u32 __u32 - 内核源码 - Linux论坛

最新推荐文章于 2023-04-24 19:59:17 发布

原创最新推荐文章于 2023-04-24 19:59:17 发布 · 7.1k 阅读

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文章标签：

#linux #types #alignment #underscore #exchange #network

本文探讨了Linux内核中为何使用__u32和u32代替unsigned int等标准类型。其目的是确保数据类型的位数在不同硬件、系统和编译器环境下保持一致，以实现平台无关性。通过这种方式，可以确保变量如a的值始终为4。

导读：

看到内核中有的 .h 文件中有下面的定义

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CODE:

typedef unsigned int __u32;
typedef unsigned int u32;

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？

QUOTE:

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 , which in turn is included by :

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 ). 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)。

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CODE:

#if PLATFORM_INT_SIZE!=32
#if PLATFORM_LONG_SIZE==32
typedef long u32;
#elif PLATFORM_LONG_LONG_SIZE==32
typedef long long u32;
#endif
#else
typedef int u32;
#endif

int a=sizeof(u32);

这样可以保证a的值肯定是4...

本文转自
http://linux.chinaunix.net/bbs/viewthread.php?tid=672196