6.4.1 Background

According to K&R, The C Programming Language (First Edition), unsigned specified exactly one type; there were no unsigned chars, unsigned shorts, or unsigned longs, but most C compilers added these very soon thereafter. Some compilers did not implement unsigned long but included the other two. Naturally, implementations chose different rules for type promotions when these new types mixed with others in expressions.

In most C compilers, the simpler rule, “unsigned preserving,” is used: 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 ISO C, is known as “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.