u32 __u32

platinum

看到内核中有的 .h 文件中有下面的定义

1. 
   
2. typedef unsigned int __u32;
   
3. typedef unsigned int u32;
   

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我不明白
1、程序为什么有的用 u32，而有的却用 __u32 呢？
2、为什么不直接 unsigned int xxx 或者 unsigned char xxx，而用 u32 u8 定义变量呢？

[ 本帖最后由 platinum 于 2005-12-20 11:13 编辑 ]

zalem

引自LDD 第10章

Assigning an Explicit Size to Data Items

Sometimes kernel code requires data items of a specific size, either to match predefined binary structures[39] or to align data within structures by inserting "filler'' fields (but please refer to "Data Alignment" later in this chapter for information about alignment issues).

    [39]This happens when reading partition tables, when executing a binary file, or when decoding a network packet.

The kernel offers the following data types to use whenever you need to know the size of your data. All the types are declared in <asm/types.h>, which in turn is included by <linux/types.h>:

    u8;   /* unsigned byte (8 bits) */
    u16;  /* unsigned word (16 bits) */
    u32;  /* unsigned 32-bit value */
    u64;  /* unsigned 64-bit value */

These data types are accessible only from kernel code (i.e., _ _KERNEL_ _ must be defined before including <linux/types.h>). The corresponding signed types exist, but are rarely needed; just replace u with s in the name if you need them.

If a user-space program needs to use these types, it can prefix the names with a double underscore: _ _u8 and the other types are defined independent of _ _KERNEL_ _. If, for example, a driver needs to exchange binary structures with a program running in user space by means of ioctl, the header files should declare 32-bit fields in the structures as _ _u32.


It's important to remember that these types are Linux specific, and using them hinders porting software to other Unix flavors. Systems with recent compilers will support the C99-standard types, such as uint8_t and uint32_t; when possible, those types should be used in favor of the Linux-specific variety. If your code must work with 2.0 kernels, however, use of these types will not be possible (since only older compilers work with 2.0).

基本就是说为了一些需要精确位数的数据类型独立于平台(包括硬件，系统和编译器)。

1. 
   
2. #if PLATFORM_INT_SIZE!=32
   
3.     #if PLATFORM_LONG_SIZE==32
   
4.         typedef long u32;
   
5.     #elif PLATFORM_LONG_LONG_SIZE==32
   
6.          typedef long long u32;
   
7.     #endif
   
8. #else
   
9.    typedef int u32;
   
10. #endif
    
11. 
    
12.    int a=sizeof(u32);
    

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这样可以保证a的值肯定是4...

[ 本帖最后由 zalem 于 2005-12-19 22:41 编辑 ]

platinum

谢谢 zalem，分析得很透彻，我终于明白原来上大学时老师说的“一个 WORD 一般情况下是 16 bit，至少在 PC 机中是这样”的意思了

那么，为什么不用 unsigned int 而用 u16/u32 定义呢？是不是因为网络编程的环境比较特殊，必须严格按多少 bit 算？
还有，为什么有的程序里面用 u32，有的是 __u32 呢？二者难道不一样吗？为何要单独分开呢？

圆点坐标

u32给用户用的，__u32在内核用的，不知道对不对？

xwzss

原帖由 圆点坐标 于 2005-12-20 09:27 发表
u32给用户用的，__u32在内核用的，不知道对不对？

恰恰相反，你仔细看看zalem的回贴。

platinum

我明白了，谢谢大家 ^_^