从“long long unsigned int”转换到“long long unsigned int:40”从“0xFFFFFFFFFFFFFFFFFF”更改值到“0xFFFFFFFFFF” [-Werror=溢出]

Question

我有这个示例代码，当我尝试修复 GCC 警告之一时，它会抛出错误

#include <stdint.h>

//
typedef union someStruct
{
   uint64_t All;
   struct
   {
      uint64_t Foo  : 40;
      uint64_t Bar  : 24;
   } Field;
} someStruct;

#define bits_64 ((uint64_t)(-1))

//
typedef union bits
{
   uint64_t oneBit: 1;
   uint64_t twoBits: 2;
   uint64_t threeBits: 3;
   uint64_t fourBits: 4;
   uint64_t fiveBits: 5;
   uint64_t sixBits: 6;
   uint64_t sevenBits: 7;
   uint64_t fourtyBits: 40;
   uint64_t All;
} bits;

#define bits_40 (((bits)(-1)).fourtyBits)

//
int main()
{
    someStruct x;
    someStruct y;

    x.Field.Foo = bits_64; //-Woverflow warning

    //trying to fix the warning with using the bits union
    y.Field.Foo = bits_40; // but this throws the error msg below

    /*
        <source>:30:19: error: cast to union type from type not present in union
        30 | #define bits_40 (((bits)(-1)).fourtyBits)
           |                   ^
    */

    return 0;
}

如何使用联合来定义任意数量的位并将其分配给任何结构字段？

PS 我不能使用枚举和/或定义联合变量；我必须以这种方式使用宏来适应代码库。

Answer 1

bits_40 的 #define 应该如下所示：

#define bits_40 (((bits){.All = -1)).fourtyBits)

您也可以这样做：

#define bits_40 ((1ULL << 40) - 1)

并完全跳过 bits 结构。或者您可以定义一个 BIT_MASK 宏，如下所示：

#define BIT_MASK(bits) ((1uLL << bits) - 1)
    :
    :
x.Field.Foo = BIT_MASK(40);