Data Types in the Kernel Ted Baker Andy Wang CIS 4930 / COP 5641

Data Types in the Kernel

Ted Baker Andy Wang

CIS 4930 / COP 5641

Kernel Data Types

For portability Should compile with –Wall –Wstrict-

prototypes flags Three main classes

Standard C types (e.g., int) Explicitly sized types (e.g., u32) Types for specific kernel objects (e.g.,

pid_t)

Use of Standard C Types

Normal C types are not the same size on all architectures

Try misc-progs/database% misc-progs/datasize

arch Size: char short int long ptr long-long u8 u16 u32 u64

i686 1 2 4 4 4 8 1 2 4 8

Use of Standard C Types

64-bit platforms have different data type representations

arch Size: char short int long ptr long-long u8 u16 u32 u64

i386 1 2 4 4 4 8 1 2 4 8

alpha 1 2 4 8 8 8 1 2 4 8

armv4l 1 2 4 4 4 8 1 2 4 8

ia64 1 2 4 8 8 8 1 2 4 8

m68k 1 2 4 4 4 8 1 2 4 8

mips 1 2 4 4 4 8 1 2 4 8

ppc 1 2 4 4 4 8 1 2 4 8

sparc 1 2 4 4 4 8 1 2 4 8

sparc64 1 2 4 4 4 8 1 2 4 8

x86_64 1 2 4 8 8 8 1 2 4 8

Use of Standard C Types

Knowing that pointers and long integers have the same size Using unsigned long for kernel

addresses prevents unintended pointer dereferencing

Assigning an Explicit Size to Data Items

See <asm/types.h> u8; /* unsigned byte (8-bits) */ u16; /* unsigned word (16-bits) */ u32; /* unsigned 32-bit value */ u64; /* unsigned 64-bit value */

If a user-space program needs to use these types, use __ prefix (e.g., __u8)

Assigning an Explicit Size to Data Items

Kernel also uses conventional types, such as unsigned int Usually done for backward compatibility

Interface-Specific Types

Interface-specific type: defined by a library to provide an interface to specific data structure (e.g., pid_t)

Interface-Specific Types

Many _t types are defined in <linux/types.h> Problematic in printk statements One solution is to cast the value to the

biggest possible type (e.g., unsigned long) Avoids warning messages Will not lose data bits

Other Portability Issues

Be suspicious of explicit constant values

Most values are parameterized

Timer Intervals

Do not assume 1000 jiffies per second Scale times using HZ (number of

interrupts per second) For example, check against a timeout of half

a second, compare the elapsed time against HZ/2

Number of jiffies corresponding to msec second is always msec*HZ/1000

Page Size

Memory page is PAGE_SIZE bytes, not 4KB Can vary from 4KB to 64KB PAGE_SHIFT contains the number of bits

to shift an address to get its page number See <asm/page.h> User-space program can use

getpagesize library function

Page Size

Example To allocate 16KB

Should not specify an order of 2 to get_free_pages

Use get_order#include <asm/page.h>

int order = get_order(16*1024);

buf = get_free_pages(GFP_KERNEL, order);

Byte Order

PC stores multibyte values low-byte first (little-endian)

Some platforms use big-endian Use predefined macros

<linux/byteorder/big_endian.h> <linux/byteorder/little_endian.h>

Byte Order

Examples u32 cpu_to_le32(u32);

cpu = internal CPU representation le = little endian

u64 be64_to_cpu(u64); be = big endian

U16 cpu_to_le16p(u16); p = pointer

Data Alignment

How to read a 4-byte value stored at an address that is not a multiple of 4 bytes? i386 permits this kind of access Not all architectures permit it

Can raise exceptions

Data Alignment

Use the following typeless macros #include <asm/unaligned.h> get_unaligned(ptr); put_unaligned(val, ptr);

Data Alignment

Another issue is the portability of data structures Compiler rearranges structure fields to be

aligned according to platform-specific conventions

Automatically add padding to make things aligned May no longer match the intended format

Data Alignment

Use natural alignment 8-byte items go in an address multiple of

8 Use filler fields that avoid leaving holes in

the data structure Not all platforms align 64-bit values on 64-bit

boundaries Might waste memory

Data Alignment

Tell the compiler to pack the data structure with no fillers added

Example: <linux/edd.h>struct {

u16 id;

u64 lun;

u16 reserved1;

u32 reserved2;

} __attribute__ ((packed)) scsi;

