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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.
[5] [6] [7] An X-Macro is a header file. Commonly, these use the extension .def instead of the traditional .h. This file contains a list of similar macro calls, which can be referred to as "component macros." The include file is then referenced repeatedly. Many compilers define additional, non-standard macros.
Another solution is to use an include guard in each header file. [4] The C standard library is declared as a collection of header files. The C++ standard library is similar, but the declarations may be provided by the compiler without reading an actual file. C standard header files are named with a .h file name extension, as in #include <stdio ...
The C preprocessor processes inclusion directives like #include "foo.h" to include "foo.h" and transcludes the code of that file into a copy of the main file often called the translation unit. However, if an #include directive for a given file appears multiple times during compilation, the code will effectively be duplicated in that file.
The C POSIX library is a specification of a C standard library for POSIX systems. It was developed at the same time as the ANSI C standard. Some effort was made to make POSIX compatible with standard C; POSIX includes additional functions to those introduced in standard C.
Berkeley sockets originated with the 4.2BSD Unix operating system, released in 1983, as a programming interface.Not until 1989, however, could the University of California, Berkeley release versions of the operating system and networking library free from the licensing constraints of AT&T Corporation's proprietary Unix.
getopt is a system dependent function, and its behavior depends on the implementation in the C library. Some custom implementations like gnulib are available, however. [6]The conventional (POSIX and BSD) handling is that the options end when the first non-option argument is encountered, and that getopt would return -1 to signal that.
A file descriptor open when an exec call is made remains open in the new process image, unless was fcntl ed with FD_CLOEXEC or opened with O_CLOEXEC (the latter was introduced in POSIX.1-2001). This aspect is used to specify the standard streams (stdin, stdout and stderr) of the new program.