c - stdint.h中的{U，} INTn_C宏有什么意义？

何时真正需要这些宏？

我的系统(gcc/glibc/linux/x86_64)stdint.h使用以下(__ -prefixed)变体来定义:

# define INT64_MIN      (-__INT64_C(9223372036854775807)-1)
# define INT64_MAX      (__INT64_C(9223372036854775807))
# define UINT64_MAX     (__UINT64_C(18446744073709551615))
# define INT_LEAST64_MIN    (-__INT64_C(9223372036854775807)-1)
# define INT_LEAST64_MAX    (__INT64_C(9223372036854775807))
# define UINT_LEAST64_MAX   (__UINT64_C(18446744073709551615))
# define INT_FAST64_MIN     (-__INT64_C(9223372036854775807)-1)
# define INT_FAST64_MAX     (__INT64_C(9223372036854775807))
# define UINT_FAST64_MAX    (__UINT64_C(18446744073709551615))
# define INTMAX_MIN     (-__INT64_C(9223372036854775807)-1)
# define INTMAX_MAX     (__INT64_C(9223372036854775807))
# define UINTMAX_MAX        (__UINT64_C(18446744073709551615))

然而，对于limits.h，它似乎可以解决:

#   define LONG_MAX 9223372036854775807L
#   define ULONG_MAX    18446744073709551615UL

为什么stdint.h不能忘记_C宏而只做:

#   define INT_LEAST64_MAX  9223372036854775807 //let it grow as needed
#   define UINT_LEAST64_MAX 18446744073709551615U //just the U

这些宏的用例是什么？

我唯一能想到的就是我想要一个足够宽的常量可以在cpp条件中使用，而同时我又不想让它太宽:

//C guarantees longs are at least 32 bits wide
#define THREE_GIGS_BUT_MAYBE_TOO_WIDE (1L<<30)
#define THREE_GIGS (INT32_C(1)<<30) //possibly narrower than the define above

最佳答案

它们确保最小的字体宽度和符号常数。

它们“扩展为与(u)int_leastN_t.类型相对应的整数常量表达式”

123 << 50                // likely int overflow (UB)
INT32_C(123) << 50       // likely int overflow (UB)
INT64_C(123) << 50       // well defined.
INT32_C(123)*2000000000  // likely int overflow (UB)
UINT32_C(123)*2000000000 // well defined - even though it may mathematically overflow

在定义computed constants时很有用。

// well defined, but the wrong product when unsigned is 32-bit
#define TBYTE (1024u*1024*1024*1024)

// well defined, and specified to be 1099511627776u
#define TBYTE (UINT64_C(1024)*1024*1024*1024)

它还通过_Generic影响代码。下面可以将代码引导至unsigned long，unsigned和unsigned long long。

(unsigned long) 123
UINT32_C(123)
UINT64_C(123)

关于c - stdint.h中的{U，} INTn_C宏有什么意义？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/53243019/