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前言
声明一个变量，经常要考虑的问题是这个类型的变量能不能装的下。今天MilkCu就总结下吧，以解除后顾之忧。
关于变量取值范围的问题，在Kernighan的《C程序设计语言》第28页练习2-1就提到过。
编写一个程序以确定分别由signed及unsigned限定的char、short、int与long类型变量的取值范围。采用打印标准头文件中的相应值以及直接计算两种方式实现。后一种方式的实现较困难一些，因为要确定各种浮点类型的取值范围。
这书是针对C89的，随着C99及C++的广泛应用long long int类型也是一种美妙的选择。
实现方案
标准规定：各种类型的取值范围必须在头文件<limits.h>中定义。不同类型在不同的硬件上有不同的长度，所以它们在不同机器上的取值范围也往往会不同。我们可以通过头文件确定类型取值范围。
函数sizeof()
可以通过函数sizeof()用字节计算参数并返回的字节数，间接得到类型宽度，源代码如下：
# include <stdio.h>
int main(void)
{
printf("byte of short = %d\n", sizeof(short));
printf("byte of int = %d\n", sizeof(int));
printf("byte of long = %d\n", sizeof(long));
printf("byte of long long int = %d\n", sizeof(long long int));
return 0;
}
打印常量
可以通过打印头文件的方法获得每种类型的取值范围：
# include <stdio.h>
# include <limits.h>
//determine ranges of types
int main(void)
{
//signed types
printf("signed char min = %d\n", SCHAR_MIN);
printf("signed char max = %d\n", SCHAR_MAX);
printf("signed short min = %d\n", SHRT_MIN);
printf("signed short max = %d\n", SHRT_MIN);
printf("signed short min = %d\n", INT_MIN);
printf("signed long min = %ld\n", LONG_MIN);
printf("signed long max = %ld\n", LONG_MAX);
printf("signed long long int min = %lld\n", LONG_LONG_MIN);
printf("signed long long int max = %lld\n", LONG_LONG_MAX);
//unsigned types
printf("unsigned char max = %u\n", UCHAR_MAX);
printf("unsigned short max = %u\n", USHRT_MAX);
printf("unsigned int max = %u\n", UINT_MAX);
printf("unsigned long max = %lu\n", ULONG_MAX);
printf("unsigned long long int max = %llu\n", ULONG_LONG_MAX);
return 0;
}
打印结果如下：
signed char min = -128 signed char max = 127 signed short min = -32768 signed short max = -32768 signed short min = -2147483648 signed long min = -2147483648 signed long max = 2147483647 signed long long int min = -9223372036854775808 signed long long int max = 9223372036854775807 unsigned char max = 255 unsigned short max = 65535 unsigned int max = 4294967295 unsigned long max = 4294967295 unsigned long long int max = 18446744073709551615
头文件
可以从头文件<limits.h>中更详细的看到它们的宏定义，<limits.h>（来自Dev-Cpp 5.4.0 MinGW 4.7.2）如下：
/*
* limits.h
* This file has no copyright assigned and is placed in the Public Domain.
* This file is a part of the mingw-runtime package.
* No warranty is given; refer to the file DISCLAIMER within the package.
*
* Functions for manipulating paths and directories (included from io.h)
* plus functions for setting the current drive.
*
* Defines constants for the sizes of integral types.
*
* NOTE: GCC should supply a version of this header and it should be safe to
* use that version instead of this one (maybe safer).
*
*/
#ifndef _LIMITS_H_
#define _LIMITS_H_
/* All the headers include this file. */
#include <_mingw.h>
/*
* File system limits
*
* TODO: NAME_MAX and OPEN_MAX are file system limits or not? Are they the
* same as FILENAME_MAX and FOPEN_MAX from stdio.h?
* NOTE: PATH_MAX is the POSIX equivalent for Microsoft's MAX_PATH; the two
* are semantically identical, with a limit of 259 characters for the
* path name, plus one for a terminating NUL, for a total of 260.
*/
#define PATH_MAX 260
/*
* Characteristics of the char data type.
*
* TODO: Is MB_LEN_MAX correct?
*/
#define CHAR_BIT 8
#define MB_LEN_MAX 2
#define SCHAR_MIN (-128)
#define SCHAR_MAX 127
#define UCHAR_MAX 255
/* TODO: Is this safe? I think it might just be testing the preprocessor,
* not the compiler itself... */
#if ('\x80' < 0)
#define CHAR_MIN SCHAR_MIN
#define CHAR_MAX SCHAR_MAX
#else
#define CHAR_MIN 0
#define CHAR_MAX UCHAR_MAX
#endif
/*
* Maximum and minimum values for ints.
*/
#define INT_MAX 2147483647
#define INT_MIN (-INT_MAX-1)
#define UINT_MAX 0xffffffff
/*
* Maximum and minimum values for shorts.
*/
#define SHRT_MAX 32767
#define SHRT_MIN (-SHRT_MAX-1)
#define USHRT_MAX 0xffff
/*
* Maximum and minimum values for longs and unsigned longs.
*
* TODO: This is not correct for Alphas, which have 64 bit longs.
*/
#define LONG_MAX 2147483647L
#define LONG_MIN (-LONG_MAX-1)
#define ULONG_MAX 0xffffffffUL
#ifndef __STRICT_ANSI__
/* POSIX wants this. */
#define SSIZE_MAX LONG_MAX
#endif
#if (defined (__STDC_VERSION__) && __STDC_VERSION__ >= 199901L) \
|| !defined(__STRICT_ANSI__)
/* ISO C9x macro names */
#define LLONG_MAX 9223372036854775807LL
#define LLONG_MIN (-LLONG_MAX - 1)
#define ULLONG_MAX (2ULL * LLONG_MAX + 1)
#endif
/*
* The GNU C compiler also allows 'long long int'
*/
#if !defined(__STRICT_ANSI__) && defined(__GNUC__)
#define LONG_LONG_MAX 9223372036854775807LL
#define LONG_LONG_MIN (-LONG_LONG_MAX-1)
#define ULONG_LONG_MAX (2ULL * LONG_LONG_MAX + 1)
/* MSVC compatibility */
#define _I64_MIN LONG_LONG_MIN
#define _I64_MAX LONG_LONG_MAX
#define _UI64_MAX ULONG_LONG_MAX
#endif /* Not Strict ANSI and GNU C compiler */
#endif /* not _LIMITS_H_ */
阅读头文件，在注释的帮助下，我们可以更明确的看到数据类型的定义。
总结
简而言之，对于MinGW32：
int类型，能完整表示9位；
unsigned long long int类型，能完整表示19位。
后记
又一轮新生活开始了，要充实快乐每一天。
借用奥斯托洛夫斯基在《钢铁是怎样炼成》中的几句话吧：
人最宝贵的东西是生命
生命属于人只有一次
一个人的生命是应该这样度过的
当他回首往事的时候
他不会因虚度年华而悔恨
也不会因碌碌无为而羞耻
这样在临死的时候
他才能够说：“我的生命和全部的经历
都献给世界上最壮丽的事业——为人类的解放而斗争”。
（全文完）