utility with equivalent flags, and which makes use of interfaces that are only made available by c17 when the -l pthread option is used or makes use of SIGEV_THREAD notifications.
A process that contains more than one thread.
A program whose executable file was produced by compiling with c17 using the flags output by getconf POSIX_V8_THREADS_CFLAGS, and linking with c17 using the flags output by getconf POSIX_V8_THREADS_LDFLAGS and the -l pthread option, or by compiling and linking using a non-standard utility with equivalent flags. Execution of a multi-threaded program initially creates a single-threaded process; the process can create additional threads using pthread_create(), thrd_create(), or SIGEV_THREAD notifications.
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.
There are two types of mutex: those of type pthread_mutex_t which are initialized using pthread_mutex_init() and those of type mtx_t which are initialized using
mtx_init(). If an application attempts to use the two types interchangeably (that
is, pass a mutex of type pthread_mutex_t to a function that takes a mtx_t, or vice versa), the behavior is undefined.
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.
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.
A computer user's spoken or written language, such as American English, British English, Danish, Dutch, French, German, Italian, Japanese, Norwegian, or Swedish.
When describing a value (not a sign), less than zero. Note that in the phrase "negative zero" it describes a sign, and therefore negative zero (also represented as -0.0) is not a negative value.
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.
A collection of interconnected hosts.
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.
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.
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.
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.
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.
A character, such as a character representing a diacritical mark in the ISO/IEC 6937:2001 standard coded graphic character set, which is used in combination with other characters to form composite graphic symbols.
A character with all bits set to zero.
A byte with all bits set to zero.
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. POSIX.1-2024 additionally guarantees that any pointer object whose representation has all bits set to zero, perhaps by memset() to 0 or by calloc(), is interpreted as a null pointer.
See Empty String in 3.121 Empty String (or Null String) .
A term used for the null byte when used as a terminator for a string.
A wide-character code with all bits set to zero.
The character '#', also known as hash sign.
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.
Unit of data representation that consists of eight contiguous bits.
A record lock owned by an open file description. OFD-owned file locks are obtained through the use of fcntl() with F_OFD_SETLK or F_OFD_SETLKW. Whenever a file descriptor associated with the owning open file description is inherited these locks remain in effect. OFD-owned file locks are automatically released on the last close of the open file description. These locks are only shared among file descriptors associated with the same open file description. Thus, a multi-threaded process can use multiple open file descriptions (such as by open()) to create independent OFD-owned locks that can then be used to coordinate access patterns to the same file, while multiple file descriptors associated with the same open file description (such as by dup()) share lock actions among all other descriptors associated with the same open file description.
An attribute of an open file description representing the largest value that can be used as a file offset.
An address such that the entity making use of it requires no details about its contents or format.
A file that is currently associated with a file descriptor.
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.
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.
In the shell command language, either a control operator or a redirection operator.
An argument to a command that is generally used to specify changes in the utility's default behavior.
A parameter that follows certain options. In some cases an option-argument immediately follows the option character
within the same argument string as the option; otherwise the option-argument is the next argument string.
A stream has one of three orientations: unoriented, byte-oriented, or wide-oriented.
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.
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.
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.
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.
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.
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.
The process which created (or inherited) the process under discussion.
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.
A string that is used to identify a file. In the context of POSIX.1-2024, 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 it is implementation-defined whether
the case of exactly two leading <slash> characters is treated specially.
Pathname Resolution is defined in detail in 4.16 Pathname Resolution .
See Filename in 3.146 Filename .
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.
A sequence of characters used either with regular expression notation or with shell pattern matching notation.
Shell pattern matching notation is defined in detail in 2.14 Pattern Matching Notation .
The syntaxes of the two types of patterns are similar, but not identical; POSIX.1-2024 always indicates the type of pattern being referred to in the immediate context of the use of the term.
The character '.'. The term "period" is contrasted with dot (see also 3.110 Dot ), which is used to describe a specific directory entry.
Attributes of an object that determine the privilege necessary to access or manipulate the object.
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.
An object identical to a FIFO which has no links in the file hierarchy.
A scheduling scheme whereby the local process periodically checks until the pre-specified events (for example, read, write) have occurred.
The collection of characters that are required to be present in all locales supported by conforming systems.
This term is contrasted against the smaller portable filename character set; see also 3.265 Portable Filename Character Set .
A filename consisting only of characters from the 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-minus> characters, respectively. See also 3.254 Pathname .
A text file containing messages and directives. A portable messages object source file can be compiled into a
messages object by the msgfmt utility.
In the shell command language, a parameter denoted by a decimal representation of a positive integer.
When describing a value (not a sign), greater than zero. Note that in the common phrase "positive zero" (which is not used in this standard, although the representation +0.0 is) it describes a sign, and therefore positive zero (+0.0) is not a positive value.
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.
