The compiler and library now agree on a single definition of bool and _Bool in all modes; its format is equivalent to "unsigned char." Unfortunately, this represents a backward incompatibility with older object files which match all of these conditions: - C source code - includes stdbool.h - compiled in strict C89 mode (the default in older compilers) - module interface uses type _Bool or bool (i.e. a global variable,   function argument, or function return value of type derived from   bool, or struct containing a type derived from bool.) To work around the problem, either recompile with the latest version of the compiler, or ensure that you aren"t using any _Bool or bool objects in the module interface.