Intel XENIX 286 Programmers Guide (86) - Tenox.tc

C Language Portability XENIX Programming
Program Portability
A program is portable if it can be compiled and run successfully on different machines
without alteration. There are many ways to write portable programs. The first is to
avoid using inherently nonportable language features. The second is to isolate any
nonportable interactions with the environment, such as 1/0 to nonstandard devices. For
example, programs should avoid hard-coding path names unless a path name is common
to all systems (e.g., /etc/passwd).
Files required at compile time (i.e., include files) may also introduce nonportability if
the path names are not the same on all machines. In some cases, include files
containing machine parameters can be used to make the source code itself portable.
Machine Hardwa re
Differences in the hardware of the various target machines and differences in the
corresponding C compilers cause the greatest number of portability problems. This
section lists problems commonly encountered on XENIX systems.
Byte Length
By definition, the char data type in C must be large enough to hold as positive integers
all members of a machine's character set. For the machines described in this appendix,
the char size is exactly an 8-bit byte.
Word Length
In C, the size of the basic data types for a given implementation are not formally
defined. Thus they often follow the most natural size for the underlying machine. It is
safe to assume that short is no longer than long. Beyond that no assumptions are
portable. For example, on some machines short is the same length as int, whereas on
others long is the same length as int.
Programs that need to know the size of a particular data type should avoid hard-coded
constants where possible. Such information can usually be written in a fairly portable
way. For example, the maximum positive integer (on a two's complement machine) can
be obtained with
A-2
#defi ne MAXPOS ((int) (((unsigned)-1) >> 1))