Without __attribute__ ((packed)), lun would be preceded by 2-6 bytes

of fillers

Data Alignment

No compiler optimizations

Some compiler optimizations

__attribute__ ((packed))

Pointers and Error Values

Functions that return pointers cannot report negative error values Return NULL on failure

Some kernel interfaces encode error code in a pointer value Cannot be compared against NULL To use this feature, include

<linux/err.h>

Pointers and Error Values

To return an error, use void *ERR_PTR(long error);

To test whether a returned pointer is an error code, use long IS_ERR(const void *ptr);

To access the error code, use long PTR_ERR(const void *ptr);

Linked Lists

Linux provides a standard implementation of circular, doubly linked lists List functions perform no locking

To use the list mechanism, include <linux/list.h>, which containsstruct list_head {

struct list_head *next, *prev;

};

Linked Lists

To use the Linux list facility Need to embed a list_head in the

structures that make up the liststruct todo_struct {

struct list_head list;

int priority; /* driver specific */

/* ... add other driver-specific fields */

};

Linked Lists

More Fun with Linked Lists

list_head sorted_by_char

list_head sorted_by_num

A

3

B

1

C

2

Can allocate list elements as

an array

What if a structure owns its own list?

Linked Lists

The head of the list is usually a standalone structure

To declare and initialize a list head, callstruct list_head todo_list;

INIT_LIST_HEAD(&todo_list);

To initialize at compile time, callLIST_HEAD(todo_list);

Linked Lists

See <linux/list.h> for a list of list functions

/* add the new entry after the list head */

/* use it to build stacks */

list_add(struct list_head *new, struct list_head *head);

/* add the new entry before the list head (tail) */

/* use it to build FIFO queues */

list_add_tail(struct list_head *new, struct list_head *head);

Linked Lists

/* the given entry is removed from the list */

/* if the entry might be reinserted into another list, call list_del_init */

list_del(struct list_head *entry);

list_del_init(struct list_head *entry);

/* remove the entry from one list and insert into another list */

list_move(struct list_head *entry, struct list_head *head);

list_move_tail(struct list_head *entry,

struct list_head *head);

/* return a nonzero value if the given list is empty */

list_empty(struct list_head *head);

Linked Lists

/* insert a list immediately after head */list_splice(struct list_head *list, struct list_head *head);

To access the data structure itself, uselist_entry(struct list_head *ptr, type_of_struct, field_name);

Same as container_of() ptr is a pointer to a struct

list_head entry

Linked Lists

type_of_struct is the type of the structure containing the ptr

field_name is the name of the list field within the structure

Examplestruct todo_struct *todo_ptr = list_entry(listptr, struct todo_struct, list);

#define container_of(ptr, type, member) ({ const typeof(((type *)0->member) *__mptr = (ptr); (type *) ((char *)__mptr – offsetof(type, member)); })Type

checking

Linked Lists

To traverse the linked list, one can follow the prev and next pointers

void todo_add_entry(struct todo_struct *new) {

struct list_head *ptr;

struct todo_struct *entry;

for (ptr = todo_list.next; ptr != &todo_list;

ptr = ptr->next) {

entry = list_entry(ptr, struct todo_struct, list);

if (entry->priority < new->priority) {

list_add_tail(&new->list, ptr);

return;

}

}

list_add_tail(&new->list, &todo_struct)

}

Linked Lists

One can also use predefined macrosvoid todo_add_entry(struct todo_struct *new) {

struct list_head *ptr;

struct todo_struct *entry;

list_for_each(ptr, &todo_list) {

entry = list_entry(ptr, struct todo_struct, list);

if (entry->priority < new->priority) {

list_add_tail(&new->list, ptr);

return;

}

}

list_add_tail(&new->list, &todo_struct)

}

Linked Lists

Predefined macros avoid simple programming errors See <linux/list.h>

/* creates a loop that executes once with cursor pointing at each successive entry */

/* be careful about changing the list while iterating */

list_for_each(struct list_head *cursor,

struct list_head *list)

/* iterates backward */

list_for_each_prev(struct list_head *cursor,

struct list_head *list)

Linked Lists

/* for deleting entries in the list */

/* stores the next entry in next at the beginning of the loop */

list_for_each_safe(struct list_head *cursor,

struct list_head *next,

struct list_head *list)

/* ease the process of dealing with a list containing a given type */

/* no need to call list_entry inside the loop */

list_for_each_entry(type *cursor, struct list_head *list,

member)

list_for_each_entry_safe(type *cursor, type *next,

struct list_head *list, member)