## 内核的数据类型

### 基本数据类型

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。

内核基本数据类型



C语言类型（int）

        char、short、int、long long在不同的平台上大小不变。

        long、ptr(指针)平台不同其大小不同，但二者的大小始终相同。

        char的符号问题：

                大多数平台上char默认是signed，但有些平台上默认是 unsigned。
                char i = -1; 大部分平台上i是-1，有些平台上是255。
                应该使用：signed char i = -1;   unsigned char i = 255;
确定大小的类型（u32）

        u8、u16、u32、u64、 s8、s16、s32、s64是Linux内核确定大小的类型。
        __u8等式linux用户态确定大小的类型。（头文件linux/types.h）

        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。
特定内核对象的类型（pid_t）

        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。
        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：

        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）。

        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。
字节序

大端、小端

        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）

        数值0x00000001，内存从低到高依次存储：01 00 00 00 为小端。

转换函数 

        u32 __cpu_to_be32(u32);    /* 把cpu字节序转为大端字节序 */

        u32 __be32_to_cpu(u32);    /* 把大端字节序转为cpu字节序 */
        u32 __cpu_to_le32(u32);      /* 把cpu字节序转为小端字节序 */
        u32 __le32_to_cpu(u32);      /* 把小端字节序转为cpu字节序 */
        在头文件<linux/byteorder.h>中


时间间隔

        使用HZ代表一秒。

        不能假定每秒就1000个jiffies。

        与msec毫秒对应的jiffies数目总是msec*HZ/1000。

页大小

        页大小为PAGE_SIZE个字节，而不是4KB。

        分配16KB的空间临时存储数据，如下：


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。

#### 基本数据类型的长度

![df](./image/ssd.png)


#### 对齐数据类型长度

![fs](./image/dfd.png)


####  types.h 中的重要数据类型

```
typedef __u32 __kernel_dev_t;

typedef __kernel_fd_set		fd_set;
typedef __kernel_dev_t		dev_t;
typedef __kernel_ino_t		ino_t;
typedef __kernel_mode_t		mode_t;
typedef unsigned short		umode_t;
typedef __u32			nlink_t;
typedef __kernel_off_t		off_t;
typedef __kernel_pid_t		pid_t;
typedef __kernel_daddr_t	daddr_t;
typedef __kernel_key_t		key_t;
typedef __kernel_suseconds_t	suseconds_t;
typedef __kernel_timer_t	timer_t;
typedef __kernel_clockid_t	clockid_t;
typedef __kernel_mqd_t		mqd_t;

typedef _Bool			bool;

typedef __kernel_uid32_t	uid_t;
typedef __kernel_gid32_t	gid_t;
typedef __kernel_uid16_t        uid16_t;
typedef __kernel_gid16_t        gid16_t;

typedef unsigned long		uintptr_t;

#ifdef CONFIG_HAVE_UID16
/* This is defined by include/asm-{arch}/posix_types.h */
typedef __kernel_old_uid_t	old_uid_t;
typedef __kernel_old_gid_t	old_gid_t;
#endif /* CONFIG_UID16 */

#if defined(__GNUC__)
typedef __kernel_loff_t		loff_t;
#endif

/*
 * The following typedefs are also protected by individual ifdefs for
 * historical reasons:
 */
#ifndef _SIZE_T
#define _SIZE_T
typedef __kernel_size_t		size_t;
#endif

#ifndef _SSIZE_T
#define _SSIZE_T
typedef __kernel_ssize_t	ssize_t;
#endif

#ifndef _PTRDIFF_T
#define _PTRDIFF_T
typedef __kernel_ptrdiff_t	ptrdiff_t;
#endif

#ifndef _TIME_T
#define _TIME_T
typedef __kernel_time_t		time_t;
#endif

#ifndef _CLOCK_T
#define _CLOCK_T
typedef __kernel_clock_t	clock_t;
#endif

#ifndef _CADDR_T
#define _CADDR_T
typedef __kernel_caddr_t	caddr_t;
#endif

/* bsd */
typedef unsigned char		u_char;
typedef unsigned short		u_short;
typedef unsigned int		u_int;
typedef unsigned long		u_long;

/* sysv */
typedef unsigned char		unchar;
typedef unsigned short		ushort;
typedef unsigned int		uint;
typedef unsigned long		ulong;

#ifndef __BIT_TYPES_DEFINED__
#define __BIT_TYPES_DEFINED__

typedef		__u8		u_int8_t;
typedef		__s8		int8_t;
typedef		__u16		u_int16_t;
typedef		__s16		int16_t;
typedef		__u32		u_int32_t;
typedef		__s32		int32_t;

#endif /* !(__BIT_TYPES_DEFINED__) */

typedef		__u8		uint8_t;
typedef		__u16		uint16_t;
typedef		__u32		uint32_t;

#if defined(__GNUC__)
typedef		__u64		uint64_t;
typedef		__u64		u_int64_t;
typedef		__s64		int64_t;
#endif

/* this is a special 64bit data type that is 8-byte aligned */
#define aligned_u64 __u64 __attribute__((aligned(8)))
#define aligned_be64 __be64 __attribute__((aligned(8)))
#define aligned_le64 __le64 __attribute__((aligned(8)))

/**
 * The type used for indexing onto a disc or disc partition.
 *
 * Linux always considers sectors to be 512 bytes long independently
 * of the devices real block size.
 *
 * blkcnt_t is the type of the inode's block count.
 */
#ifdef CONFIG_LBDAF
typedef u64 sector_t;
typedef u64 blkcnt_t;
#else
typedef unsigned long sector_t;
typedef unsigned long blkcnt_t;
#endif

/*
 * The type of an index into the pagecache.
 */
#define pgoff_t unsigned long

/*
 * A dma_addr_t can hold any valid DMA address, i.e., any address returned
 * by the DMA API.
 *
 * If the DMA API only uses 32-bit addresses, dma_addr_t need only be 32
 * bits wide.  Bus addresses, e.g., PCI BARs, may be wider than 32 bits,
 * but drivers do memory-mapped I/O to ioremapped kernel virtual addresses,
 * so they don't care about the size of the actual bus addresses.
 */
#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
typedef u64 dma_addr_t;
#else
typedef u32 dma_addr_t;
#endif

typedef unsigned __bitwise__ gfp_t;
typedef unsigned __bitwise__ fmode_t;
typedef unsigned __bitwise__ oom_flags_t;

#ifdef CONFIG_PHYS_ADDR_T_64BIT
typedef u64 phys_addr_t;
#else
typedef u32 phys_addr_t;
#endif

typedef phys_addr_t resource_size_t;

/*
 * This type is the placeholder for a hardware interrupt number. It has to be
 * big enough to enclose whatever representation is used by a given platform.
 */
typedef unsigned long irq_hw_number_t;

typedef struct {
	int counter;
} atomic_t;

#ifdef CONFIG_64BIT
typedef struct {
	long counter;
} atomic64_t;
#endif

struct list_head {
	struct list_head *next, *prev;
};

struct hlist_head {
	struct hlist_node *first;
};

struct hlist_node {
	struct hlist_node *next, **pprev;
};

struct ustat {
	__kernel_daddr_t	f_tfree;
	__kernel_ino_t		f_tinode;
	char			f_fname[6];
	char			f_fpack[6];
};

/**
 * struct callback_head - callback structure for use with RCU and task_work
 * @next: next update requests in a list
 * @func: actual update function to call after the grace period.
 *
 * The struct is aligned to size of pointer. On most architectures it happens
 * naturally due ABI requirements, but some architectures (like CRIS) have
 * weird ABI and we need to ask it explicitly.
 *
 * The alignment is required to guarantee that bits 0 and 1 of @next will be
 * clear under normal conditions -- as long as we use call_rcu(),
 * call_rcu_bh(), call_rcu_sched(), or call_srcu() to queue callback.
 *
 * This guarantee is important for few reasons:
 *  - future call_rcu_lazy() will make use of lower bits in the pointer;
 *  - the structure shares storage spacer in struct page with @compound_head,
 *    which encode PageTail() in bit 0. The guarantee is needed to avoid
 *    false-positive PageTail().
 */
struct callback_head {
	struct callback_head *next;
	void (*func)(struct callback_head *head);
} __attribute__((aligned(sizeof(void *))));
#define rcu_head callback_head

typedef void (*rcu_callback_t)(struct rcu_head *head);
typedef void (*call_rcu_func_t)(struct rcu_head *head, rcu_callback_t func);

/* clocksource cycle base type */
typedef u64 cycle_t;

```


####  list.h