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）

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。