A running thread whose execution is suspended due to another thread becoming runnable at a higher priority.
In the context of job control, the job used as the default for the fg or bg utilities if the current job exits. There is at most one previous job; see also 3.182 Job ID .
One of the characters included in the print character classification of the LC_CTYPE category in the
current locale.
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.
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.
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.
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.
Scheduling in which the selection of a running thread is determined by the priorities of the runnable processes or threads.
See Appropriate Privileges in 3.21 Appropriate Privileges .
A live process (see 3.189 Live Process ) or a zombie process (see 3.426 Zombie Process ). The lifetime of a process is described in 3.285 Process Lifetime .
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.
The unique positive integer identifier representing a process group during its lifetime.
A process whose process ID is the same as its process group ID.
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.
The unique positive integer identifier representing a process during its lifetime.
The period of time that begins when a process is created and ends when its process ID is returned to the system.
See also 3.189 Live Process , 3.287 Process Termination , and
3.426 Zombie Process .
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.
There are two kinds of process termination:
The measurement of time in units elapsed by the system clock while a process is executing.
A record lock owned by a process. Process-owned file locks are obtained through the use of fcntl() with F_SETLK or F_SETLKW, or the use of lockf(). Process-owned file locks are not inherited by child processes, but are preserved across the exec family of functions. A process-owned file lock is released when the process exits, or when any file descriptor in the process referring to the same file is closed (even if via a different open file description). These locks are shared among all open file descriptions referring to the same file in the process, making the use of process-owned file locks unsuitable for use for coordination of record access among multiple threads in a process.
The transfer of data between processes.
A prepared sequence of instructions to the system to accomplish a defined task. The term "program" in POSIX.1-2024 encompasses applications written in the Shell Command Language, complex utility input languages (for example, awk, lex, sed, and so on), and high-level languages.
A set of semantic and syntactic rules for exchanging information.
A facility that provides an interface that is identical to the terminal subsystem, except where noted otherwise in POSIX.1-2024. A pseudo-terminal is composed of two devices: the "manager device" and a "subsidiary device". The subsidiary 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 manager device is presented to the subsidiary as an input and anything written on the subsidiary device is presented as an input on the manager side.
The character that separates the integer part of a number from the fractional part.
A file system that has implementation-defined characteristics restricting modifications.
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.
The attribute of a process that, at the time of process creation, identifies the group of the user who created the process; see also 3.165 Group ID .
Time measured as total units elapsed by the system clock without regard to which thread is executing.
A determinism improvement facility to enable asynchronous signal notifications to an application to be queued without impacting compatibility with the existing signal functions.
The attribute of a process that, at the time of process creation, identifies the user who created the process; see also 3.408 User ID .
A collection of related data units or words which is treated as a unit.
A file lock held on a record within a file. A record lock can be used to lock a whole file by specifying a special record with starting offset zero and length zero. (This special record extends to any future end-of-file, not just the current end-of-file.) This includes an OFD-owned file lock (see 3.237 OFD-Owned File Lock ) or a process-owned file lock (see 3.289 Process-Owned File Lock ). It is unspecified whether an implementation will detect and prevent deadlocks caused by two competing lock owners holding separate locks where each tries to obtain a lock that is blocked by the other's lock.
In the shell command language, a method of associating files with the input or output of commands.
In the shell command language, a token that performs a redirection function. It is one of the following symbols:
< > >| << >> <& >& <<- <>
A shared memory object that is open or has one or more mappings defined on it.
Make the information on the user's terminal screen up-to-date.
A pattern that selects specific strings from a set of character strings.
In the context of the address space of a process, a sequence of addresses.
In the context of a file, a sequence of offsets.
A file that is a randomly accessible sequence of bytes, with no further structure imposed by the system.
A pathname not beginning with a <slash> character.
A file holding code or data suitable for linking with other object files to create an executable or a shared object file.
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.
The minimum time interval that a clock can measure or whose passage a timer can detect.
A mutex with the robust attribute set.
A directory, associated with a process, that is used in pathname resolution for pathnames that begin with a <slash> character.
A thread that is capable of being a running thread, but for which no processor is available.
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.
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().
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().
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().
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.
The set of processors on which an individual thread can be scheduled at any given time.
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.1-2024, a thread has scheduling contention scope of either PTHREAD_SCOPE_SYSTEM or PTHREAD_SCOPE_PROCESS.
A set of rules that is used to determine the order of execution of processes or threads to achieve some goal.
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.
To move the representation of data vertically or horizontally relative to the terminal screen. There are two types of scrolling:
A minimum synchronization primitive to serve as a basis for more complex synchronization mechanisms to be defined
by the application program.
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.
A process that has created a session.
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.
An object that represents memory that can be mapped concurrently into the address space of more than one process.
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.
The Shell Command Language Interpreter; a specific instance of a shell.
