Global Behavior: Value versus unsigned Preserving

A program that depends on unsigned preserving arithmetic conversions behaves differently. This is considered to be the most serious change made by ANSI/ISO C.

In the first edition of K&R, The C Programming Language (Prentice-Hall, 1978), unsigned specified exactly one type; there were no unsigned chars, unsigned shorts, or unsigned longs, but most C compilers added these very soon thereafter.

In previous C compilers, the unsigned preserving rule is used for promotions: when an unsigned type needs to be widened, it is widened to an unsigned type; when an unsigned type mixes with a signed type, the result is an unsigned type.

The other rule, specified by ANSI/ISO C, came to be called "value preserving," in which the result type depends on the relative sizes of the operand types. When an unsigned char or unsigned short is widened, the result type is int if an int is large enough to represent all the values of the smaller type. Otherwise, the result type is unsigned int. The value preserving rule produces the least surprise arithmetic result for most expressions.

Only in the -Xt and -Xs modes does the compiler use the unsigned preserving promotions; in the other modes, -Xc and -Xa, the value preserving promotion rules are used. When the -xtransition option is used, the compiler warns about each expression whose behavior might depend on the promotion rules used.