c，unsigned char 和 char 上的相等运算符

Question

当我将 int 与 unsigned int 进行比较时，为什么我可以得到 "x==y"

那么，为什么当我比较char和unsigned char时，我得到的是"a!=b"，虽然它们确实有相同的位模式“0xff”

在应用相等运算符时，是否考虑变量类型？

代码:

#include <stdio.h>

int main() 
{
    unsigned int x = 0xFFFFFFFF;
    int y = 0xFFFFFFFF;
    printf("unsigned int x = 0xFFFFFFFF;\n");
    printf("int y = 0xFFFFFFFF;\n");
    if (x < 0)
        printf("x < 0\n");
    else
        printf("x > 0\n");
    if (y < 0)
        printf("y < 0\n");
    else
        printf("y > 0\n");
    if(x==y)
        printf("x==y\n\n");
    ///////////-- char --////////////////////////
    unsigned char a = 0xFF;
    char b = 0xFF;    
    printf("unsigned char a = 0xFF\n");
    printf("char b = 0xFF\n");
    if (a < 0)
        printf("a < 0\n");
    else
        printf("a > 0\n");
    if (b < 0)
        printf("b < 0\n");
    else
        printf("b > 0\n");
    if(a==b)
        printf("a==b\n");   
    else
        printf("a!=b\n");

}

输出:

unsigned int x = 0xFFFFFFFF;
int y = 0xFFFFFFFF;
x > 0
y < 0
x==y

unsigned char a = 0xFF
char b = 0xFF
a > 0
b < 0
a!=b

Answer 1

来自 C11 ISO/IEC 9899:201x 标准:

Otherwise, if the operand that has unsigned integer type has rank greater or equal to the rank of the type of the other operand, then the operand with signed integer type is converted to the type of the operand with unsigned integer type.

在与 unsigned int x = 0xFFFFFFFF 比较之前，提升应用于 int y = 0xFFFFFFFF。将 int y 提升为 unsigned int 将保留值 0xFFFFFFFF，这会导致 x == y。

另一方面:

If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int ; otherwise, it is converted to an unsigned int . These are called the integer promotions . All other types are unchanged by the integer promotions. The integer promotions preserve value including sign. As discussed earlier, whether a ‘‘plain’’ char is treated as signed is implementation-defined.

这意味着 unsigned char a = 0xFF 和 char b = 0xFF 在比较之前都被转换为 signed int。但是转换 b 会导致符号扩展，这意味着 b 的值被扩展为 0xFFFFFFFF == -1 导致 int a = 255 大于 int b = -1。