c - 普通 `int` 位域的符号

Question

6.7.2/5 中的 C 标准说:

... it is implementation-defined whether the specifier int designates the same type as signed int or the same type as unsigned int.

x86-64 ABI 3.1.2中规定:

Bit-fields that are neither signed nor unsigned always have non-negative values. Although they may have type char, short, int, or long (which can have negative values), these bit-fields have the same range as a bit-field of the same size with the corresponding unsigned type.

因此，在下面的程序中，我希望断言能够通过(至少在 Linux 上)。

#include <assert.h>

struct S {
  int x : 3;
};

struct S s;
int main() {
  s.x = 6;
  assert(s.x == 6);
}

但它在 Clang 和 Gcc 中都失败了。为什么？

此外，Clang 会发出警告:

warning: implicit truncation from 'int' to bit-field changes value from 6 to -2

因此它的行为就好像位域被声明为signed int。如果我明确地设置它unsigned int，则警告消失并且断言通过。

Answer 1

此行为记录在 GCC manual 中:

Whether a “plain” int bit-field is treated as a signed int bit-field or as an unsigned int bit-field (C90 6.5.2, C90 6.5.2.1, C99 and C11 6.7.2, C99 and C11 6.7.2.1).

默认情况下，它被视为有符号整数，但这可以通过 -funsigned-bitfields 选项更改。