What is the meaning of POSIX

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What is POSIX  I have read the Wikipedia article and I read it every time I encounter the term  The fact is that I never really understood what it is   Can anyone please explain it to me by explaining  the need for POSIX  too

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Let me give the churlish  unofficial  explanation   POSIX is a set of standards which attempts to distinguish  UNIX  and UNIX-like systems from those which are incompatible with them   It was created by the U S  government for procurement purposes   The idea was that the U S  federal procurements needed a way to legally specify the requirements for various sorts of bids and contracts in a way that could be used to exclude systems to which a given existing code base or programming staff would NOT be portable   Since POSIX was written post facto     to describe a loosely similar set of competing systems     it was NOT written in a way that could be implemented   So  for example  Microsoft s NT was written with enough POSIX conformance to qualify for some bids     even though the POSIX subsystem was essentially useless in terms of practical portability and compatibility with UNIX systems   Various other standards for UNIX have been written over the decades   Things like the SPEC1170  specified eleven hundred and seventy function calls which had to be implemented compatibly  and various incarnations of the SUS  Single UNIX Specification    For the most part these  standards  have been inadequate to any practical technical application   They most exist for argumentation  legal wrangling and other dysfunctional reasons

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The most important things POSIX 7 defines   C API  Greatly extends ANSI C with things like    more file operations  mkdir  dirname  symlink  readlink  link  hardlinks   poll    stat  sync  nftw   process and threads  fork  execl  wait  pipe  semaphors sem    shared memory  shm     kill  scheduling parameters  nice  sched     sleep  mkfifo  setpgid   networking  socket   memory management  mmap  mlock  mprotect  madvise  brk   utilities  regular expressions  reg     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      Many of those syscalls also have Linux-specific extensions however   Major Linux desktop implementation  glibc  which in many cases just provides a shallow wrapper to system calls  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  Shell language  E g   a b  echo   a   Major Linux desktop implementation  GNU Bash  Environment variables  E g   HOME  PATH   PATH search semantics are specified  including how slashes prevent PATH search   Program exit status  ANSI C says 0 or EXIT SUCCESS for success  EXIT FAILURE for failure  and leaves the rest implementation defined   POSIX adds    126  command found but not executable  127  command not found   gt  128  terminated by a signal   But POSIX does not seem to specify the 128   SIGNAL ID rule used by Bash  https   unix stackexchange com questions 99112 default-exit-code-when-process-is-terminated  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  Directory structure  E g    dev null   tmp  The Linux FHS greatly extends POSIX  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  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   POSIX ACLs   Access Control Lists   e g  as used as backend for setfacl   This was withdrawn but it was implemented in several OSes  including in Linux with setxattr    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  Inspur s K-UX and Huawei s EulerOS are two certified examples   The official list of certified systems be found at  https   www opengroup org openbrand register  and also at 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    Where does Microsoft Windows  39  7 POSIX implementation currently stand  https   superuser com questions 495360 does-windows-8-still-implement-posix Feature request  https   windows uservoice com forums 265757-windows-feature-suggestions suggestions 6573649-full-posix-support   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  Android has its own C library  Bionic  which does not fully support POSIX as of Android O  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   Where is the list of the POSIX C API functions

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A specification  blueprint  about how to make an OS compatible with late UNIX OS  may God bless him    This is why macOS and GNU Linux have very similar terminal command lines  GUI s  libraries  etc  Because they both were designed according to POSIX blueprint   POSIX does not tell engineers and programmers how to code but what to code

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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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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

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POSIX stands for Portable Operating System Interface  and is an IEEE standard designed to facilitate application portability  POSIX is an attempt by a consortium of vendors to create a single standard version of UNIX

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POSIX is      POSIX  pronounced   p z ks   or    Portable Operating System Interface    for Unix  1 is the name of a family   of related standards specified by the   IEEE to define the application   programming interface  API   along   with shell and utilities interfaces   for software compatible with variants   of the Unix operating system  although   the standard can apply to any   operating system    Basically it was a set of measures to ease the pain of development and usage of different flavours of UNIX by having a  mostly  common API and utilities  Limited POSIX compliance also extended to various versions of Windows

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This standard provides a common basis for Unix-like operating systems  It specifies how the shell should work  what to expect from commands like ls and grep  and a number of C libraries that C authors can expect to have available   For example  the pipes that command-line users use to string together commands are specified in detail here  which means C   s popen  pipe open  function is POSIX-standard  not ISO C-standard

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POSIX is a standard for operating systems that was supposed to make it easier to write cross-platform software  It s an especially big deal in the world of Unix

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Some facts about POSIX that are not so bright    POSIX is also the system call interface or API  and it is nearly 30 years old    It was designed for serialized data access to local storage  using single computers with single CPUs   Security was not a major concern in POSIX by design  leading to numerous race condition attacks over the years and forcing programmers to work around these limitations    Serious bugs are still being discovered  bugs that could have been averted with a more secure POSIX API design    POSIX expects users to issue one synchronous call at a time and wait for its results before issuing the next  Today s programmers expect to issue many asynchronous requests at a time to improve overall throughput    This synchronous API is particularly bad for accessing remote and cloud objects  where high latency matters

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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  amp  Tinkertim for pointing out my error - my bad

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In 1985  individuals from companies throughout the computer industry   joined together to develop the POSIX  Portable Operating System   Interface for Computer Environments  standard  which is based largely   on the UNIX System V Interface Definition  SVID  and other earlier   standardization efforts  These efforts were spurred by the U S    government  which needed a standard computing environment to minimize   its training and procurement costs  Released in 1988  POSIX is a group   of IEEE standards that define the API  shell  and utility interfaces   for an operating system  Although aimed at UNIX-like systems  the   standards can apply to any compatible operating system  Now that these   stan- dards have gained acceptance  software developers are able to   develop applications that run on all conforming versions of UNIX    Linux  and other operating systems    From the book  A Practical Guide To Linux

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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

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POSIX is a set of standards put forth by IEEE and The Open Group that describes how an ideal Unix would operate  Programmers  users  and administrators can all become familiar with the POSIX document  and expect a POSIX-complaint Unix to provide all of the standard facilities mentioned   Since every Unix does things a little differently -- Solaris  Mac OS X  IRIX  BSD  and Linux all have their quirks -- POSIX is especially useful to those in the industry as it defines a standard environment to operate in   For example  most of the functions in the C library are based in POSIX  a programmer can  therefore  use one in his application and expect it to behave the same across most Unices   However  the divergent areas of Unix are typically the focus  rather than the standard ones   The great thing about POSIX is that you re welcome to read it yourself      The Open Group Base Specifications Issue 7   Issue 7 is known as POSIX 1-2008  and there are new things in there -- however  Google-fu for POSIX 1 and such will allow you to see the entire history behind what Unix is

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