I never really understood: what is POSIX?

Question

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?

Answer 1

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.

Answer 2

POSIX is:

POSIX (pronounced /ˈpɒzɪks/) 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.

Answer 3

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.

Answer 4

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.

Answer 5

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. Best regards, Tom.

Thanks to Jed Smith & Tinkertim for pointing out my error - my bad!!! :(

Answer 6

Get 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.