C99 added type _Bool (also called bool if you #include <stdbool.h>) -- but it doesn't actually use it much. Operators that yield logically "boolean" values (<,

C99 added type _Bool (also called bool if you #include <stdbool.h>) -- but it doesn't actually use it much. Operators that yield logically "boolean" values (<, <=, >, >=, ==, !=, &&, ||, !) yield values of type int, not _Bool. The value is always 0 or 1 -- in contrast with isdigit(), for example, which is specified to return 0 for false and some unspecified non-zero value for true. Converting any scalar value (that includes pointers) to _Bool yields 0 or 1 (false or true).