5

我正在尝试检测整数类型族（char, unsigned char, short, unsigned short, int, ...）的值是否是 C 中的负数。如果可能的话，可以使用可以使用任何兼容的 C 编译器编译的宏（因此，不允许gcc使用 -tricks ) 并且没有警告！

过了一段时间，我带来了以下内容：

#define ISNEG(X) ((X) && (X-1) && ((X <= 0) && (~X >= 0)))

我尝试了以下示例：

void
display_result(int arg, int result)
{
  printf("ISNEG(%d) is %stive\n", arg, (result ? "nega" : "posi"));
}

void
display_uresult(unsigned int arg, int result)
{
  printf("ISNEG(%u) is %stive\n", arg, (result ? "nega" : "posi"));
}

int main ()
{
  short shrt =  5;
  short nshrt = -5;
  unsigned short ushrt = 5;

  display_result(shrt, ISNEG(shrt));
  display_result(nshrt, ISNEG(nshrt));
  display_uresult(ushrt, ISNEG(ushrt));

  int ni = -5;
  int i = 5;
  int zero = 0;

  display_result(ni, ISNEG(ni));
  display_result(i, ISNEG(i));
  display_result(zero, ISNEG(zero));
  display_result(~zero, ISNEG(~zero));  // wrong

  unsigned int uzero = 0;
  unsigned int ui = 5;

  display_uresult(uzero, ISNEG(uzero));
  display_uresult(~uzero, ISNEG(~uzero));
  display_uresult(ui, ISNEG(ui));

  long int li = -5;
  unsigned long int uli = 5;

  display_result(li, ISNEG(li));
  display_uresult(uli, ISNEG(uli));

  long long int lli = -5;
  unsigned long long int ulli = 5;

  display_result(lli, ISNEG(lli));
  display_uresult(ulli, ISNEG(ulli));

  return EXIT_SUCCESS;
}

而且，结果是：

ISNEG(5) is positive
ISNEG(-5) is negative
ISNEG(5) is positive
ISNEG(-5) is negative
ISNEG(5) is positive
ISNEG(0) is positive
ISNEG(-1) is negative
ISNEG(0) is positive
ISNEG(4294967295) is positive
ISNEG(5) is positive
ISNEG(-5) is negative
ISNEG(5) is positive
ISNEG(-5) is negative
ISNEG(5) is positive

它工作得很好，但问题是，当使用所有警告 ( -Wall -Wextra) 编译时，我收到以下消息：

signedness.c: In function ‘main’:
signedness.c:27:3: warning: promoted ~unsigned is always non-zero [-Wsign-compare]
   display_uresult(ushrt, ISNEG(ushrt));
   ^
signedness.c:4:49: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
 #define ISNEG(X) ((X) && (X-1) && ((X <= 0) && (~X >= 0)))
                                                 ^
signedness.c:41:26: note: in expansion of macro ‘ISNEG’
   display_uresult(uzero, ISNEG(uzero));
                          ^
signedness.c:4:49: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
 #define ISNEG(X) ((X) && (X-1) && ((X <= 0) && (~X >= 0)))
                                                 ^
signedness.c:42:27: note: in expansion of macro ‘ISNEG’
   display_uresult(~uzero, ISNEG(~uzero));
                           ^
signedness.c:4:49: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
 #define ISNEG(X) ((X) && (X-1) && ((X <= 0) && (~X >= 0)))
                                                 ^
signedness.c:43:23: note: in expansion of macro ‘ISNEG’
   display_uresult(ui, ISNEG(ui));
                       ^
signedness.c:4:49: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
 #define ISNEG(X) ((X) && (X-1) && ((X <= 0) && (~X >= 0)))
                                                 ^
signedness.c:49:24: note: in expansion of macro ‘ISNEG’
    display_uresult(uli, ISNEG(uli));
                         ^
signedness.c:4:49: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
 #define ISNEG(X) ((X) && (X-1) && ((X <= 0) && (~X >= 0)))
                                             ^
signedness.c:55:25: note: in expansion of macro ‘ISNEG’
    display_uresult(ulli, ISNEG(ulli));
                          ^

所以，我的问题是：

  1. 有没有更好的方法来检测我们在所有可能的 C 语言整数类型中都有一个负变量？

  2. 如何在不停用它（并且不使用 GCC 技巧）的情况下摆脱所有这些警告？

4

2 回答 2

5

这个定义似乎对我有用，不会产生任何警告：

#define ISNEG(X) (!((X) > 0) && ((X) != 0))

我在IDEONE 上将您的测试用例与此宏一起使用。

如果您使用的系统支持 C.11 的_Generic选择功能，那么您可以执行以下操作（这有点简单，我相信可以通过利用积分提升规则来简化它）：

#define ISNEG(X) \
    _Generic((X), \                 
             char: !((X) > 0) && (X) != 0, \
             signed char: (X) < 0, \
             short: (X) < 0, \
             int: (X) < 0, \
             long: (X) < 0, \
             long long: (X) < 0, \
             float: (X) < 0, \
             double: (X) < 0, \
             long double: (X) < 0, \
             default: 0)
于 2013-08-19T23:37:28.253 回答
0

对我来说，这很好用：

定义 ISNEG(X) ( (X) < ( (int)0 ) )

有了这个结果（http://ideone.com/9SIoHQ）

ISNEG(5) is positive
ISNEG(-5) is negative
ISNEG(5) is positive
ISNEG(-5) is negative
ISNEG(5) is positive
ISNEG(0) is positive
ISNEG(-1) is negative
ISNEG(0) is positive
ISNEG(4294967295) is positive
ISNEG(5) is positive
ISNEG(-5) is negative
ISNEG(5) is positive
ISNEG(-5) is negative
ISNEG(5) is positive
于 2016-04-21T14:05:23.217 回答