由<limits.h>看整型范围

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。

方案一

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;
}

方案二

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;
}

方案三

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。

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）