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。