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

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]
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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。