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.
The sh utility is defined in detail in the Shell and Utilities volume of POSIX.1-2024.
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.
Memory established for a thread, in which signal handlers catching signals sent to that thread are executed.
The character designated by '\'' in the C language, also known as <apostrophe>.
A process that contains a single thread.
A program whose executable file was produced by compiling with c17 without using the flags output by getconf POSIX_V8_THREADS_CFLAGS and linking with c17 using neither the flags output by getconf POSIX_V8_THREADS_LDFLAGS nor the -l pthread option, or by compiling and linking using a non-standard utility with equivalent flags. Execution of a single-threaded program creates a single-threaded process; if the process attempts to create additional threads using pthread_create(), thrd_create(), or SIGEV_THREAD notifications, the behavior is undefined. If the process uses dlopen() to load a multi-threaded library, the behavior is undefined.
The character '/', also known as solidus.
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.1-2024.
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.
A resource limitation established for each process that the process may set to any value less than or equal to the hard limit.
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, ex, lex, localedef, make, sed, and yacc.
Source code can also refer to a collection of sources meeting any or all of these meanings.
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 3.413 White Space .
A file that contains more holes than just the virtual hole at the end of the file.
A process creation primitive useful for systems that have difficulty with fork() and as an efficient replacement for fork()/exec.
See Built-In Utility in 3.54 Built-In Utility (or Built-In) .
In the shell command language, a parameter named by a single character from the following list:
* @ # ? ! - $ 0
A synchronization object used to allow multiple threads to serialize their access to shared data.
A scheduling policy for threads and processes that reserves a certain amount of execution capacity for processing aperiodic events at a given priority level.
In the context of file descriptors (see 3.141 File Descriptor ), file descriptor number 2.
In the context of standard I/O streams (see XSH 2.5 Standard I/O
Streams ), an output stream usually intended to be used for diagnostic messages, and accessed using the global variable
stderr.
In the context of file descriptors (see 3.141 File Descriptor ), file descriptor number 0.
In the context of standard I/O streams (see XSH 2.5 Standard I/O
Streams ), an input stream usually intended to be used for primary data input, and accessed using the global variable
stdin.
In the context of file descriptors (see 3.141 File Descriptor ), file descriptor number 1.
In the context of standard I/O streams (see XSH 2.5 Standard I/O
Streams ), an output stream usually intended to be used for primary data output, and accessed using the global variable
stdout.
The utilities described in the Shell and Utilities volume of POSIX.1-2024.
Appearing in lowercase, a stream is an ordered sequence of bytes, as described by the ISO C standard.
In the shell command language, each stream is associated with a file descriptor. These can be opened using
redirection operators.
In the C language, each stream is accessed via a file access object and is either a stream associated with a file
descriptor or a memory stream. A file access object associated with a file descriptor can be created by the fdopen(), fopen(), or popen() functions. A file access object for a memory stream can be created by the fmemopen() or open_memstream()
functions. A stream provides the additional services of user-selectable buffering and formatted input and output.
The fdopen(), fmemopen(), fopen(), open_memstream(), and popen() functions are defined in detail in the System Interfaces volume of POSIX.1-2024.
A contiguous sequence of bytes terminated by and including the first null byte.
A shell execution environment, distinguished from the main or current shell execution environment.
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.
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.
In the context of the System Interfaces volume of POSIX.1-2024, a job that has received a SIGSTOP, SIGTSTP, SIGTTIN, or SIGTTOU signal that caused the process group to stop.
In the context of the shell, a job, other than a non-job-control background job, that became suspended when a process returned a wait status to the shell indicating that the process was stopped by a SIGSTOP, SIGTSTP, SIGTTIN, or SIGTTOU signal.
A suspended job is a job-control background job, but a job-control background job is not necessarily a suspended
job. A non-job-control background job is never a suspended job, even if it includes processes that have been stopped by a SIGSTOP,
SIGTSTP, SIGTTIN, or SIGTTOU signal.
An object-like macro defined with a constant value.
Unless stated otherwise, the following shall apply to every symbolic constant:
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.
An operation that synchronizes memory. See 4.15 Memory Ordering and Synchronization .
A determinism and robustness improvement mechanism to enhance the data input and output mechanisms, so that an application can be assured that the data being manipulated is physically present on secondary mass storage devices.
The state of an I/O operation that has either been successfully transferred or diagnosed as unsuccessful.
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.
For the purpose of this definition, an operation that reads or searches a directory is considered to be a read operation, an operation that modifies a directory is considered to be a write operation, and a directory's entries are considered to be the data read or written.
This standard provides no way to synchronize the contents or attributes of a symbolic link.
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.
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.
An I/O operation performed on a file that provides the application assurance of the integrity of its data and files.
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.
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.
An implementation of POSIX.1-2024.
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.
A clock with at least one second resolution that contains seconds since the Epoch.
