The realization that the operating systems of the target machines
were as great an obstacle to portability as their hardware architecture
led us to a seemingly radical suggestion: to evade that part of the
problem altogether by moving the operating system itself.
--
<authorgroup>Steve Johnson
Dennis Ritchie</authorgroup>
Portability of C Programs and the UNIX System (1978)
Unix was the first production operating system to be ported
between differing processor families (Version 6 Unix, 1976-77). Today,
Unix is routinely ported to every new machine powerful enough to sport
a memory-management unit. Unix applications are routinely moved
between Unixes running on wildly differing hardware; in fact, it is
unheard of for a port to fail.
Portability has always been one of Unix's principal advantages.
Unix programmers tend to write on the assumption that hardware is
evanescent and only the Unix API is stable, making as few assumptions
as possible about machine specifics such as word length, endianness or
memory architecture. In fact, code that is hardware-dependent in any
way that goes beyond the abstract machine model of
C is considered
bad form in Unix circles, and only really tolerated in very
special cases like operating system kernels.
Unix programmers have learned that it is easy to be wrong when
anticipating that a software project will have a short
lifetime.[141] Thus, they
tend to avoid making software dependent on specific and perishable
technologies, and to lean heavily on open standards. These habits of
writing for portability are so ingrained in the Unix tradition that
they are applied even to small single-use projects that are thought of as
throwaway code. They have had secondary effects all through the design
of the Unix development toolkit, and on programming languages like
Perl and Python and Tcl that were developed under Unix.
The direct benefit of portability is that it is normal for Unix
software to outlive its original hardware platform, so tools and
applications don't have to be reinvented every few years. Today,
applications originally written for Version 7 Unix (1979) are
routinely used not merely on Unixes genetically descended from V7, but
on variants like Linux in which the operating system API was
written from a Unix specification and shares no code with the Bell
Labs source tree.
The indirect benefits are less obvious but may be more important.
The discipline of portability tends to exert a simplifying influence on
architectures, interfaces, and implementations. This both increases
the odds of project success and reduces life-cycle maintenance
costs.
In this chapter, we'll survey the scope and history of Unix
standards. We'll discuss which ones are still relevant today and
describe the areas of greater and lesser variance in the Unix API.
We'll examine the tools and practices that Unix developers use to keep
code portable, and develop some guides to good practice.
