# Difference in Single Unix Specification, POSIX and X/Open



## zubairahmed_cae (Apr 28, 2010)

Hi,

From the point of view of a UNIX programmer, whats the difference between Single Unix Specification, POSIX and X/Open standard?

Thanks
Zubair


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## graudeejs (Apr 28, 2010)

Correct me if I'm wrong, but I think you have to pay big $ for POSIX specification (~1K per lisence, so I've read [Will check again and update you])
On other hand single Unix specification is free.

Don't know about X/Open


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## SirDice (Apr 28, 2010)

> After 1997, the Austin Group developed the POSIX revisions. The specifications are known under the name Single UNIX Specification, before they become a POSIX standard when formally approved by the ISO.


Wikipedia: POSIX



> The last version of the XPG, the X/Open Portability Guide Issue 4  (also known as the Common Applications Environment Specification Issue 4 (CAE4)), was published in July 1992 by The Open Group. The Single UNIX Specification was based on the XPG4 standard.


Wikipedia: X/Open


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## graudeejs (Apr 28, 2010)

I'll try to translate (very loosely) from Russian to English, so bear with me. My native language is Latvian 

POSIX is for portability, it's based on UNIX, but is not limited to it.... (Windows claims to be POSIX compatible), The problem is that it defines various services, but doesn't specify how to implement them, which results in hard to write portable code, because of various implementations

Single UNIX Specification is based on POSIX.1, and fills it's holes. The full equipment of system interfaces is called _X/Open System Interface_ (XSI)

only XSI compatible realisations can be called UNIX systems


Can't find info about pricing ...


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## SirDice (Apr 28, 2010)

killasmurf86 said:
			
		

> (Windows claims to be POSIX compatible)


Errr.. It's actually POSIX certified, certain versions of Windows are at least. Something FreeBSD isn't, FreeBSD is (partly) POSIX compliant.


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## sossego (Apr 28, 2010)

Why isn't it POSIX compliant?

I know that FreeBSD passes my standards of POPTART compliance and Single Coffee Specification.


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## SirDice (Apr 28, 2010)

sossego said:
			
		

> Why isn't it POSIX compliant?


It's mostly compliant.
http://www.freebsd.org/projects/c99/index.html


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## ckester (Apr 30, 2010)

sossego said:
			
		

> Why isn't it POSIX compliant?



As far as I know, the cost of certifying POSIX compliance is the biggest obstacle.

I no longer care.  Instead of asking whether something is POSIX  compliant, I ask if it's BSD compatible.  :e


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