严格的ANSI C - 8位结构元素是否违规？

Question

I have defined a struct as follows:

struct Display7
{
    U8 a : 1;
    U8 b : 1;
    U8 c : 1;
    U8 d : 1;
    U8 e : 1;
    U8 f : 1;
    U8 g : 1;
}Display7;

If I disable Strict ANSI C check in Keil IDE, there is no errors. If enable Strict ANSI C, I get the error "non standard type for a bit field". What is causing the problem? If I make each struct element as U32,then ANSI C doesn't complain.

Answer 1

Standard is clear about it:

6.7.2.1 Structure and union specifiers : 5/ A bit-field shall have a type that is a qualified or unqualified version of _Bool , signed int , unsigned int , or some other implementation-defined type. It is implementation-defined whether atomic types are permitted.

Answer 2

在C99中，具有除signed或unsigned int或_Bool之外的类型的位字段是可选的。