POSIX C & ANSI C

• 以下部分转载自：http://blog.youkuaiyun.com/skylj0209/article/details/3071507
• ANSI C

• 这一标准是 ANSI（美国国家标准局）于 1989 年制定的 C 语言标准。 后来被 ISO（国际标准化组织）接受为标准，因此也称为 ISO C。
  ANSI C 的目标是为各种操作系统上的 C 程序提供可移植性保证，而不仅仅限于 UNIX。 该标准不仅定义了 C 编程语言的语发和语义，而且还定义了一个标准库。这个库可以根据 头文件划分为 15 个部分，其中包括：字符类型 (<ctype.h>)、错误码 (<errno.h>)、 浮点常数 (<float.h>)、数学常数 (<math.h>)、标准定义 (<stddef.h>)、 标准 I/O (<stdio.h>)、工具函数 (<stdlib.h>)、字符串操作 (<string.h>)、 时间和日期 (<time.h>)、可变参数表 (<stdarg.h>)、信号 (<signal.h>)、 非局部跳转 (<setjmp.h>)、本地信息 (<local.h>)、程序断言 (<assert.h>) 等等。

• POSIX C

• 该标准最初由 IEEE 开发的标准族，部分已经被 ISO 接受为国际标准。该标准的具体内容 见 1.1.3。POSIX.1 和 POSIX.2 分别定义了 POSIX 兼容操作系统的 C 语言系统接口 以及 shell 和工具标准。这两个标准是通常提到的标准。POSIX 表示可移植操作系统接口（Portable Operating System Interface ，缩写为 POSIX 是为了读音更 像 UNIX。电气和电子工程师协会（Institute of Electrical and Electronics Engineers，IEEE） 最初开发 POSIX 标准，是为了提高 UNIX 环境下应用程序的可移植性。然而，POSIX 并不局限于 UNIX。 许多其它的操作系统，例如 DEC OpenVMS 和 Microsoft Windows NT，都支持 POSIX 标准，尤其是 IEEE Std. 1003.1-1990（1995 年修订）或 POSIX.1，POSIX.1 提供了源代码级别的 C 语言应用编程 接口（API）给操作系统的服务程序，例如读写文件。POSIX.1 已经被国际标准化组织（International Standards Organization，ISO）所接受，被命名为 ISO/IEC 9945-1:1990 标准。
• 以下转载自：http://www.thejaywalker.net/2010/12/difference-between-posix-and-standard-c.html
  

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• Difference Between POSIX and Standard C Library

  If everything you learn about Unix development using C/C++ is POSIX then what the hell does the standard C library do?
  
  First things first. POSIX stands for "Portable Operating System Interface (for UniX)." Now that's a fancy name but the requirement becomes very clear when you see the disparity amongst so many flavors of Unix. There was a need to unify the programming of these flavors of Unix (include Linux); POSIX provides this unified API. Though the standard is not tied to just C, there is no known full implementation of POSIX in any other environment.
  
  
  

  What does POSIX contain?

  
  A lot of things! For example, threads, semaphores, file system access API, etc.
  
  

  What's wrong with/ limiting in Standard C Library?

  
  Standard C library (i.e., ANSI/ ISO C) assumes a very minimalist operating system. For example, no standard C library functions exist to manage threads. Heck, there is no IPC (inter process communication); in other words, ISO C doesn't assume that the program would be running on a multi-processing operating system!
  
  

  Again, what exactly does POSIX add to the standard C library?

  
  To answer this, one must know which header files ISO/ ANSI C contains. There are 24 header files including the well known  <stdio.h>,  <stdlib.h>, etc. 
  
  

  <assert.h>	<complex.h>	<ctype.h>	<errno.h>	<fenv.h>
  <float.h>	<inttypes.h>	<iso646.h>	<limits.h>	<locale.h>
  <math.h>	<setjmp.h>	<signal.h>	<stdarg.h>	<stdbool.h>
  <stddef.h>	<stdint.h>	<stdio.h>	<stdlib.h>	<string.h>
  <tgmath.h>	<time.h>	<whcar.h>	<wctype.h>

  
  
  As you can see, there is nothing like  <socket.h>,  <thread.h>,  <sharedmemory.h>, <graphics.h>, etc., since this kind of functionality is heavily dependent on the kind of environment the application would run in while C was designed to be a "portable" language.
  
  Not having this "advanced" functionality, C becomes severely limited and each operating environment is bound to introduce its own standard for defining multi-processing, interprocess communication, advanced memory management, etc. That's where POSIX (and other kinds of libraries such as graphics toolkit) kicks in.
  
  If you have programmed for Unix/ Linux, you know that there are several library functions beyond system calls which are not in the standard C list. This includes functions like  fork(), which is neither a system call nor part of the standard C library (remember C doesn't assume a multi-processing OS?). In fact, POSIX goes well beyond a programming API and defines the "environment" in which the application would run. For example, POSIX defines that there shall be a root directory "/" in the operating environment---no such definition is part of C standard library.
  
  The complete list of  POSIX header files should be referred as well. Below is a listing of the header files additional to ISO C:
  
  

  <aio.h>	<arpa/inet.h>	<assert.h>	<ctype.h>
  <dirent.h>	<dlfcn.h>	<fcntl.h>	<fmtmsg.h>
  <fnmatch.h>	<ftw.h>	<glob.h>	<grp.h>
  <iconv.h>	<langinfo.h>	<libgen.h>	<monetary.h>
  <mqueue.h>	<ndbm.h>	<net/if.h>	<netdb.h>
  <netinet/in.h>	<netinet/tcp.h>	<nl_types.h>	<poll.h>
  <pthread.h>	<pwd.h>	<regex.h>	<sched.h>
  <search.h>	<semaphore.h>	<spawn.h>	<strings.h>
  <stropts.h>	<sys/ipc.h>	<sys/mman.h>	<sys/msg.h>
  <sys/resource.h>	<sys/select.h>	<sys/sem.h>	<sys/shm.h>
  <sys/socket.h>	<sys/stat.h>	<sys/statvfs.h>	<sys/time.h>
  <sys/times.h>	<sys/types.h>	<sys/uio.h>	<sys/un.h>
  <sys/utsname.h>	<sys/wait.h>	<syslog.h>	<tar.h>
  <termios.h>	<trace.h>	<ulimit.h>
  <unistd.h>	<utime.h>	<utmpx.h>	<wordexp.h>

  
  
  

  Is POSIX a superset of standard C library?

  
  Yes! POSIX, for example, redefines all standard C header files (sometimes augmenting them with advanced functionality), and uses the following statement in its documentation:
  

  The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2008 defers to the ISO C standard.

  
  
  

  What's the use of standard C library, then?

  
  Nothing if you are programming for a POSIX system but not all systems are POSIX compliant. To ensure maximum portability---ranging from desktop machines to handheld devices running a C runtime---restrict yourself to the standard C library and you can fly quite a distance with that.