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c/c++: uint8_t uint16_t uint32_t uint64_t size_t ssize_t数据类型

2011-12-13 14:26 363 查看
原文写的不错,转来收藏,转自:http://wangyisouhuxin.blog.163.com/blog/static/761966592011072348700/?fromdm&fromSearch&isFromSearchEngine=yes

在nesc的代码中,你会看到很多你不认识的数据类型,比如uint8_t等。咋一看,好像是个新的数据类型,不过C语言(nesc是C的扩展)里面好像没有这种数据类型啊!怎么又是u又是_t的?很多人有这样的疑问。论坛上就有人问:以*_t结尾的类型是不是都是long型的?在baidu上查一下,才找到答案,这时才发觉原来自己对C掌握的太少。
那么_t的意思到底表示什么?具体的官方答案没有找到,不过我觉得有个答案比较接近。它就是一个结构的标注,可以理解为type/typedef的缩写,表示它是通过typedef定义的,而不是其它数据类型。
uint8_t,uint16_t,uint32_t等都不是什么新的数据类型,它们只是使用typedef给类型起的别名,新瓶装老酒的把戏。不过,不要小看了typedef,它对于你代码的维护会有很好的作用。比如C中没有bool,于是在一个软件中,一些程序员使用int,一些程序员使用short,会比较混乱,最好就是用一个typedef来定义,如:

typedef char bool;

一般来说,一个C的工程中一定要做一些这方面的工作,因为你会涉及到跨平台,不同的平台会有不同的字长,所以利用预编译和typedef可以让你最有效的维护你的代码。为了用户的方便,C99标准的C语言硬件为我们定义了这些类型,我们放心使用就可以了。
按照posix标准,一般整形对应的*_t类型为:

1字节 uint8_t

2字节 uint16_t

4字节 uint32_t

8字节 uint64_t
附:inttypes.h的内容(不同的服务器会有不同的源文件结构,但原理是一样的,我这里sun服务器inttypes.h引用了int_type.h)
bash-3.00$ vi int_types.h
"int_types.h" [Read only] 176 lines, 4367 characters
/*
* Copyright 2004 Sun Microsystems, Inc.  All rights reserved.
* Use is subject to license terms.
*/

#ifndef _SYS_INT_TYPES_H
#define _SYS_INT_TYPES_H

#pragma ident   "@(#)int_types.h        1.10    04/09/28 SMI"

/*
* This file, <sys/int_types.h>, is part of the Sun Microsystems implementation
* of <inttypes.h> defined in the ISO C standard, ISO/IEC 9899:1999
* Programming language - C.
*
* Programs/Modules should not directly include this file.  Access to the
* types defined in this file should be through the inclusion of one of the
* following files:
*
*      <sys/types.h>           Provides only the "_t" types defined in this
*                              file which is a subset of the contents of
*                              <inttypes.h>.  (This can be appropriate for
*                              all programs/modules except those claiming
*                              ANSI-C conformance.)
*
*      <sys/inttypes.h>        Provides the Kernel and Driver appropriate
*                              components of <inttypes.h>.
*
*      <inttypes.h>            For use by applications.
*
* See these files for more details.
*/

#include <sys/feature_tests.h>

#ifdef __cplusplus
extern "C" {
#endif

/*
* Basic / Extended integer types
*
* The following defines the basic fixed-size integer types.
*
* Implementations are free to typedef them to Standard C integer types or
* extensions that they support. If an implementation does not support one
* of the particular integer data types below, then it should not define the
* typedefs and macros corresponding to that data type.  Note that int8_t
* is not defined in -Xs mode on ISAs for which the ABI specifies "char"
* as an unsigned entity because there is no way to define an eight bit
* signed integral.
*/
#if defined(_CHAR_IS_SIGNED)
typedef char                    int8_t;
#else
#if defined(__STDC__)
typedef signed char             int8_t;
#endif
#endif
typedef short                   int16_t;
typedef int                     int32_t;
#ifdef  _LP64
#define _INT64_TYPE
typedef long                    int64_t;
#else   /* _ILP32 */
#if defined(_LONGLONG_TYPE)
#define _INT64_TYPE
typedef long long               int64_t;
#endif
#endif

typedef unsigned char           uint8_t;
typedef unsigned short          uint16_t;
typedef unsigned int            uint32_t;
#ifdef  _LP64
typedef unsigned long           uint64_t;
#else   /* _ILP32 */
#if defined(_LONGLONG_TYPE)
typedef unsigned long long      uint64_t;
#endif
#endif

/*
* intmax_t and uintmax_t are to be the longest (in number of bits) signed
* and unsigned integer types supported by the implementation.
*/
#if defined(_INT64_TYPE)
typedef int64_t                 intmax_t;
typedef uint64_t                uintmax_t;
#else
typedef int32_t                 intmax_t;
typedef uint32_t                uintmax_t;
#endif

/*
* intptr_t and uintptr_t are signed and unsigned integer types large enough
* to hold any data pointer; that is, data pointers can be assigned into or
* from these integer types without losing precision.
*/
#if defined(_LP64) || defined(_I32LPx)
typedef long                    intptr_t;
typedef unsigned long           uintptr_t;
#else
typedef int                     intptr_t;
typedef unsigned int            uintptr_t;
#endif

/*
* The following define the fastest integer types that can hold the
* specified number of bits.
*/
#if defined(_CHAR_IS_SIGNED)
typedef char                    int_fast8_t;
#else
#if defined(__STDC__)
typedef signed char             int_fast8_t;
#endif
#endif
typedef int                     int_fast16_t;
typedef int                     int_fast32_t;
#ifdef  _LP64
typedef long                    int_fast64_t;
#else   /* _ILP32 */
#if defined(_LONGLONG_TYPE)
typedef long long               int_fast64_t;
#endif
#endif

typedef unsigned char           uint_fast8_t;
typedef unsigned int            uint_fast16_t;
typedef unsigned int            uint_fast32_t;
#ifdef  _LP64
typedef unsigned long           uint_fast64_t;
#else   /* _ILP32 */
#if defined(_LONGLONG_TYPE)
typedef unsigned long long      uint_fast64_t;
#endif
#endif

/*
* The following define the smallest integer types that can hold the
* specified number of bits.
*/
#if defined(_CHAR_IS_SIGNED)
typedef char                    int_least8_t;
#else
#if defined(__STDC__)
typedef signed char             int_least8_t;
#endif
#endif
typedef short                   int_least16_t;
typedef int                     int_least32_t;
#ifdef  _LP64
typedef long                    int_least64_t;
#else   /* _ILP32 */
#if defined(_LONGLONG_TYPE)
typedef long long               int_least64_t;
#endif
#endif

typedef unsigned char           uint_least8_t;
typedef unsigned short          uint_least16_t;
typedef unsigned int            uint_least32_t;
#ifdef  _LP64
typedef unsigned long           uint_least64_t;
#else   /* _ILP32 */
#if defined(_LONGLONG_TYPE)
typedef unsigned long long      uint_least64_t;
#endif
#endif

#ifdef __cplusplus
}
#endif

#endif /* _SYS_INT_TYPES_H */

同理在types.h文件中会有size_t ssize_t的定义,篇幅较长不贴出了。
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