 | NOTE: Many older compilers, including previous releases of
HP C/HP-UX, performed integral promotions in a slightly different
way, following unsigned preserving rules. In
order to avoid "breaking" programs that may rely on this non-ANSI
behavior, compatibility mode continues to follow the unsigned preserving
rules. Under these rules, the only difference is that unsigned char
and unsigned short are promoted to unsigned int, rather than int. In the vast majority of cases, results are the same. However,
if the promoted result is used in a context where its sign is significant
(such as a division or comparison operation), results can be different
between ANSI mode and compatibility mode. The following program
shows two expressions that are evaluated differently in the two
modes. #include <stdio.h>
main ()
{
unsigned short us = 1;
printf ("Quotient = %d\n",-us/2);
printf ("Comparison = %d\n",us<-1);
} |
In compatibility mode, as with many pre-ANSI compilers, the
results are: Quotient = 2147483647
Comparison = 1 |
ANSI C gives the following results: Quotient = 0
Comparison = 0 |
To avoid situations where unsigned preserving and value preserving
promotion rules yield different results, you could refrain from
using an unsigned char or unsigned short in an expression that is
used as an operand of one of the following operators: >>, /, %, <, <=, >,
or >=. Or remove
the ambiguity by using an explicit cast to specify the conversion
you want. If you enable ANSI migration warnings, the compiler will warn
you of situations where differences in the promotion rules might
cause different results. See Chapter 9 for information on enabling
ANSI migration warnings. |
