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

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）

[cpp] view
plaincopy

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。