C 标准中 switch 语句的用语
考虑以下代码示例：
char c = 0xff;
char mask = 0xfe;
switch ((unsigned)(c & mask)) {
case -2: /* do task 1 */ break;
default: /* do task 2 */
}
让我们假设 CHAR_BIT = 8，并且实现定义的对 c 和掩码的分配是通过位模式的解释：11111111 和 11111110，并且允许负零。因此，此代码的行为是：
如果 char 有符号且实现使用 2 的补码，则 c = -1, mask = -2, c &掩码 = -2，（无符号）（c 和掩码）= UINT_MAX - 1。
如果 char 有符号且实现使用 1 的补码，则 c = 0, mask = -1, c &掩码 = 0，（无符号）（c 和掩码）= 0。 c 为零且不是负零，因为 C 不允许通过赋值创建负零。
如果 char 有符号且实现使用有符号大小，则 c = -127，掩码 = -126，c & mask = -126, (unsigned)(c & mask) = UINT_MAX - 125
如果 char 是无符号的 c = 255, mask = 254, c & mask = 254, (unsigned)(c & mask) = 254
case 常量 -2 转换为与控制表达式相同的类型，因此值为 UINT_MAX - 1。因此，仅当 char 有符号且实现使用 2 的补码时，它才会匹配。
根据 C 标准，这是否正确，或者是否需要添加其他假设？
Consider this code example:
char c = 0xff;
char mask = 0xfe;
switch ((unsigned)(c & mask)) {
case -2: /* do task 1 */ break;
default: /* do task 2 */
}
Let us assume that CHAR_BIT = 8 and the implementation defined assignment to c and mask is via interpretation of the bit patterns: 11111111 and 11111110, and negative zeros are allowed. The behaviour of this code is therefore:
if char is signed and implementation uses 2's complement, c = -1, mask = -2, c & mask = -2, (unsigned)(c & mask) = UINT_MAX - 1.
if char is signed and implementation uses 1's complement, c = 0, mask = -1, c & mask = 0, (unsigned)(c & mask) = 0.c is zero and not negative zero because C does not allow creation of negative zero via an assignment.
if char is signed and implementation uses signed magnitude, c = -127, mask = -126, c & mask = -126, (unsigned)(c & mask) = UINT_MAX - 125
if char is unsigned c = 255, mask = 254, c & mask = 254, (unsigned)(c & mask) = 254
The case constant -2 is converted to the same type as the controlling expression, so the value is UINT_MAX - 1. Hence it would only match if char is signed and implementation uses 2's complement.
Is this correct according to the C standard or are there additional assumptions that needs to be added?
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不正确。如果
char有符号且为 8 位（即CHAR_MAX小于 255），则该行是实现定义的。它可能会按照你说的做，但也可能不会。
C标准6.3.1.3：
Not really. If
charis signed and 8-bit (ie.CHAR_MAXis less than 255), then the lineis implementation-defined. It might do what you say, but it might not.
The C standard 6.3.1.3: