310 lines
8.0 KiB
Markdown
310 lines
8.0 KiB
Markdown
## 内核的数据类型
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### 基本数据类型
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我们写了两个小的模块来测试实际数据类型和内存对齐的长度。
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内核基本数据类型
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C语言类型（int）
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char、short、int、long long在不同的平台上大小不变。
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long、ptr(指针)平台不同其大小不同，但二者的大小始终相同。
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char的符号问题：
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大多数平台上char默认是signed，但有些平台上默认是 unsigned。
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char i = -1; 大部分平台上i是-1，有些平台上是255。
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应该使用：signed char i = -1; unsigned char i = 255;
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确定大小的类型（u32）
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u8、u16、u32、u64、 s8、s16、s32、s64是Linux内核确定大小的类型。
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__u8等式linux用户态确定大小的类型。（头文件linux/types.h）
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uint8_t、uint32_t是新编译器支持的C99标准确定大小的类型，可以跨平台。
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特定内核对象的类型（pid_t）
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进程标识符使用pid_t类型，而不使用int，屏蔽了实际的数据类型中任何可能的差异。
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特定内核对象的类型，打印时，不太好选择printk或printf的输出格式：
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1. 一些平台上排除的警告，在另一平台上可能会出现（size_t在一些平台上是unsigned long，在一些平台上是unsigned int）。
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2. 将其强制转换成可能的最大类型，然后用响应的格式打印输出。
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字节序
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大端、小端
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数值0x01020304，内存从低到高依次存储：04 03 02 01 为小端。 存储顺序反过来为大端）
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数值0x00000001，内存从低到高依次存储：01 00 00 00 为小端。
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转换函数
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u32 __cpu_to_be32(u32); /* 把cpu字节序转为大端字节序 */
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u32 __be32_to_cpu(u32); /* 把大端字节序转为cpu字节序 */
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u32 __cpu_to_le32(u32); /* 把cpu字节序转为小端字节序 */
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u32 __le32_to_cpu(u32); /* 把小端字节序转为cpu字节序 */
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在头文件<linux/byteorder.h>中
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时间间隔
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使用HZ代表一秒。
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不能假定每秒就1000个jiffies。
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与msec毫秒对应的jiffies数目总是msec*HZ/1000。
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页大小
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页大小为PAGE_SIZE个字节，而不是4KB。
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分配16KB的空间临时存储数据，如下：
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以上，只是基本数据类型中最简单的一部分，绝大多数都是细节问题，要注意。
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#### 基本数据类型的长度
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#### 对齐数据类型长度
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#### types.h 中的重要数据类型
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```
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typedef __u32 __kernel_dev_t;
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typedef __kernel_fd_set fd_set;
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typedef __kernel_dev_t dev_t;
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typedef __kernel_ino_t ino_t;
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typedef __kernel_mode_t mode_t;
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typedef unsigned short umode_t;
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typedef __u32 nlink_t;
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typedef __kernel_off_t off_t;
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typedef __kernel_pid_t pid_t;
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typedef __kernel_daddr_t daddr_t;
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typedef __kernel_key_t key_t;
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typedef __kernel_suseconds_t suseconds_t;
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typedef __kernel_timer_t timer_t;
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typedef __kernel_clockid_t clockid_t;
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typedef __kernel_mqd_t mqd_t;
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typedef _Bool bool;
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typedef __kernel_uid32_t uid_t;
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typedef __kernel_gid32_t gid_t;
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typedef __kernel_uid16_t uid16_t;
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typedef __kernel_gid16_t gid16_t;
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typedef unsigned long uintptr_t;
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#ifdef CONFIG_HAVE_UID16
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/* This is defined by include/asm-{arch}/posix_types.h */
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typedef __kernel_old_uid_t old_uid_t;
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typedef __kernel_old_gid_t old_gid_t;
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#endif /* CONFIG_UID16 */
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#if defined(__GNUC__)
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typedef __kernel_loff_t loff_t;
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#endif
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/*
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* The following typedefs are also protected by individual ifdefs for
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* historical reasons:
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*/
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#ifndef _SIZE_T
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#define _SIZE_T
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typedef __kernel_size_t size_t;
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#endif
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#ifndef _SSIZE_T
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#define _SSIZE_T
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typedef __kernel_ssize_t ssize_t;
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#endif
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#ifndef _PTRDIFF_T
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#define _PTRDIFF_T
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typedef __kernel_ptrdiff_t ptrdiff_t;
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#endif
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#ifndef _TIME_T
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#define _TIME_T
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typedef __kernel_time_t time_t;
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#endif
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#ifndef _CLOCK_T
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#define _CLOCK_T
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typedef __kernel_clock_t clock_t;
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#endif
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#ifndef _CADDR_T
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#define _CADDR_T
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typedef __kernel_caddr_t caddr_t;
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#endif
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/* bsd */
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typedef unsigned char u_char;
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typedef unsigned short u_short;
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typedef unsigned int u_int;
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typedef unsigned long u_long;
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/* sysv */
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typedef unsigned char unchar;
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typedef unsigned short ushort;
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typedef unsigned int uint;
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typedef unsigned long ulong;
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#ifndef __BIT_TYPES_DEFINED__
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#define __BIT_TYPES_DEFINED__
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typedef __u8 u_int8_t;
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typedef __s8 int8_t;
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typedef __u16 u_int16_t;
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typedef __s16 int16_t;
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typedef __u32 u_int32_t;
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typedef __s32 int32_t;
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#endif /* !(__BIT_TYPES_DEFINED__) */
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typedef __u8 uint8_t;
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typedef __u16 uint16_t;
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typedef __u32 uint32_t;
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#if defined(__GNUC__)
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typedef __u64 uint64_t;
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typedef __u64 u_int64_t;
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typedef __s64 int64_t;
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#endif
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/* this is a special 64bit data type that is 8-byte aligned */
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#define aligned_u64 __u64 __attribute__((aligned(8)))
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#define aligned_be64 __be64 __attribute__((aligned(8)))
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#define aligned_le64 __le64 __attribute__((aligned(8)))
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/**
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* The type used for indexing onto a disc or disc partition.
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*
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* Linux always considers sectors to be 512 bytes long independently
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* of the devices real block size.
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*
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* blkcnt_t is the type of the inode's block count.
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*/
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#ifdef CONFIG_LBDAF
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typedef u64 sector_t;
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typedef u64 blkcnt_t;
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#else
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typedef unsigned long sector_t;
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typedef unsigned long blkcnt_t;
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#endif
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/*
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* The type of an index into the pagecache.
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*/
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#define pgoff_t unsigned long
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/*
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* A dma_addr_t can hold any valid DMA address, i.e., any address returned
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* by the DMA API.
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*
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* If the DMA API only uses 32-bit addresses, dma_addr_t need only be 32
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* bits wide. Bus addresses, e.g., PCI BARs, may be wider than 32 bits,
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* but drivers do memory-mapped I/O to ioremapped kernel virtual addresses,
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* so they don't care about the size of the actual bus addresses.
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*/
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#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
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typedef u64 dma_addr_t;
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#else
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typedef u32 dma_addr_t;
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#endif
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typedef unsigned __bitwise__ gfp_t;
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typedef unsigned __bitwise__ fmode_t;
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typedef unsigned __bitwise__ oom_flags_t;
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#ifdef CONFIG_PHYS_ADDR_T_64BIT
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typedef u64 phys_addr_t;
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#else
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typedef u32 phys_addr_t;
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#endif
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typedef phys_addr_t resource_size_t;
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/*
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* This type is the placeholder for a hardware interrupt number. It has to be
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* big enough to enclose whatever representation is used by a given platform.
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*/
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typedef unsigned long irq_hw_number_t;
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typedef struct {
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int counter;
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} atomic_t;
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#ifdef CONFIG_64BIT
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typedef struct {
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long counter;
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} atomic64_t;
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#endif
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struct list_head {
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struct list_head *next, *prev;
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};
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struct hlist_head {
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struct hlist_node *first;
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};
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struct hlist_node {
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struct hlist_node *next, **pprev;
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};
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struct ustat {
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__kernel_daddr_t f_tfree;
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__kernel_ino_t f_tinode;
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char f_fname[6];
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char f_fpack[6];
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};
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/**
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* struct callback_head - callback structure for use with RCU and task_work
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* @next: next update requests in a list
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* @func: actual update function to call after the grace period.
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*
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* The struct is aligned to size of pointer. On most architectures it happens
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* naturally due ABI requirements, but some architectures (like CRIS) have
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* weird ABI and we need to ask it explicitly.
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*
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* The alignment is required to guarantee that bits 0 and 1 of @next will be
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* clear under normal conditions -- as long as we use call_rcu(),
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* call_rcu_bh(), call_rcu_sched(), or call_srcu() to queue callback.
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*
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* This guarantee is important for few reasons:
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* - future call_rcu_lazy() will make use of lower bits in the pointer;
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* - the structure shares storage spacer in struct page with @compound_head,
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* which encode PageTail() in bit 0. The guarantee is needed to avoid
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* false-positive PageTail().
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*/
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struct callback_head {
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struct callback_head *next;
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void (*func)(struct callback_head *head);
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} __attribute__((aligned(sizeof(void *))));
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#define rcu_head callback_head
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typedef void (*rcu_callback_t)(struct rcu_head *head);
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typedef void (*call_rcu_func_t)(struct rcu_head *head, rcu_callback_t func);
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/* clocksource cycle base type */
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typedef u64 cycle_t;
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```
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#### list.h |