Unistd.h
Encyclopedia
In the C and C++
programming languages,
that provides access to the POSIX
operating system API
. It is defined by the POSIX.1 standard, the base of the Single Unix Specification
, and should therefore be available in any conforming (or quasi-conforming
) operating system/compiler
(all official versions of Unix
, including Mac OS X
, Linux
, etc.).
On Unix-like systems, the interface defined by
primitives (
Unix compatibility layers as Cygwin
and MinGW
also provide their own versions of unistd.h. In fact, those systems provide it along with the translation libraries that implement its functions in terms of Win32 functions. E.g. In Cygwin, a header file can be found in
and many more. Thus, unistd.h is only a generically defined adaptive layer that might be based upon already existing system and compiler specific definitions. This has the general advantage of not having a possibly concurrent set of header file defined, but one that is built upon the same root which, for this reason, will raise much fewer concerns in combined usage cases.
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
programming languages,
unistd.h is the name of the header fileHeader file
Some programming languages use header files. These files allow programmers to separate certain elements of a program's source code into reusable files. Header files commonly contain forward declarations of classes, subroutines, variables, and other identifiers...
that provides access to the POSIX
POSIX
POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...
operating system API
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...
. It is defined by the POSIX.1 standard, the base of the Single Unix Specification
Single UNIX Specification
The Single UNIX Specification is the collective name of a family of standards for computer operating systems to qualify for the name "Unix"...
, and should therefore be available in any conforming (or quasi-conforming
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
) operating system/compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
(all official versions of Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
, including Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
, Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
, etc.).
On Unix-like systems, the interface defined by
unistd.h is typically made up largely of system call wrapper functions such as forkFork (operating system)In computing, when a process forks, it creates a copy of itself. More generally, a fork in a multithreading environment means that a thread of execution is duplicated, creating a child thread from the parent thread....
, pipePipeline (software)In software engineering, a pipeline consists of a chain of processing elements , arranged so that the output of each element is the input of the next. Usually some amount of buffering is provided between consecutive elements...
and I/OInput/output
In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...
primitives (
read, write, close, etc.).Unix compatibility layers as Cygwin
Cygwin
Cygwin is a Unix-like environment and command-line interface for Microsoft Windows. Cygwin provides native integration of Windows-based applications, data, and other system resources with applications, software tools, and data of the Unix-like environment...
and MinGW
MinGW
MinGW , formerly mingw32, is a native software port of the GNU Compiler Collection and GNU Binutils for use in the development of native Microsoft Windows applications; MinGW can function either as a cross compiler targeting Windows or as a native toolchain run on Windows itself...
also provide their own versions of unistd.h. In fact, those systems provide it along with the translation libraries that implement its functions in terms of Win32 functions. E.g. In Cygwin, a header file can be found in
/usr/include that sub-includes a file of the same name in /usr/include/sys. Not everything is defined in there but some definitions are done by references to the GNU C standard library headers (like stddef.h) which provide the type size tSize t
size_t is an unsigned data type defined by several C and C++ standards that is defined in stddef.h. It can be further imported by inclusion of stdlib.h as this file internally sub includes stddef.h....
and many more. Thus, unistd.h is only a generically defined adaptive layer that might be based upon already existing system and compiler specific definitions. This has the general advantage of not having a possibly concurrent set of header file defined, but one that is built upon the same root which, for this reason, will raise much fewer concerns in combined usage cases.