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I never really understood: what is POSIX?

栈溢出 James Webster 16℃ 0评论
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1.原始问题:I never really understood: what is POSIX?

What is POSIX? I read the Wikipedia article and I read it ever time I encounter the term. Fact is that I never really understood what it is.

Can anyone please explain it to me by explaining “the need for POSIX” too?

2.被采纳答案

POSIX is a family of standards, specified by the IEEE, to clarify and make uniform the application programming interfaces (and ancillary issues, such as commandline shell utilities) provided by Unix-y operating systems. When you write your programs to rely on POSIX standards, you can be pretty sure to be able to port them easily among a large family of Unix derivatives (including Linux, but not limited to it!); if and when you use some Linux API that’s not standardized as part of Posix, you will have a harder time if and when you want to port that program or library to other Unix-y systems (e.g., MacOSX) in the future.

3.其他高票答案

3.1.第1个答案

Most important things POSIX 7 defines

  1. C API

    Greatly extends ANSI C with things like:

    • more file operations: mkdir, dirname, symlink, readlink, link (hardlinks), poll, sync
    • process and threads: fork, execl, pipe, semaphors sem_*, shared memory (shm_*), kill, scheduling parameters (nice, sched_), sleep
    • networking
    • regular expressions
    • advanced memory management: mmap, mlock, mprotect, madvise

    Those APIs also determine underlying system concepts on which they depend, e.g. fork requires a concept of a process.

    Many Linux system calls exist to implement a specific POSIX C API function and make Linux compliant, e.g. sys_write, sys_read, …

    Major Linux desktop implementation: glibc, which in many cases just provides a shallow wrapper to system calls.

  2. CLI utilities

    E.g.: cd, ls, echo, …

    Many utilities are direct shell front ends for a corresponding C API function, e.g. mkdir.

    Major Linux desktop implementation: GNU Coreutils for the small ones, separate GNU projects for the big ones: sed, grep, awk, … Some CLI utilities are implemented by Bash as built-ins.

  3. Shell language

    E.g., a=b; echo "$a"

    Major Linux desktop implementation: GNU Bash.

  4. Environment variables

    E.g.: HOME, PATH.

  5. Program exit status

    ANSI C says 0 or EXIT_SUCCESS for success, EXIT_FAILURE for failure, and leaves the rest implementation defined.

    POSIX adds:

  6. Regular expression

    There are two types: BRE (Basic) and ERE (Extended). Basic is deprecated and only kept to not break APIs.

    Those are implemented by C API functions, and used throughout CLI utilities, e.g. grep accepts BREs by default, and EREs with -E.

    E.g.: echo 'a.1' | grep -E 'a.[[:digit:]]'

    Major Linux implementation: glibc implements the functions under regex.h which programs like grep can use as backend.

  7. Directory struture

    E.g.: /dev/null, /tmp

    The Linux FHS greatly extends POSIX.

  8. Filenames

    • / is the path separator
    • NUL cannot be used
    • . is cwd, .. parent
    • portable filenames
      • use at most max 14 chars and 256 for the full path
      • can only contain: a-zA-Z0-9._-

    See also: what is posix compliance for filesystem?

  9. Command line utility API conventions

    Not mandatory, used by POSIX, but almost nowhere else, notably not in GNU. But true, it is too restrictive, e.g. single letter flags only (e.g. -a), no double hyphen long versions (e.g. --all).

    A few widely used conventions:

    • - means stdin where a file is expected
    • -- terminates flags, e.g. ls -- -l to list a directory named -l

    See also: Are there standards for Linux command line switches and arguments?

Who conforms to POSIX?

Many systems follow POSIX closely, but few are actually certified by the Open Group which maintains the standard. Notable certified ones include:

  • OS X (Apple) X stands for both 10 and UNIX. Was the first Apple POSIX system, released circa 2001. See also: Is OSX a POSIX OS?
  • AIX (IBM)
  • HP-UX (HP)
  • Solaris (Oracle)

Most Linux distros are very compliant, but not certified because they don’t want to pay the compliance check.

See the wiki page.

Windows

Windows implemented POSIX on some of its professional distributions.

Since it was an optional feature, programmers could not rely on it for most end user applications.

Support was deprecated in Windows 8:

In 2016 a new official Linux-like API called “Windows Subsystem for Linux” was announced. It includes Linux system calls, ELF running, parts of the /proc filesystem, Bash, GCC, (TODO likely glibc?), apt-get and more: https://channel9.msdn.com/Events/Build/2016/P488 so I believe that it will allow Windows to run much, if not all, of POSIX. However, it is focused on developers / deployment instead of end users. In particular, there were no plans to allow access to the Windows GUI.

Historical overview of the official Microsoft POSIX compatibility: http://brianreiter.org/2010/08/24/the-sad-history-of-the-microsoft-posix-subsystem/

Cygwin is a well known GPL third-party project for that “provides substantial POSIX API functionality” for Windows, but requires that you “rebuild your application from source if you want it to run on Windows”. MSYS2 is a related project that seems to add more functionality on top of Cygwin.

Android

Is Android POSIX-compatible?

Bonus level

The Linux Standard Base further extends POSIX.

Use the non-frames indexes, they are much more readable and searchable: http://pubs.opengroup.org/onlinepubs/9699919799/nfindex.html

Get a full zipped version of the HTML pages for grepping: Is there a listing of the POSIX API / functions?

3.2.第2个答案

Posix is more as an OS, it is an “OS standard”. You can imagine it as an imaginary OS, which actually doesn’t exist but it has a documentation. These papers are the “posix standard”, defined by the IEEE, which is the big standard organization of the USA.
The OSes implementing this specification are “Posix-compliant”.

Government regulations prefer Posix-compliant solutions in their investments, thus being Posix-compliant has a significant financial advantage, particularly for the big IT companies of the USA.

The reward for an OS being fully posix compliant, that it is a guarantee that it will compile and run all Posix-compliant applications seamlessly.

Linux is the most well-known one. OSX, Solaris, NetBSD and Windows NT play here as well. Free- and OpenBSD are only “nearly” Posix-compliant. The posix-compliance of the WinNT is only a pseudo-solution to avoid this government regulation above.

3.3.第3个答案

Posix governs interoperability, portability, and in other areas such as the usage and mechanism of fork, permissions and filesystem standards such as /etc, /var, /usr and so on . Hence, when developers write a program under a Posix compliant system such as for example Linux, it is generally, not always, guaranteed to run on another posix compliant system such as IBM’s AIX system or other commercial variants of Unix. Posix is a good thing to have as such it eases the software development for maximum portability which it strives for. Hope this answer makes sense.

Thanks to Jed Smith & Tinkertim for pointing out my error – my bad!!! 🙁

3.4.第4个答案

POSIX defines set of standards for an operating system or a program.
The goal is to write new software that is compatible with UNIX-like systems.

For example a program runs on Linux is also can be compile and run on other UNIX-like systems like Solaris, HP-UX, and AIX etc..

The most popular examples are GNU Bash which is 100% POSIX compliance and gawk utility.

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