HP C Programmer's Guide (92434-90009)

146 Chapter5
Programming for Portability
Silent Changes for ANSI C
Silent Changes for ANSI C
Non-ANSI mode HP C is different from ANSI mode HP C in ways that generally go
unnoticed. On HP-UX, many of these silent differences can be found by running the lint(1)
program. The following list provides some of these silent changes:
• Trigraphs are new in ANSI C. A trigraph is a three character sequence that is replaced
by a corresponding single character. For example, ??= is replaced by #. For more
information on trigraphs, refer to "Preprocessing Directives" in the HP C/HP-UX
Reference Manual.
• Promotion rules for unsigned char and unsigned short have changed. Non-ANSI
mode rules specify when an unsigned char or unsigned short is used with an integer
the result is unsigned. ANSI mode rules specify the result is signed. The following
program example illustrates a case where these rules differ:
main(){
unsigned short us = 1;
int i = -2;
printf("%s\n",(i+us)>0 ? "non-ANSI mode" : "ANSI mode");
}
Note that differences in promotion rules can occur under the following conditions:
1
— An expression involving an unsigned char or unsigned short produces an
integer-wide result in which the sign bit is set: that is, either a unary operation on
such a type, or a binary operation in which the other operand is int or a "narrower"
type.
— The result of the preceding expression is used in a context in which its condition of
being signed is significant: it is the left operand of the right-shift operator or either
operand of /,%,<,<=,>, or >=.
• Floating-point expressions with float operands may be computed as float precision in
ANSI mode. In non-ANSI mode they will always be computed in double precision.
• Initialization rules are different in some cases when braces are omitted in an
initialization.
• Unsuffixed integer constants may have different types. In non-ANSI mode, unsuffixed
constants have type int. In the ANSI mode, unsuffixed constants less than or equal to
2147483647 have type int. Constants larger than 2147483647 have type unsigned.For
example:
-2147483648
has type unsigned in the ANSI mode and int in non-ANSI mode. The above constant is
unsigned in the ANSI mode because 2147483648 is unsigned, and the - is a unary
operator.
1. Rationale for Proposed American National Standard for Information Systems - Programming
Language C (311 First Street, N.W., Suite 500, Washington, DC 20001-2178; X3 Secretariat:
Computer and Business Equipment Manufacturers Association), pages 34-35