C语言中的bool类型
C99之前没有定义bool型，在C99中在stdbool.h中定义了_Bool为bool，当然true 和 false也没有定义。但是C++中是有bool型的，并且true 和 false也是有定义的。
所以如果是C99之前的标准，需要我们自己定义bool，如
typedef enum {false = 0, true = 1} bool;
/* Copyright (C) 1998, 1999, 2000 Free Software Foundation, Inc.
This file is part of GCC.
GCC is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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GCC is distributed in the hope that it will be useful,
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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the Free Software Foundation, 51 Franklin Street, Fifth Floor,
Boston, MA 02110-1301, USA. */
/* As a special exception, if you include this header file into source
files compiled by GCC, this header file does not by itself cause
the resulting executable to be covered by the GNU General Public
License. This exception does not however invalidate any other
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/*
* ISO C Standard: 7.16 Boolean type and values <stdbool.h>
*/
#ifndef _STDBOOL_H
#define _STDBOOL_H
#ifndef __cplusplus
#define bool _Bool
#define true 1
#define false 0
#else /* __cplusplus */
/* Supporting <stdbool.h> in C++ is a GCC extension. */
#define _Bool bool
#define bool bool
#define false false
#define true true
#endif /* __cplusplus */
/* Signal that all the definitions are present. */
#define __bool_true_false_are_defined 1
#endif /* stdbool.h */
1.FALSE/TRUE与false/true的区别：
false/true是标准C++语言里新增的关键字，而FALSE/TRUE是通过#define，这要用途
是解决程序在C与C++中环境的差异,以下是FALSE/TRUE在windef.h的定义：
#ifndef FALSE
#define FALSE 0
#endif
#ifndef TRUE
#define TRUE 1
#endif
也就是说FALSE/TRUE是int类型，而false/true是bool类型；所以两者不一样的，只不过
我们在使用中没有这种感觉，因为C++会帮你做隐式转换。
2.bool的大小与BOOL的区别：
bool在C++里是占用1字节,而BOOL是int类型，int类型的大小是视具体环境而定的；所以来说：
false/true只占用1个字节,而TRUE/FALSE视具体环境而言，以下是BOOL在windef .h中的定义：typedef int BOOL;
3.NULL与0的区别：
还是让我们看一下windef.h中NULL的定义：
#ifndef NULL
#ifdef __cplusplus//这个是指示是用C++来编译程序
#define NULL 0
#else
#define NULL ((void *)0)
#endif
#endif
所以说：它们没有区别，只不过在C里面会做一个强制类型转换。