A device that receives messages sent by the syslog() function, and
the fmtmsg() function when the MM_CONSOLE flag is set.
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.1-2024.
An implementation provides two system databases: the "group database" (see also 3.164 Group Database ) and the "user database" (see also 3.407 User Database ).
All documentation provided with an implementation except for the conformance document. Electronically distributed documents for an implementation are considered part of the system documentation.
An object other than a process executing an application, that is provided by the system and has a process ID.
See System Boot defined in 3.374 System Boot .
Pertaining to events occurring in all processes existing in an implementation at a given point in time.
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.
A character special file that obeys the specifications of the general terminal interface.
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.
A named collection of messages objects (one messages object per supported language) for internationalization and
localization purposes. A text domain is often named after the application or library that provides the collection, but may have a
more general name if it is intended to be shared by multiple applications or libraries.
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.1-2024 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.
A live thread (see 3.190 Live Thread ) or a zombie thread (see 3.427 Zombie Thread ). The lifetime of a thread is described in 3.390 Thread Lifetime .
A value that uniquely identifies each thread in a process during the thread's lifetime. The value shall be unique across all threads in a process, regardless of whether the thread is:
The period of time that begins when a thread is created and ends when its thread ID is returned to the process.
See also Live Thread in 3.190 Live Thread , Thread Termination in 3.392 Thread Termination , and Zombie Thread in 3.427 Zombie Thread .
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.
Thread termination occurs when a thread executes pthread_exit()
or thrd_exit(), when it returns from the start_routine function passed to
pthread_create() or from the func function passed to thrd_create(), or when it acts on a cancellation request initiated by pthread_cancel().
A thread-safe function shall avoid data races with other calls to the same function, and with calls to any other thread-safe functions, by multiple threads. Each function defined in the System Interfaces volume of POSIX.1-2024 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.
A function that is not required to be thread-safe need not avoid data races with other calls to the same function, nor with calls to any other function (including thread-safe functions), by multiple threads, unless explicitly stated otherwise.
A process global handle which is used for naming thread-specific data. There are two types of key: those of type pthread_key_t which are created using pthread_key_create() and those of type tss_t which are created using tss_create(). If an application attempts to use the two types of key interchangeably (that is, pass a key of type pthread_key_t to a function that takes a tss_t, or vice versa), the behavior is undefined.
Although the same key value can be used by different threads, the values bound to the key by pthread_setspecific() for keys of type pthread_key_t, and by tss_set() for keys of type tss_t, are maintained on a per-thread basis and persist
for the life of the calling thread.
The character '~'.
A method of limiting the length of time an interface will block; see also 3.47 Blocked Process (or Thread) .
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.
A condition that occurs each time a timer, for which there is already an expiration signal queued to the process, expires.
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.
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.
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.
An extent of memory with the same operational characteristics. Typed memory pools may be contained within each other.
A hardware access path to one or more typed memory pools.
Remove the association between a network address and an endpoint.
See Datagram in 3.96 Datagram .
The conversion of a lowercase character that has a single-character uppercase representation into this uppercase representation.
A system database that contains at least the following information for each user ID:
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.
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. The value (uid_t)-1 shall not be a valid user ID, but does have a defined use in some interfaces defined in this standard.
A string that is used to identify a user; see also 3.407 User Database . To be portable across systems conforming to POSIX.1-2024, the value is composed of characters from the portable filename character set. The <hyphen-minus> character should not be used as the first character of a portable user name.
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.
In the shell command language, a named parameter.
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.
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 or a specified 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.
A single-byte white-space character; that is, a character for which the isspace() or isspace_l() function returns a non-zero value.
A character that belongs to the space character class as defined via the LC_CTYPE category in the current locale or a specified locale.
A wide-character code that belongs to the space character class as defined via the LC_CTYPE category in the current locale or a specified locale.
An integer value corresponding to a single graphic symbol or control code.
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().
A contiguous sequence of wide-character codes terminated by and including the first null wide-character code.
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.
A directory, associated with a process, that is used in pathname resolution for pathnames that do not begin with a <slash> character.
Functions for handling characters in a codeset-independent manner.
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 3.107 Display .
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.1-2024.
A system which allows an application to be built using a set of services that are consistent across all systems
that conform to POSIX.1-2024 and that support the XSI option.
The remains of a live process (see 3.189 Live Process ) after it terminates (see 3.287 Process Termination ) and before its status information (see XSH 2.12 Status Information ) is consumed by its parent process.
The remains of a joinable live thread (see 3.183 Joinable Thread and 3.190 Live Thread ) after it terminates (see 3.392 Thread Termination ) and before it
has been joined with pthread_join() or thrd_join() or detached with pthread_detach() or thrd_detach().
The algebraic sign provides additional information about any variable that has the value zero when the representation allows the sign to be determined.