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

XENIX Programming C Language Portability
Compiler Differences
A number of C compilers are available for various XENIX systems. The "Ritchie"
compiler is available for PDP-11 systems. Available for the PDP-11 and most other
XENIX systems is the Portable C Compiler.
Signed/Unsigned char, Sign Extension
The current state of the signed versus unsigned char problem is best described as
unsatisfactory.
The sign extension problem is a serious barrier to writing portable C, and the best
solution at present is to write defensive code that does not rely on particular
implementation features.
Shift Operations
The left shift operator >, when applied to an
unsigned quantity, performs a logical shift operation. When applied to a signed quantity,
the vacated bits may be filled with zero (logical shift) or with sign bits (arithmetic
shift). The decision is implementation dependent, and code that uses knowledge of a
particular implementation is nonportable.
The PDP-11 compilers use arithmetic right shift. To avoid sign extension, you must
shift and mask out the appropriate number of high order bits:
char c;
c = (c > > 3) & Ox1f;
You can also avoid sign extension by using the divide operator:
char c;
c = c I 8;
Identifier Length
The use of long symbols and identifier names will cause portability problems with some
compilers. To avoid these problems, a program should keep the following symbols as
short as possible:
• C preprocessor symbols
• C local symbols
• C external symbols
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