Section 3.1. Linux Standards

3.1. Linux Standards

Almost all Linux distributors acknowledge the values that standardization and portability provide and work with the various Linux implementers/maintainers to meet those standards. The primary Linux industry standard is the Linux Standard Base (LSB), which is a project overseen by the Free Standards Group (FSG). The LSB is based heavily on the IEEE POSIX and Open Group Single UNIX Specification standards, but it also documents places where Linux is not UNIX- or POSIX-compatible. Almost all the major Linux distributors have certified to the LSB.

The LSB, in addition to providing a source code API specification, also provides a binary interface (ABI) specification for each of several common Linux hardware platforms. The goal of the ABI is to ensure binary portability, for LSB-portable applications, between Linux distributions on that hardware platform. Distributors must certify to the ABI as well as the API. Independent software vendors (ISVs) can also certify their applications as "LSB portable" using tools provided by the FSG.

Table 3-1 shows the Linux supported standards based on the LSB specification version 2.0.1.

Table 3-1. Supported Standards on Linux

Standard	Linux
IEEE POSIX 1003.1-2001 Base Function	Supported as described in the LSB
IEEE POSIX 1003.1 Threads (Pthreads)	Supported
Single UNIX Specification Base Function (sometimes known as XPG5 or XPG6)	Supported as described in the LSB
IEEE POSIX 1003.12001 Real-time scheduler, clocks and timers, asynchronous I/O, semaphores, memory locking, message queues, real-time signals, shared memory	Asynchronous I/O, real-time signals, shared memory, semaphores
IEEE POSIX 1003.1-2001 (commands and utilities)	Supported
System V Interface Definition (SVID3)	Referenced[1]
File System Directory Layout	Filesystem Hierarchy Standard, www.pathname.com/fhs/
X11 Window System, Font Server, and Clients	X11R6.4
OSF/Motif	Supported through third-party libraries (see LessTif, www.lesstif.org)[2]
RPC function interfaces	Supported
NFS+ Diskless and ONC+, NFS+, version 4.2	Referenced standards: SVID Issue 4, SVID Issue 3
OSF/Motif Application Environment Specification (AES)	Supported through third-party products[3]
LP64, 64-bit data model	Supported

^[1] A referenced standard means that not all interfaces within the standard are supported.

^[2] Not included within the LSB specification

^[3] See Open Motif, www.opengroup.org/openmotif/faq.html

Although there is standardization within Linux, Linux itself is not totally compliant to either the IEEE POSIX or Open Group SUS specifications.^[4] This means that applications that may be totally portable under these common commercial UNIX standards may still need some work to move successfully to Linux. However, this work will be much less than for an application that meets no standards at all, or one that uses operating systemspecific extensions.

^[4] Conflicts between ISO/IEC 9945 (POSIX) and the Linux Standard Base (www.opengroup.org/personal/ajosey/tr01-04-2005.txt)