HP C A.06.05 Reference Manual

Type Conversions
Integral Promotions
Chapter 476
Integral Promotions
Wherever an int or an unsigned int may be used in an expression, a narrower integral type
may also be used. The narrow type will generally be widened by means of a conversion called
an integral promotion. All ANSI C compilers follow what are called value preserving rules for
the conversion. In HP C the value preserving integral promotion takes place as follows: a
char, a short int, a bit-field, or their signed or unsigned varieties, are widened to an int; all
other arithmetic types are unchanged by the integral promotion.
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