unistd.h

In the C and C++ programming languages, unistd.h is the name of the header file 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 POSIX-compliant operating system and compiler. For instance, this includes Unix and Unix-like operating systems, such as GNU variants, distributions of Linux and BSD, and macOS, and compilers such as GCC and LLVM.

On Unix-like systems, the interface defined by unistd.h is typically made up largely of system call wrapper functions such as fork, pipe and I/O primitives (read, write, close, etc.).

Unix compatibility layers such 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 /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_t 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.

Overview of functions

	Function	Description
	crypt	password and data encryption
	encrypt	encrypt 64-byte messages
	gethostid	get the unique identifier of the current host
	gethostname	get hostname
	getopt	parse command-line options
	swab	swap adjacent bytes
	sysconf	get configuration at run time
Signals	alarm	schedule an alarm signal
	pause	wait for signal
Filesystem	access faccessat faccessat2	check user's permissions for a file or whether it exists
	chdir fchdir	change working directory
	chown fchown lchown fchownat	change owner and group of a file
	close	close a file descriptor
	dup dup2	duplicate a file descriptor
	fsync fdatasync	synchronize a file's in-core state with storage device
	fpathconf pathconf	get configuration values for files
	ftruncate truncate	truncate a file to a specified length
	getcwd getwd get_current_dir_name	get current working directory
	isatty	test whether a file descriptor refers to a terminal
	link linkat	make a new name for a file
	lockf	apply, test or remove a POSIX lock on an open file
	lseek	reposition read/write file offset
	pipe pipe2	create pipe
	pread pwrite	read from or write to a file descriptor at a given offset
	read	read from a file descriptor
	readlink readlinkat	read value of a symbolic link
	rmdir	delete a directory
	symlink symlinkat	make a new name for a file
	sync syncfs	commit filesystem caches to disk
	ttyname ttyname_r	return name of a terminal
	unlink unlinkat	delete a name and possibly the file it refers to
	write	write to a file descriptor
Process	_exit	terminate the calling process
	execl execlp execle execv execvp execvpe	execute a file
	fexecve	execute program specified via file descriptor
	fork	create a new process
	setpgid getpgid setpgrp getpgrp	set/get process group
	getpid getppid	get process identification
	getsid	get session ID
	nice	change process priority
	setsid	creates a session and sets the process group ID
	sleep	sleep for a specified number of seconds
	tcgetpgrp tcsetpgrp	get and set terminal foreground process group
User/Group	getgid getegid	get group identity
	getuid geteuid	get user identity
	getgroups	get list of supplementary group IDs
	getlogin getlogin_r	get username
	seteuid setegid	set effective user or group ID
	setgid	set group identity
	setreuid setregid	set real and/or effective user or group ID
	setuid	set user identity

External links

• unistd.h: standard symbolic constants and types – Base Definitions Reference, The Single UNIX Specification, Version 5 from The Open Group