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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. [1] 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.
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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.